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evolution | scientific theory | Britannica.com

Alternative Title: descent

Evolution, theory in biology postulating that the various types of plants, animals, and other living things on Earth have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory.

The diversity of the living world is staggering. More than 2 million existing species of organisms have been named and described; many more remain to be discoveredfrom 10 million to 30 million, according to some estimates. What is impressive is not just the numbers but also the incredible heterogeneity in size, shape, and way of lifefrom lowly bacteria, measuring less than a thousandth of a millimetre in diameter, to stately sequoias, rising 100 metres (300 feet) above the ground and weighing several thousand tons; from bacteria living in hot springs at temperatures near the boiling point of water to fungi and algae thriving on the ice masses of Antarctica and in saline pools at 23 C (9 F); and from giant tube worms discovered living near hydrothermal vents on the dark ocean floor to spiders and larkspur plants existing on the slopes of Mount Everest more than 6,000 metres (19,700 feet) above sea level.

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heredity (genetics): Heredity and evolution

At the centre of the theory of evolution as proposed by Charles Darwin and Alfred Russell Wallace were the concepts of variation and natural selection. Hereditary variants were thought to arise naturally in populations, and then these were either selected for or against by the contemporary environmental conditions. In this way, subsequent generations either became enriched or impoverished for…

The virtually infinite variations on life are the fruit of the evolutionary process. All living creatures are related by descent from common ancestors. Humans and other mammals descend from shrewlike creatures that lived more than 150 million years ago; mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived 600 million years ago; and all plants and animals derive from bacteria-like microorganisms that originated more than 3 billion years ago. Biological evolution is a process of descent with modification. Lineages of organisms change through generations; diversity arises because the lineages that descend from common ancestors diverge through time.

The 19th-century English naturalist Charles Darwin argued that organisms come about by evolution, and he provided a scientific explanation, essentially correct but incomplete, of how evolution occurs and why it is that organisms have featuressuch as wings, eyes, and kidneysclearly structured to serve specific functions. Natural selection was the fundamental concept in his explanation. Natural selection occurs because individuals having more-useful traits, such as more-acute vision or swifter legs, survive better and produce more progeny than individuals with less-favourable traits. Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution. Beginning in the 1960s, a related scientific discipline, molecular biology, enormously advanced knowledge of biological evolution and made it possible to investigate detailed problems that had seemed completely out of reach only a short time previouslyfor example, how similar the genes of humans and chimpanzees might be (they differ in about 12 percent of the units that make up the genes).

This article discusses evolution as it applies generally to living things. For a discussion of human evolution, see the article human evolution. For a more complete treatment of a discipline that has proved essential to the study of evolution, see the articles genetics, human and heredity. Specific aspects of evolution are discussed in the articles coloration and mimicry. Applications of evolutionary theory to plant and animal breeding are discussed in the articles plant breeding and animal breeding. An overview of the evolution of life as a major characteristic of Earths history is given in community ecology: Evolution of the biosphere. A detailed discussion of the life and thought of Charles Darwin is found in the article Darwin, Charles.

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Darwin and other 19th-century biologists found compelling evidence for biological evolution in the comparative study of living organisms, in their geographic distribution, and in the fossil remains of extinct organisms. Since Darwins time, the evidence from these sources has become considerably stronger and more comprehensive, while biological disciplines that emerged more recentlygenetics, biochemistry, physiology, ecology, animal behaviour (ethology), and especially molecular biologyhave supplied powerful additional evidence and detailed confirmation. The amount of information about evolutionary history stored in the DNA and proteins of living things is virtually unlimited; scientists can reconstruct any detail of the evolutionary history of life by investing sufficient time and laboratory resources.

Evolutionists no longer are concerned with obtaining evidence to support the fact of evolution but rather are concerned with what sorts of knowledge can be obtained from different sources of evidence. The following sections identify the most productive of these sources and illustrate the types of information they have provided.

Paleontologists have recovered and studied the fossil remains of many thousands of organisms that lived in the past. This fossil record shows that many kinds of extinct organisms were very different in form from any now living. It also shows successions of organisms through time (see faunal succession, law of; geochronology: Determining the relationships of fossils with rock strata), manifesting their transition from one form to another.

When an organism dies, it is usually destroyed by other forms of life and by weathering processes. On rare occasions some body partsparticularly hard ones such as shells, teeth, or bonesare preserved by being buried in mud or protected in some other way from predators and weather. Eventually, they may become petrified and preserved indefinitely with the rocks in which they are embedded. Methods such as radiometric datingmeasuring the amounts of natural radioactive atoms that remain in certain minerals to determine the elapsed time since they were constitutedmake it possible to estimate the time period when the rocks, and the fossils associated with them, were formed.

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Radiometric dating indicates that Earth was formed about 4.5 billion years ago. The earliest fossils resemble microorganisms such as bacteria and cyanobacteria (blue-green algae); the oldest of these fossils appear in rocks 3.5 billion years old (see Precambrian time). The oldest known animal fossils, about 700 million years old, come from the so-called Ediacara fauna, small wormlike creatures with soft bodies. Numerous fossils belonging to many living phyla and exhibiting mineralized skeletons appear in rocks about 540 million years old. These organisms are different from organisms living now and from those living at intervening times. Some are so radically different that paleontologists have created new phyla in order to classify them. (See Cambrian Period.) The first vertebrates, animals with backbones, appeared about 400 million years ago; the first mammals, less than 200 million years ago. The history of life recorded by fossils presents compelling evidence of evolution.

The fossil record is incomplete. Of the small proportion of organisms preserved as fossils, only a tiny fraction have been recovered and studied by paleontologists. In some cases the succession of forms over time has been reconstructed in detail. One example is the evolution of the horse. The horse can be traced to an animal the size of a dog having several toes on each foot and teeth appropriate for browsing; this animal, called the dawn horse (genus Hyracotherium), lived more than 50 million years ago. The most recent form, the modern horse (Equus), is much larger in size, is one-toed, and has teeth appropriate for grazing. The transitional forms are well preserved as fossils, as are many other kinds of extinct horses that evolved in different directions and left no living descendants.

Using recovered fossils, paleontologists have reconstructed examples of radical evolutionary transitions in form and function. For example, the lower jaw of reptiles contains several bones, but that of mammals only one. The other bones in the reptile jaw unmistakably evolved into bones now found in the mammalian ear. At first, such a transition would seem unlikelyit is hard to imagine what function such bones could have had during their intermediate stages. Yet paleontologists discovered two transitional forms of mammal-like reptiles, called therapsids, that had a double jaw joint (i.e., two hinge points side by side)one joint consisting of the bones that persist in the mammalian jaw and the other composed of the quadrate and articular bones, which eventually became the hammer and anvil of the mammalian ear. (See also mammal: Skeleton.)

For skeptical contemporaries of Darwin, the missing linkthe absence of any known transitional form between apes and humanswas a battle cry, as it remained for uninformed people afterward. Not one but many creatures intermediate between living apes and humans have since been found as fossils. The oldest known fossil homininsi.e., primates belonging to the human lineage after it separated from lineages going to the apesare 6 million to 7 million years old, come from Africa, and are known as Sahelanthropus and Orrorin (or Praeanthropus), which were predominantly bipedal when on the ground but which had very small brains. Ardipithecus lived about 4.4 million years ago, also in Africa. Numerous fossil remains from diverse African origins are known of Australopithecus, a hominin that appeared between 3 million and 4 million years ago. Australopithecus had an upright human stance but a cranial capacity of less than 500 cc (equivalent to a brain weight of about 500 grams), comparable to that of a gorilla or a chimpanzee and about one-third that of humans. Its head displayed a mixture of ape and human characteristicsa low forehead and a long, apelike face but with teeth proportioned like those of humans. Other early hominins partly contemporaneous with Australopithecus include Kenyanthropus and Paranthropus; both had comparatively small brains, although some species of Paranthropus had larger bodies. Paranthropus represents a side branch in the hominin lineage that became extinct. Along with increased cranial capacity, other human characteristics have been found in Homo habilis, which lived about 1.5 million to 2 million years ago in Africa and had a cranial capacity of more than 600 cc (brain weight of 600 grams), and in H. erectus, which lived between 0.5 million and more than 1.5 million years ago, apparently ranged widely over Africa, Asia, and Europe, and had a cranial capacity of 800 to 1,100 cc (brain weight of 800 to 1,100 grams). The brain sizes of H. ergaster, H. antecessor, and H. heidelbergensis were roughly that of the brain of H. erectus, some of which species were partly contemporaneous, though they lived in different regions of the Eastern Hemisphere. (See also human evolution.)

The skeletons of turtles, horses, humans, birds, and bats are strikingly similar, in spite of the different ways of life of these animals and the diversity of their environments. The correspondence, bone by bone, can easily be seen not only in the limbs but also in every other part of the body. From a purely practical point of view, it is incomprehensible that a turtle should swim, a horse run, a person write, and a bird or a bat fly with forelimb structures built of the same bones. An engineer could design better limbs in each case. But if it is accepted that all of these skeletons inherited their structures from a common ancestor and became modified only as they adapted to different ways of life, the similarity of their structures makes sense.

Comparative anatomy investigates the homologies, or inherited similarities, among organisms in bone structure and in other parts of the body. The correspondence of structures is typically very close among some organismsthe different varieties of songbirds, for instancebut becomes less so as the organisms being compared are less closely related in their evolutionary history. The similarities are less between mammals and birds than they are among mammals, and they are still less between mammals and fishes. Similarities in structure, therefore, not only manifest evolution but also help to reconstruct the phylogeny, or evolutionary history, of organisms.

Comparative anatomy also reveals why most organismic structures are not perfect. Like the forelimbs of turtles, horses, humans, birds, and bats, an organisms body parts are less than perfectly adapted because they are modified from an inherited structure rather than designed from completely raw materials for a specific purpose. The imperfection of structures is evidence for evolution and against antievolutionist arguments that invoke intelligent design (see below Intelligent design and its critics).

Darwin and his followers found support for evolution in the study of embryology, the science that investigates the development of organisms from fertilized egg to time of birth or hatching. Vertebrates, from fishes through lizards to humans, develop in ways that are remarkably similar during early stages, but they become more and more differentiated as the embryos approach maturity. The similarities persist longer between organisms that are more closely related (e.g., humans and monkeys) than between those less closely related (humans and sharks). Common developmental patterns reflect evolutionary kinship. Lizards and humans share a developmental pattern inherited from their remote common ancestor; the inherited pattern of each was modified only as the separate descendant lineages evolved in different directions. The common embryonic stages of the two creatures reflect the constraints imposed by this common inheritance, which prevents changes that have not been necessitated by their diverging environments and ways of life.

The embryos of humans and other nonaquatic vertebrates exhibit gill slits even though they never breathe through gills. These slits are found in the embryos of all vertebrates because they share as common ancestors the fish in which these structures first evolved. Human embryos also exhibit by the fourth week of development a well-defined tail, which reaches maximum length at six weeks. Similar embryonic tails are found in other mammals, such as dogs, horses, and monkeys; in humans, however, the tail eventually shortens, persisting only as a rudiment in the adult coccyx.

A close evolutionary relationship between organisms that appear drastically different as adults can sometimes be recognized by their embryonic homologies. Barnacles, for example, are sedentary crustaceans with little apparent likeness to such free-swimming crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-swimming larval stage, the nauplius, which is unmistakably similar to that of other crustacean larvae.

Embryonic rudiments that never fully develop, such as the gill slits in humans, are common in all sorts of animals. Some, however, like the tail rudiment in humans, persist as adult vestiges, reflecting evolutionary ancestry. The most familiar rudimentary organ in humans is the vermiform appendix. This wormlike structure attaches to a short section of intestine called the cecum, which is located at the point where the large and small intestines join. The human vermiform appendix is a functionless vestige of a fully developed organ present in other mammals, such as the rabbit and other herbivores, where a large cecum and appendix store vegetable cellulose to enable its digestion with the help of bacteria. Vestiges are instances of imperfectionslike the imperfections seen in anatomical structuresthat argue against creation by design but are fully understandable as a result of evolution.

Darwin also saw a confirmation of evolution in the geographic distribution of plants and animals, and later knowledge has reinforced his observations. For example, there are about 1,500 known species of Drosophila vinegar flies in the world; nearly one-third of them live in Hawaii and nowhere else, although the total area of the archipelago is less than one-twentieth the area of California or Germany. Also in Hawaii are more than 1,000 species of snails and other land mollusks that exist nowhere else. This unusual diversity is easily explained by evolution. The islands of Hawaii are extremely isolated and have had few colonizersi.e, animals and plants that arrived there from elsewhere and established populations. Those species that did colonize the islands found many unoccupied ecological niches, local environments suited to sustaining them and lacking predators that would prevent them from multiplying. In response, these species rapidly diversified; this process of diversifying in order to fill ecological niches is called adaptive radiation.

Each of the worlds continents has its own distinctive collection of animals and plants. In Africa are rhinoceroses, hippopotamuses, lions, hyenas, giraffes, zebras, lemurs, monkeys with narrow noses and nonprehensile tails, chimpanzees, and gorillas. South America, which extends over much the same latitudes as Africa, has none of these animals; it instead has pumas, jaguars, tapir, llamas, raccoons, opossums, armadillos, and monkeys with broad noses and large prehensile tails.

These vagaries of biogeography are not due solely to the suitability of the different environments. There is no reason to believe that South American animals are not well suited to living in Africa or those of Africa to living in South America. The islands of Hawaii are no better suited than other Pacific islands for vinegar flies, nor are they less hospitable than other parts of the world for many absent organisms. In fact, although no large mammals are native to the Hawaiian islands, pigs and goats have multiplied there as wild animals since being introduced by humans. This absence of many species from a hospitable environment in which an extraordinary variety of other species flourish can be explained by the theory of evolution, which holds that species can exist and evolve only in geographic areas that were colonized by their ancestors.

The field of molecular biology provides the most detailed and convincing evidence available for biological evolution. In its unveiling of the nature of DNA and the workings of organisms at the level of enzymes and other protein molecules, it has shown that these molecules hold information about an organisms ancestry. This has made it possible to reconstruct evolutionary events that were previously unknown and to confirm and adjust the view of events already known. The precision with which these events can be reconstructed is one reason the evidence from molecular biology is so compelling. Another reason is that molecular evolution has shown all living organisms, from bacteria to humans, to be related by descent from common ancestors.

A remarkable uniformity exists in the molecular components of organismsin the nature of the components as well as in the ways in which they are assembled and used. In all bacteria, plants, animals, and humans, the DNA comprises a different sequence of the same four component nucleotides, and all the various proteins are synthesized from different combinations and sequences of the same 20 amino acids, although several hundred other amino acids do exist. The genetic code by which the information contained in the DNA of the cell nucleus is passed on to proteins is virtually everywhere the same. Similar metabolic pathwayssequences of biochemical reactions (see metabolism)are used by the most diverse organisms to produce energy and to make up the cell components.

This unity reveals the genetic continuity and common ancestry of all organisms. There is no other rational way to account for their molecular uniformity when numerous alternative structures are equally likely. The genetic code serves as an example. Each particular sequence of three nucleotides in the nuclear DNA acts as a pattern for the production of exactly the same amino acid in all organisms. This is no more necessary than it is for a language to use a particular combination of letters to represent a particular object. If it is found that certain sequences of lettersplanet, tree, womanare used with identical meanings in a number of different books, one can be sure that the languages used in those books are of common origin.

Genes and proteins are long molecules that contain information in the sequence of their components in much the same way as sentences of the English language contain information in the sequence of their letters and words. The sequences that make up the genes are passed on from parents to offspring and are identical except for occasional changes introduced by mutations. As an illustration, one may assume that two books are being compared. Both books are 200 pages long and contain the same number of chapters. Closer examination reveals that the two books are identical page for page and word for word, except that an occasional wordsay, one in 100is different. The two books cannot have been written independently; either one has been copied from the other, or both have been copied, directly or indirectly, from the same original book. Similarly, if each component nucleotide of DNA is represented by one letter, the complete sequence of nucleotides in the DNA of a higher organism would require several hundred books of hundreds of pages, with several thousand letters on each page. When the pages (or sequences of nucleotides) in these books (organisms) are examined one by one, the correspondence in the letters (nucleotides) gives unmistakable evidence of common origin.

The two arguments presented above are based on different grounds, although both attest to evolution. Using the alphabet analogy, the first argument says that languages that use the same dictionarythe same genetic code and the same 20 amino acidscannot be of independent origin. The second argument, concerning similarity in the sequence of nucleotides in the DNA (and thus the sequence of amino acids in the proteins), says that books with very similar texts cannot be of independent origin.

The evidence of evolution revealed by molecular biology goes even farther. The degree of similarity in the sequence of nucleotides or of amino acids can be precisely quantified. For example, in humans and chimpanzees, the protein molecule called cytochrome c, which serves a vital function in respiration within cells, consists of the same 104 amino acids in exactly the same order. It differs, however, from the cytochrome c of rhesus monkeys by 1 amino acid, from that of horses by 11 additional amino acids, and from that of tuna by 21 additional amino acids. The degree of similarity reflects the recency of common ancestry. Thus, the inferences from comparative anatomy and other disciplines concerning evolutionary history can be tested in molecular studies of DNA and proteins by examining their sequences of nucleotides and amino acids. (See below DNA and protein as informational macromolecules.)

The authority of this kind of test is overwhelming; each of the thousands of genes and thousands of proteins contained in an organism provides an independent test of that organisms evolutionary history. Not all possible tests have been performed, but many hundreds have been done, and not one has given evidence contrary to evolution. There is probably no other notion in any field of science that has been as extensively tested and as thoroughly corroborated as the evolutionary origin of living organisms.

All human cultures have developed their own explanations for the origin of the world and of human beings and other creatures. Traditional Judaism and Christianity explain the origin of living beings and their adaptations to their environmentswings, gills, hands, flowersas the handiwork of an omniscient God. The philosophers of ancient Greece had their own creation myths. Anaximander proposed that animals could be transformed from one kind into another, and Empedocles speculated that they were made up of various combinations of preexisting parts. Closer to modern evolutionary ideas were the proposals of early Church Fathers such as Gregory of Nazianzus and Augustine, both of whom maintained that not all species of plants and animals were created by God; rather, some had developed in historical times from Gods creations. Their motivation was not biological but religiousit would have been impossible to hold representatives of all species in a single vessel such as Noahs Ark; hence, some species must have come into existence only after the Flood.

The notion that organisms may change by natural processes was not investigated as a biological subject by Christian theologians of the Middle Ages, but it was, usually incidentally, considered as a possibility by many, including Albertus Magnus and his student Thomas Aquinas. Aquinas concluded, after detailed discussion, that the development of living creatures such as maggots and flies from nonliving matter such as decaying meat was not incompatible with Christian faith or philosophy. But he left it to others to determine whether this actually happened.

The idea of progress, particularly the belief in unbounded human progress, was central to the Enlightenment of the 18th century, particularly in France among such philosophers as the marquis de Condorcet and Denis Diderot and such scientists as Georges-Louis Leclerc, comte de Buffon. But belief in progress did not necessarily lead to the development of a theory of evolution. Pierre-Louis Moreau de Maupertuis proposed the spontaneous generation and extinction of organisms as part of his theory of origins, but he advanced no theory of evolutioni.e., the transformation of one species into another through knowable, natural causes. Buffon, one of the greatest naturalists of the time, explicitly consideredand rejectedthe possible descent of several species from a common ancestor. He postulated that organisms arise from organic molecules by spontaneous generation, so that there could be as many kinds of animals and plants as there are viable combinations of organic molecules.

The English physician Erasmus Darwin, grandfather of Charles Darwin, offered in his Zoonomia; or, The Laws of Organic Life (179496) some evolutionary speculations, but they were not further developed and had no real influence on subsequent theories. The Swedish botanist Carolus Linnaeus devised the hierarchical system of plant and animal classification that is still in use in a modernized form. Although he insisted on the fixity of species, his classification system eventually contributed much to the acceptance of the concept of common descent.

The great French naturalist Jean-Baptiste de Monet, chevalier de Lamarck, held the enlightened view of his age that living organisms represent a progression, with humans as the highest form. From this idea he proposed, in the early years of the 19th century, the first broad theory of evolution. Organisms evolve through eons of time from lower to higher forms, a process still going on, always culminating in human beings. As organisms become adapted to their environments through their habits, modifications occur. Use of an organ or structure reinforces it; disuse leads to obliteration. The characteristics acquired by use and disuse, according to this theory, would be inherited. This assumption, later called the inheritance of acquired characteristics (or Lamarckism), was thoroughly disproved in the 20th century. Although his theory did not stand up in the light of later knowledge, Lamarck made important contributions to the gradual acceptance of biological evolution and stimulated countless later studies.

The founder of the modern theory of evolution was Charles Darwin. The son and grandson of physicians, he enrolled as a medical student at the University of Edinburgh. After two years, however, he left to study at the University of Cambridge and prepare to become a clergyman. He was not an exceptional student, but he was deeply interested in natural history. On December 27, 1831, a few months after his graduation from Cambridge, he sailed as a naturalist aboard the HMS Beagle on a round-the-world trip that lasted until October 1836. Darwin was often able to disembark for extended trips ashore to collect natural specimens.

The discovery of fossil bones from large extinct mammals in Argentina and the observation of numerous species of finches in the Galapagos Islands were among the events credited with stimulating Darwins interest in how species originate. In 1859 he published On the Origin of Species by Means of Natural Selection, a treatise establishing the theory of evolution and, most important, the role of natural selection in determining its course. He published many other books as well, notably The Descent of Man and Selection in Relation to Sex (1871), which extends the theory of natural selection to human evolution.

Darwin must be seen as a great intellectual revolutionary who inaugurated a new era in the cultural history of humankind, an era that was the second and final stage of the Copernican revolution that had begun in the 16th and 17th centuries under the leadership of men such as Nicolaus Copernicus, Galileo, and Isaac Newton. The Copernican revolution marked the beginnings of modern science. Discoveries in astronomy and physics overturned traditional conceptions of the universe. Earth no longer was seen as the centre of the universe but was seen as a small planet revolving around one of myriad stars; the seasons and the rains that make crops grow, as well as destructive storms and other vagaries of weather, became understood as aspects of natural processes; the revolutions of the planets were now explained by simple laws that also accounted for the motion of projectiles on Earth.

The significance of these and other discoveries was that they led to a conception of the universe as a system of matter in motion governed by laws of nature. The workings of the universe no longer needed to be attributed to the ineffable will of a divine Creator; rather, they were brought into the realm of sciencean explanation of phenomena through natural laws. Physical phenomena such as tides, eclipses, and positions of the planets could now be predicted whenever the causes were adequately known. Darwin accumulated evidence showing that evolution had occurred, that diverse organisms share common ancestors, and that living beings have changed drastically over the course of Earths history. More important, however, he extended to the living world the idea of nature as a system of matter in motion governed by natural laws.

Before Darwin, the origin of Earths living things, with their marvelous contrivances for adaptation, had been attributed to the design of an omniscient God. He had created the fish in the waters, the birds in the air, and all sorts of animals and plants on the land. God had endowed these creatures with gills for breathing, wings for flying, and eyes for seeing, and he had coloured birds and flowers so that human beings could enjoy them and recognize Gods wisdom. Christian theologians, from Aquinas on, had argued that the presence of design, so evident in living beings, demonstrates the existence of a supreme Creator; the argument from design was Aquinass fifth way for proving the existence of God. In 19th-century England the eight Bridgewater Treatises were commissioned so that eminent scientists and philosophers would expand on the marvels of the natural world and thereby set forth the Power, wisdom, and goodness of God as manifested in the Creation.

The British theologian William Paley in his Natural Theology (1802) used natural history, physiology, and other contemporary knowledge to elaborate the argument from design. If a person should find a watch, even in an uninhabited desert, Paley contended, the harmony of its many parts would force him to conclude that it had been created by a skilled watchmaker; and, Paley went on, how much more intricate and perfect in design is the human eye, with its transparent lens, its retina placed at the precise distance for forming a distinct image, and its large nerve transmitting signals to the brain.

The argument from design seems to be forceful. A ladder is made for climbing, a knife for cutting, and a watch for telling time; their functional design leads to the conclusion that they have been fashioned by a carpenter, a smith, or a watchmaker. Similarly, the obvious functional design of animals and plants seems to denote the work of a Creator. It was Darwins genius that he provided a natural explanation for the organization and functional design of living beings. (For additional discussion of the argument from design and its revival in the 1990s, see below Intelligent design and its critics.)

Darwin accepted the facts of adaptationhands are for grasping, eyes for seeing, lungs for breathing. But he showed that the multiplicity of plants and animals, with their exquisite and varied adaptations, could be explained by a process of natural selection, without recourse to a Creator or any designer agent. This achievement would prove to have intellectual and cultural implications more profound and lasting than his multipronged evidence that convinced contemporaries of the fact of evolution.

Darwins theory of natural selection is summarized in the Origin of Species as follows:

As many more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.

Natural selection was proposed by Darwin primarily to account for the adaptive organization of living beings; it is a process that promotes or maintains adaptation. Evolutionary change through time and evolutionary diversification (multiplication of species) are not directly promoted by natural selection, but they often ensue as by-products of natural selection as it fosters adaptation to different environments.

The publication of the Origin of Species produced considerable public excitement. Scientists, politicians, clergymen, and notables of all kinds read and discussed the book, defending or deriding Darwins ideas. The most visible actor in the controversies immediately following publication was the English biologist T.H. Huxley, known as Darwins bulldog, who defended the theory of evolution with articulate and sometimes mordant words on public occasions as well as in numerous writings. Evolution by natural selection was indeed a favourite topic in society salons during the 1860s and beyond. But serious scientific controversies also arose, first in Britain and then on the Continent and in the United States.

One occasional participant in the discussion was the British naturalist Alfred Russel Wallace, who had hit upon the idea of natural selection independently and had sent a short manuscript about it to Darwin from the Malay Archipelago, where he was collecting specimens and writing. On July 1, 1858, one year before the publication of the Origin, a paper jointly authored by Wallace and Darwin was presented, in the absence of both, to the Linnean Society in Londonwith apparently little notice. Greater credit is duly given to Darwin than to Wallace for the idea of evolution by natural selection; Darwin developed the theory in considerably more detail, provided far more evidence for it, and was primarily responsible for its acceptance. Wallaces views differed from Darwins in several ways, most importantly in that Wallace did not think natural selection sufficient to account for the origin of human beings, which in his view required direct divine intervention.

A younger English contemporary of Darwin, with considerable influence during the latter part of the 19th and in the early 20th century, was Herbert Spencer. A philosopher rather than a biologist, he became an energetic proponent of evolutionary ideas, popularized a number of slogans, such as survival of the fittest (which was taken up by Darwin in later editions of the Origin), and engaged in social and metaphysical speculations. His ideas considerably damaged proper understanding and acceptance of the theory of evolution by natural selection. Darwin wrote of Spencers speculations:

His deductive manner of treating any subject is wholly opposed to my frame of mind.His fundamental generalizations (which have been compared in importance by some persons with Newtons laws!) which I dare say may be very valuable under a philosophical point of view, are of such a nature that they do not seem to me to be of any strictly scientific use.

Most pernicious was the crude extension by Spencer and others of the notion of the struggle for existence to human economic and social life that became known as social Darwinism (see below Scientific acceptance and extension to other disciplines).

The most serious difficulty facing Darwins evolutionary theory was the lack of an adequate theory of inheritance that would account for the preservation through the generations of the variations on which natural selection was supposed to act. Contemporary theories of blending inheritance proposed that offspring merely struck an average between the characteristics of their parents. But as Darwin became aware, blending inheritance (including his own theory of pangenesis, in which each organ and tissue of an organism throws off tiny contributions of itself that are collected in the sex organs and determine the configuration of the offspring) could not account for the conservation of variations, because differences between variant offspring would be halved each generation, rapidly reducing the original variation to the average of the preexisting characteristics.

The missing link in Darwins argument was provided by Mendelian genetics. About the time the Origin of Species was published, the Augustinian monk Gregor Mendel was starting a long series of experiments with peas in the garden of his monastery in Brnn, Austria-Hungary (now Brno, Czech Republic). These experiments and the analysis of their results are by any standard an example of masterly scientific method. Mendels paper, published in 1866 in the Proceedings of the Natural Science Society of Brnn, formulated the fundamental principles of the theory of heredity that is still current. His theory accounts for biological inheritance through particulate factors (now known as genes) inherited one from each parent, which do not mix or blend but segregate in the formation of the sex cells, or gametes.

Mendels discoveries remained unknown to Darwin, however, and, indeed, they did not become generally known until 1900, when they were simultaneously rediscovered by a number of scientists on the Continent. In the meantime, Darwinism in the latter part of the 19th century faced an alternative evolutionary theory known as neo-Lamarckism. This hypothesis shared with Lamarcks the importance of use and disuse in the development and obliteration of organs, and it added the notion that the environment acts directly on organic structures, which explained their adaptation to the way of life and environment of the organism. Adherents of this theory discarded natural selection as an explanation for adaptation to the environment.

Prominent among the defenders of natural selection was the German biologist August Weismann, who in the 1880s published his germ plasm theory. He distinguished two substances that make up an organism: the soma, which comprises most body parts and organs, and the germ plasm, which contains the cells that give rise to the gametes and hence to progeny. Early in the development of an egg, the germ plasm becomes segregated from the somatic cells that give rise to the rest of the body. This notion of a radical separation between germ plasm and somathat is, between the reproductive tissues and all other body tissuesprompted Weismann to assert that inheritance of acquired characteristics was impossible, and it opened the way for his championship of natural selection as the only major process that would account for biological evolution. Weismanns ideas became known after 1896 as neo-Darwinism.

The rediscovery in 1900 of Mendels theory of heredity, by the Dutch botanist and geneticist Hugo de Vries and others, led to an emphasis on the role of heredity in evolution. De Vries proposed a new theory of evolution known as mutationism, which essentially did away with natural selection as a major evolutionary process. According to de Vries (who was joined by other geneticists such as William Bateson in England), two kinds of variation take place in organisms. One is the ordinary variability observed among individuals of a species, which is of no lasting consequence in evolution because, according to de Vries, it could not lead to a transgression of the species border [i.e., to establishment of new species] even under conditions of the most stringent and continued selection. The other consists of the changes brought about by mutations, spontaneous alterations of genes that result in large modifications of the organism and give rise to new species: The new species thus originates suddenly, it is produced by the existing one without any visible preparation and without transition.

Mutationism was opposed by many naturalists and in particular by the so-called biometricians, led by the English statistician Karl Pearson, who defended Darwinian natural selection as the major cause of evolution through the cumulative effects of small, continuous, individual variations (which the biometricians assumed passed from one generation to the next without being limited by Mendels laws of inheritance [see Mendelism]).

The controversy between mutationists (also referred to at the time as Mendelians) and biometricians approached a resolution in the 1920s and 30s through the theoretical work of geneticists. These scientists used mathematical arguments to show, first, that continuous variation (in such characteristics as body size, number of eggs laid, and the like) could be explained by Mendels laws and, second, that natural selection acting cumulatively on small variations could yield major evolutionary changes in form and function. Distinguished members of this group of theoretical geneticists were R.A. Fisher and J.B.S. Haldane in Britain and Sewall Wright in the United States. Their work contributed to the downfall of mutationism and, most important, provided a theoretical framework for the integration of genetics into Darwins theory of natural selection. Yet their work had a limited impact on contemporary biologists for several reasonsit was formulated in a mathematical language that most biologists could not understand; it was almost exclusively theoretical, with little empirical corroboration; and it was limited in scope, largely omitting many issues, such as speciation (the process by which new species are formed), that were of great importance to evolutionists.

A major breakthrough came in 1937 with the publication of Genetics and the Origin of Species by Theodosius Dobzhansky, a Russian-born American naturalist and experimental geneticist. Dobzhanskys book advanced a reasonably comprehensive account of the evolutionary process in genetic terms, laced with experimental evidence supporting the theoretical argument. Genetics and the Origin of Species may be considered the most important landmark in the formulation of what came to be known as the synthetic theory of evolution, effectively combining Darwinian natural selection and Mendelian genetics. It had an enormous impact on naturalists and experimental biologists, who rapidly embraced the new understanding of the evolutionary process as one of genetic change in populations. Interest in evolutionary studies was greatly stimulated, and contributions to the theory soon began to follow, extending the synthesis of genetics and natural selection to a variety of biological fields.

The main writers who, together with Dobzhansky, may be considered the architects of the synthetic theory were the German-born American zoologist Ernst Mayr, the English zoologist Julian Huxley, the American paleontologist George Gaylord Simpson, and the American botanist George Ledyard Stebbins. These researchers contributed to a burst of evolutionary studies in the traditional biological disciplines and in some emerging onesnotably population genetics and, later, evolutionary ecology (see community ecology). By 1950 acceptance of Darwins theory of evolution by natural selection was universal among biologists, and the synthetic theory had become widely adopted.

The most important line of investigation after 1950 was the application of molecular biology to evolutionary studies. In 1953 the American geneticist James Watson and the British biophysicist Francis Crick deduced the molecular structure of DNA (deoxyribonucleic acid), the hereditary material contained in the chromosomes of every cells nucleus. The genetic information is encoded within the sequence of nucleotides that make up the chainlike DNA molecules. This information determines the sequence of amino acid building blocks of protein molecules, which include, among others, structural proteins such as collagen, respiratory proteins such as hemoglobin, and numerous enzymes responsible for the organisms fundamental life processes. Genetic information contained in the DNA can thus be investigated by examining the sequences of amino acids in the proteins.

In the mid-1960s laboratory techniques such as electrophoresis and selective assay of enzymes became available for the rapid and inexpensive study of differences among enzymes and other proteins. The application of these techniques to evolutionary problems made possible the pursuit of issues that earlier could not be investigatedfor example, exploring the extent of genetic variation in natural populations (which sets bounds on their evolutionary potential) and determining the amount of genetic change that occurs during the formation of new species.

Comparisons of the amino acid sequences of corresponding proteins in different species provided quantitatively precise measures of the divergence among species evolved from common ancestors, a considerable improvement over the typically qualitative evaluations obtained by comparative anatomy and other evolutionary subdisciplines. In 1968 the Japanese geneticist Motoo Kimura proposed the neutrality theory of molecular evolution, which assumes that, at the level of the sequences of nucleotides in DNA and of amino acids in proteins, many changes are adaptively neutral; they have little or no effect on the molecules function and thus on an organisms fitness within its environment. If the neutrality theory is correct, there should be a molecular clock of evolution; that is, the degree to which amino acid or nucleotide sequences diverge between species should provide a reliable estimate of the time since the species diverged. This would make it possible to reconstruct an evolutionary history that would reveal the order of branching of different lineages, such as those leading to humans, chimpanzees, and orangutans, as well as the time in the past when the lineages split from one another. During the 1970s and 80s it gradually became clear that the molecular clock is not exact; nevertheless, into the early 21st century it continued to provide the most reliable evidence for reconstructing evolutionary history. (See below The molecular clock of evolution and The neutrality theory of molecular evolution.)

The laboratory techniques of DNA cloning and sequencing have provided a new and powerful means of investigating evolution at the molecular level. The fruits of this technology began to accumulate during the 1980s following the development of automated DNA-sequencing machines and the invention of the polymerase chain reaction (PCR), a simple and inexpensive technique that obtains, in a few hours, billions or trillions of copies of a specific DNA sequence or gene. Major research efforts such as the Human Genome Project further improved the technology for obtaining long DNA sequences rapidly and inexpensively. By the first few years of the 21st century, the full DNA sequencei.e., the full genetic complement, or genomehad been obtained for more than 20 higher organisms, including human beings, the house mouse (Mus musculus), the rat Rattus norvegicus, the vinegar fly Drosophila melanogaster, the mosquito Anopheles gambiae, the nematode worm Caenorhabditis elegans, the malaria parasite Plasmodium falciparum, the mustard weed Arabidopsis thaliana, and the yeast Saccharomyces cerevisiae, as well as for numerous microorganisms. Additional research during this time explored alternative mechanisms of inheritance, including epigenetic modification (the chemical modification of specific genes or gene-associated proteins), that could explain an organisms ability to transmit traits developed during its lifetime to its offspring.

The Earth sciences also experienced, in the second half of the 20th century, a conceptual revolution with considerable consequence to the study of evolution. The theory of plate tectonics, which was formulated in the late 1960s, revealed that the configuration and position of the continents and oceans are dynamic, rather than static, features of Earth. Oceans grow and shrink, while continents break into fragments or coalesce into larger masses. The continents move across Earths surface at rates of a few centimetres a year, and over millions of years of geologic history this movement profoundly alters the face of the planet, causing major climatic changes along the way. These previously unsuspected massive modifications of Earths past environments are, of necessity, reflected in the evolutionary history of life. Biogeography, the evolutionary study of plant and animal distribution, has been revolutionized by the knowledge, for example, that Africa and South America were part of a single landmass some 200 million years ago and that the Indian subcontinent was not connected with Asia until geologically recent times.

Ecology, the study of the interactions of organisms with their environments, has evolved from descriptive studiesnatural historyinto a vigorous biological discipline with a strong mathematical component, both in the development of theoretical models and in the collection and analysis of quantitative data. Evolutionary ecology (see community ecology) is an active field of evolutionary biology; another is evolutionary ethology, the study of the evolution of animal behaviour. Sociobiology, the evolutionary study of social behaviour, is perhaps the most active subfield of ethology. It is also the most controversial, because of its extension to human societies.

The theory of evolution makes statements about three different, though related, issues: (1) the fact of evolutionthat is, that organisms are related by common descent; (2) evolutionary historythe details of when lineages split from one another and of the changes that occurred in each lineage; and (3) the mechanisms or processes by which evolutionary change occurs.

The first issue is the most fundamental and the one established with utmost certainty. Darwin gathered much evidence in its support, but evidence has accumulated continuously ever since, derived from all biological disciplines. The evolutionary origin of organisms is today a scientific conclusion established with the kind of certainty attributable to such scientific concepts as the roundness of Earth, the motions of the planets, and the molecular composition of matter. This degree of certainty beyond reasonable doubt is what is implied when biologists say that evolution is a fact; the evolutionary origin of organisms is accepted by virtually every biologist.

But the theory of evolution goes far beyond the general affirmation that organisms evolve. The second and third issuesseeking to ascertain evolutionary relationships between particular organisms and the events of evolutionary history, as well as to explain how and why evolution takes placeare matters of active scientific investigation. Some conclusions are well established. One, for example, is that the chimpanzee and the gorilla are more closely related to humans than is any of those three species to the baboon or other monkeys. Another conclusion is that natural selection, the process postulated by Darwin, explains the configuration of such adaptive features as the human eye and the wings of birds. Many matters are less certain, others are conjectural, and still otherssuch as the characteristics of the first living things and when they came aboutremain completely unknown.

Since Darwin, the theory of evolution has gradually extended its influence to other biological disciplines, from physiology to ecology and from biochemistry to systematics. All biological knowledge now includes the phenomenon of evolution. In the words of Theodosius Dobzhansky, Nothing in biology makes sense except in the light of evolution.

The term evolution and the general concept of change through time also have penetrated into scientific language well beyond biology and even into common language. Astrophysicists speak of the evolution of the solar system or of the universe; geologists, of the evolution of Earths interior; psychologists, of the evolution of the mind; anthropologists, of the evolution of cultures; art historians, of the evolution of architectural styles; and couturiers, of the evolution of fashion. These and other disciplines use the word with only the slightest commonality of meaningthe notion of gradual, and perhaps directional, change over the course of time.

Toward the end of the 20th century, specific concepts and processes borrowed from biological evolution and living systems were incorporated into computational research, beginning with the work of the American mathematician John Holland and others. One outcome of this endeavour was the development of methods for automatically generating computer-based systems that are proficient at given tasks. These systems have a wide variety of potential uses, such as solving practical computational problems, providing machines with the ability to learn from experience, and modeling processes in fields as diverse as ecology, immunology, economics, and even biological evolution itself.

To generate computer programs that represent proficient solutions to a problem under study, the computer scientist creates a set of step-by-step procedures, called a genetic algorithm or, more broadly, an evolutionary algorithm, that incorporates analogies of genetic processesfor instance, heredity, mutation, and recombinationas well as of evolutionary processes such as natural selection in the presence of specified environments. The algorithm is designed typically to simulate the biological evolution of a population of individual computer programs through successive generations to improve their fitness for carrying out a designated task. Each program in an initial population receives a fitness score that measures how well it performs in a specific environmentfor example, how efficiently it sorts a list of numbers or allocates the floor space in a new factory design. Only those with the highest scores are selected to reproduce, to contribute hereditary materiali.e., computer codeto the following generation of programs. The rules of reproduction may involve such elements as recombination (strings of code from the best programs are shuffled and combined into the programs of the next generation) and mutation (bits of code in a few of the new programs are changed at random). The evolutionary algorithm then evaluates each program in the new generation for fitness, winnows out the poorer performers, and allows reproduction to take place once again, with the cycle repeating itself as often as desired. Evolutionary algorithms are simplistic compared with biological evolution, but they have provided robust and powerful mechanisms for finding solutions to all sorts of problems in economics, industrial production, and the distribution of goods and services. (See also artificial intelligence: Evolutionary computing.)

Darwins notion of natural selection also has been extended to areas of human discourse outside the scientific setting, particularly in the fields of sociopolitical theory and economics. The extension can be only metaphoric, because in Darwins intended meaning natural selection applies only to hereditary variations in entities endowed with biological reproductionthat is, to living organisms. That natural selection is a natural process in the living world has been taken by some as a justification for ruthless competition and for survival of the fittest in the struggle for economic advantage or for political hegemony. Social Darwinism was an influential social philosophy in some circles through the late 19th and early 20th centuries, when it was used as a rationalization for racism, colonialism, and social stratification. At the other end of the political spectrum, Marxist theorists have resorted to evolution by natural selection as an explanation for humankinds political history.

Darwinism understood as a process that favours the strong and successful and eliminates the weak and failing has been used to justify alternative and, in some respects, quite diametric economic theories (see economics). These theories share in common the premise that the valuation of all market products depends on a Darwinian process. Specific market commodities are evaluated in terms of the degree to which they conform to specific valuations emanating from the consumers. On the one hand, some of these economic theories are consistent with theories of evolutionary psychology that see preferences as determined largely genetically; as such, they hold that the reactions of markets can be predicted in terms of largely fixed human attributes. The dominant neo-Keynesian (see economics: Keynesian economics) and monetarist schools of economics make predictions of the macroscopic behaviour of economies (see macroeconomics) based the interrelationship of a few variables; money supply, rate of inflation, and rate of unemployment jointly determine the rate of economic growth. On the other hand, some minority economists, such as the 20th-century Austrian-born British theorist F.A. Hayek and his followers, predicate the Darwinian process on individual preferences that are mostly underdetermined and change in erratic or unpredictable ways. According to them, old ways of producing goods and services are continuously replaced by new inventions and behaviours. These theorists affirm that what drives the economy is the ingenuity of individuals and corporations and their ability to bring new and better products to the market.

The theory of evolution has been seen by some people as incompatible with religious beliefs, particularly those of Christianity. The first chapters of the biblical book of Genesis describe Gods creation of the world, the plants, the animals, and human beings. A literal interpretation of Genesis seems incompatible with the gradual evolution of humans and other organisms by natural processes. Independently of the biblical narrative, the Christian beliefs in the immortality of the soul and in humans as created in the image of God have appeared to many as contrary to the evolutionary origin of humans from nonhuman animals.

Religiously motivated attacks started during Darwins lifetime. In 1874 Charles Hodge, an American Protestant theologian, published What Is Darwinism?, one of the most articulate assaults on evolutionary theory. Hodge perceived Darwins theory as the most thoroughly naturalistic that can be imagined and far more atheistic than that of his predecessor Lamarck. He argued that the design of the human eye evinces that it has been planned by the Creator, like the design of a watch evinces a watchmaker. He concluded that the denial of design in nature is actually the denial of God.

Other Protestant theologians saw a solution to the difficulty through the argument that God operates through intermediate causes. The origin and motion of the planets could be explained by the law of gravity and other natural processes without denying Gods creation and providence. Similarly, evolution could be seen as the natural process through which God brought living beings into existence and developed them according to his plan. Thus, A.H. Strong, the president of Rochester Theological Seminary in New York state, wrote in his Systematic Theology (1885): We grant the principle of evolution, but we regard it as only the method of divine intelligence. The brutish ancestry of human beings was not incompatible with their excelling status as creatures in the image of God. Strong drew an analogy with Christs miraculous conversion of water into wine: The wine in the miracle was not water because water had been used in the making of it, nor is man a brute because the brute has made some contributions to its creation. Arguments for and against Darwins theory came from Roman Catholic theologians as well.

Gradually, well into the 20th century, evolution by natural selection came to be accepted by the majority of Christian writers. Pope Pius XII in his encyclical Humani generis (1950; Of the Human Race) acknowledged that biological evolution was compatible with the Christian faith, although he argued that Gods intervention was necessary for the creation of the human soul. Pope John Paul II, in an address to the Pontifical Academy of Sciences on October 22, 1996, deplored interpreting the Bibles texts as scientific statements rather than religious teachings, adding:

New scientific knowledge has led us to realize that the theory of evolution is no longer a mere hypothesis. It is indeed remarkable that this theory has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge. The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of this theory.

Similar views were expressed by other mainstream Christian denominations. The General Assembly of the United Presbyterian Church in 1982 adopted a resolution stating that Biblical scholars and theological schoolsfind that the scientific theory of evolution does not conflict with their interpretation of the origins of life found in Biblical literature. The Lutheran World Federation in 1965 affirmed that evolutions assumptions are as much around us as the air we breathe and no more escapable. At the same time theologys affirmations are being made as responsibly as ever. In this sense both science and religion are here to stay, andneed to remain in a healthful tension of respect toward one another. Similar statements have been advanced by Jewish authorities and those of other major religions. In 1984 the 95th Annual Convention of the Central Conference of American Rabbis adopted a resolution stating: Whereas the principles and concepts of biological evolution are basic to understanding sciencewe call upon science teachers and local school authorities in all states to demand quality textbooks that are based on modern, scientific knowledge and that exclude scientific creationism.

Opposing these views were Christian denominations that continued to hold a literal interpretation of the Bible. A succinct expression of this interpretation is found in the Statement of Belief of the Creation Research Society, founded in 1963 as a professional organization of trained scientists and interested laypersons who are firmly committed to scientific special creation (see creationism):

The Bible is the Written Word of God, and because it is inspired throughout, all of its assertions are historically and scientifically true in the original autographs. To the student of nature this means that the account of origins in Genesis is a factual presentation of simple historical truths.

Many Bible scholars and theologians have long rejected a literal interpretation as untenable, however, because the Bible contains incompatible statements. The very beginning of the book of Genesis presents two different creation narratives. Extending through chapter 1 and the first verses of chapter 2 is the familiar six-day narrative, in which God creates human beingsboth male and femalein his own image on the sixth day, after creating light, Earth, firmament, fish, fowl, and cattle. But in verse 4 of chapter 2 a different narrative starts, in which God creates a male human, then plants a garden and creates the animals, and only then proceeds to take a rib from the man to make a woman.

Biblical scholars point out that the Bible is inerrant with respect to religious truth, not in matters that are of no significance to salvation. Augustine, considered by many the greatest Christian theologian, wrote in the early 5th century in his De Genesi ad litteram (Literal Commentary on Genesis):

It is also frequently asked what our belief must be about the form and shape of heaven, according to Sacred Scripture. Many scholars engage in lengthy discussions on these matters, but the sacred writers with their deeper wisdom have omitted them. Such subjects are of no profit for those who seek beatitude. And what is worse, they take up very precious time that ought to be given to what is spiritually beneficial. What concern is it of mine whether heaven is like a sphere and Earth is enclosed by it and suspended in the middle of the universe, or whether heaven is like a disk and the Earth is above it and hovering to one side.

Augustine adds later in the same chapter: In the matter of the shape of heaven, the sacred writers did not wish to teach men facts that could be of no avail for their salvation. Augustine is saying that the book of Genesis is not an elementary book of astronomy. It is a book about religion, and it is not the purpose of its religious authors to settle questions about the shape of the universe that are of no relevance whatsoever to how to seek salvation.

In the same vein, John Paul II said in 1981:

The Bible itself speaks to us of the origin of the universe and its make-up, not in order to provide us with a scientific treatise but in order to state the correct relationships of man with God and with the universe. Sacred scripture wishes simply to declare that the world was created by God, and in order to teach this truth it expresses itself in the terms of the cosmology in use at the time of the writer.Any other teaching about the origin and make-up of the universe is alien to the intentions of the Bible, which does not wish to teach how the heavens were made but how one goes to heaven.

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evolution | scientific theory | Britannica.com

Evolution | Definition of Evolution by Merriam-Webster

noun evolution e-v-l-shn, -v-

noun evolution e-v-l-shn, -v-

1 : the theory that the various kinds of existing animals and plants have come from kinds that existed in the past

2 : the process of development of an animal or a plant

noun evolution ev–l-shn also -v-

1: a process of change in a certain direction tumor evolution and progressionI. J. Fidler et al

2a: the historical development of a biological group (as a race or species) : phylogenyb: a theory that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations

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Evolution | Definition of Evolution by Merriam-Webster

Welcome to Evolution 101!

EVOLUTION 101

Introduction

Patterns

Mechanisms

Microevolution

Speciation

Macroevolution

The big issues

Welcome to Evolution 101! by the Understanding Evolution team

What is evolution and how does it work? Evolution 101 provides the nuts-and-bolts on the patterns and mechanisms of evolution. You can explore the following sections:

See the full table of contents here.

Evolution 101 For Teachers in Spanish

Evolution 101 in Turkish

Go here to see the original:

Welcome to Evolution 101!

Evolution (2001) – IMDb

Box Office Budget: $80,000,000 (estimated) Opening Weekend:

$13,408,351 (USA) (8 June 2001)

$6,927 (USA) (25 November 2016)

18 August 2017 9:01 AM, -05:00 |DailyDead Horror Highlights: Star Trek Beer, Rob Zombies 13 Nights of Halloween, Kindred Spirits and Evil Things, Screamboxs Bloody Disgusting Collection, The Evil In Us, Evolution, American Gods

4 August 2017 1:02 AM, -05:00 |Den of Geek Transformers: screenwriter Akiva Goldsman leaves franchise

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Evolution (2001) – IMDb

The Evolution Of My Dividend Growth Portfolio – Seeking Alpha

When I write articles about buying shares of companies like Alibaba (BABA) and Tencent (OTCPK:TCEHY), yet call myself a dividend growth investor, Im sure that people begin to wonder what in the world is going on? I wouldnt blame them. Those singular pieces focused on high-growth stocks dont do my focus on income justice. Ive had several people reach out to me, asking about my current holdings. Its been awhile since Ive done a portfolio review piece, so I decided to spend some time putting this together so that followers, old and new, can stay up to date on my portfolios construction.

We live in a different world now than investors did in the middle of the last century. Many of the markets that have given tremendous returns throughout much of the 20th century are very mature at this point, and therefore, I dont expect a lot more growth coming out of them. Im not saying that a company like Coca-Cola (KO) is going away. I expect continued large cash flows and capital returns, but Im not satisfied with 100% exposure to slow-growth industries. I bet that KO will continue to post low-single-digit top line growth on average over the long-term as it adds brands and takes market share. This is all a very mature company needs to do: Single-digit sales growth combined with respectable margins and a sustainable share buyback with excess cash flows is all a mature company needs to produce respectable bottom line growth. Being that these companies are typically evaluated based upon a price to earnings ratio, this bodes well for their stock prices over the long-term. With that being said, I dont expect many of the current dividend aristocrats to generate wealth for their investors over the next 50 years as they did during the last 50 years, and Ive been willing to place riskier bets to attempt to capitalize on truly wealth-building opportunities elsewhere.

I dont think that any of this could come across as a controversial or revolutionary statement. As markets mature, growth slows, and expectations of future returns should change. When seeking that same sort of generational growth that early investors in the dividend aristocrats of today experienced, Im looking at new growth frontiers. Im looking for developing markets in terms of both sectors/industries and economies. In other words, when it comes to growth, Im looking at things like software, and not soft drinks.

But before we get to the growth portion of my portfolio, lets take a look at the more conservative dividend growth portion, which makes up the vast majority of my holdings. I end up writing about my more speculative bets much more often than my conservative holdings, but thats sort of the point, isnt it? When I buy shares of a dividend aristocrat, I hope that I never have to write about them again. I dont want these companies in the news. Im quite happy to sit back and watch as they slowly and steadily grow. Im happy to watch their dividends compound via re-investment in an un-noteworthy fashion and track my monthly income, which is surely trending in the right direction. For the most part, I hope that my dividend growth holdings are boring. That would mean that theyre meeting my expectations and goals.

My dividend growth investments make up nearly 75% of my portfolio. When you consider the fact that ~8.5% of my portfolio is currently in cash, this majority appears even larger. My speculative bets basket amounts to 16.7% of my portfolio, though 5 of the 11 companies that currently comprise that basket pay a growing dividend and I imagine that in a decade or so, their yields will be high enough for me to move them up into the main dividend growth category. So without further ado, here are the graphs I put together to break down my holdings.

DGI Holdings:

Speculative Growth Basket:

As you can see, I hold more holdings than many of the other DGI portfolios that are regularly tracked here on Seeking Alpha. I think only RoseNose owns more individual names. This highly diversified strategy may not be best for other investors; Im essentially a full-time investor at this point, and I have the time/energy available to track a portfolio with 75 holdings. I know Jim Cramer says that retail investors shouldnt hold more than a dozen or so stocks because of the time it takes to properly track them. I dont think there is any one magic number in terms of the right amount of holdings. I imagine it comes down to investable capital, risk tolerance, and the aforementioned time, energy, and passion for the markets. I look at a lot of professionally managed funds/sovereign wealth funds, and these portfolios are typically highly diversified. While I ultimately make investment decisions based upon my own personal goals, I like to see what the big boys and girls are doing as I strive to become a better investor. I imagine that my portfolio will continue to grow to the point where its 100 holdings or so and plateau there due to the fact that, for me at least, money doesnt grow on trees.

I understand that my industry/sector allocations are different than many of the other DGI portfolios that youll see here on Seeking Alpha. Technology, not consumer staples, utilities, or real estate, is my largest sector allocation at more than 26% of my overall portfolio. Up next is consumer cyclical and healthcare, coming in at ~16.5% and ~15%, respectively. The rest of my major sectors/industries are currently weighted fairly equally in the 7-10% range. None of these sector allocations are set in stone. Over time I buy value where I see it (healthcare throughout 2016, for example), and I imagine these weightings will change as market sentiment ebbs and flows. When I take a step back and look at my portfolio through a wider lens, Im happy with where they all currently sit.

Maybe the most glaring difference between my portfolio and others is the fact that I have basically zero exposure to energy and utilities. Ive been a bear with regard to the energy space for some time now, having divested all of my oil/gas-related names in 2015/early 2016. Id love to add exposure to the utilities back into my portfolio, but for the time being, I believe theyre irrationally overpriced in this low-rate environment. These high valuations combined with the fact that utilities typically dont offer the dividend growth that Id like to see have caused me to avoid the sector, in general.

77% of my holdings are of the large/mega cap variety. 6% are mid-caps and less than 1% are small caps. Generally, because of my focus on shareholder returns, reliable earnings, strong balance sheets and large cash reserves, Im attracted to large-cap companies. Even when I look at growth names I find myself attracted to large-/mega-cap names because of my focus on best in breed names. The cream typically rises to the top in the markets, and once a growth company becomes profitable (something that I usually wait for before investing), their market caps are relatively large. Im OK with this. Ive seen amazing stories of investors who created generational wealth with relatively small investments in early stage companies that turned into the industry leaders that we see today, but for every one of these home runs Im sure there were numerous strike outs, and sticking to the baseball metaphor, Im content to bat for average rather than power.

Nearly 92.5% of my overall portfolio is comprised of companies domiciled in North America. I dont mind being so overweight with North American (primarily American) companies because many of them are multinationals and Im getting exposure to foreign and emerging markets through their sales anyway. I have taken steps recently to reduce this vastly overweight exposure, hoping to become a bit more diversified internationally. European companies currently make up about ~5% of my portfolio and Id probably like to see that figure rise to the 10% range. Asia makes up the last ~2.5% of my portfolio; as time moves forward, Id like to see this figure rise as well, probably to the 5-10% range. Right now Im seeing better value in Europe and Asia than I am in the U.S. markets, generally. I hope to take advantage of these value gaps as the world plays catch up to the American markets.

But heres the most important graph that Ill be including in this piece: my monthly dividend income totals. Im quite pleased with the progress that Ive made in this regard and I feel confident that Im well on my way to financial freedom because of this passive income. Every few months it seems that I cross a new monthly income threshold with potentially impactful meaning to my life. I remember a few years ago when I was excited to know that my dividends could cover my utility bills if I needed them to. Now my utilities and both of our car payments could fall under the dividend income umbrella if I decided to spend the cash rather than re-invest it. I havent had a month yet where my dividend income could have potentially covered my mortgage payment, but I expect to achieve that goal within the next year or so. Tracking dividend income, rather than the overall value of my portfolio, gives me an anchor to hold on to during market volatility. This is one of the reasons why Ive become so attracted to the DGI portfolio management strategy.

Sure, if I was to eliminate my speculative growth basket and put those funds into a handful of stocks yielding 3%, my monthly income would be even higher. But since Im still in the accumulation phase, I like having that growth exposure and the opportunity to generate outsized returns over the long-term. Although I like to focus on my income stream, I still track the major market indexes and attempt to beat them on an annual basis. The competition aspect of the stock market is a large part of what I love so much about it. Having exposure to companies like Facebook and Amazon and Alibaba and Tencent as a small piece of my portfolio gives me what I believe to be the best of both worlds: steady, reliable income and the potential to make large jumps up the social ladder due to the massive potential of a sub-set of my portfolio.

All of this just goes to show that there are many ways to skin the cat in terms of a dividend growth portfolio. I dont think it matters much what ones sector/industry allocation weights look like so long as they stay true to standard value investing principles with an extra focus on shareholder return related metrics. I look forward to hearing what everyone has to say about the portfolio that Ive constructed over the last 5 years or so. I look forward to the continued journey moving forward. Until next time, best wishes all!

Disclosure: I am/we are long AAPL, DIS, T, BA, AMGN, ABBV, BMY, MDT, MRK, PFE, NVO, JNJ, AMZN, FB, GOOGL, NVDA, MA, V, EXPE, REGN, CELG, BABA, TCEHY, KR, MKC, SJM, KO, PEP, MMM, MO, HASI, NNN, STOR, VER, VTR, SBRA, OHI, CMCSA, MSFT, DLR, AVGO, NKE, QCOM, CSCO, UPS, WHR, FDX, NSRGY, C, MS, GS, BAC, JPM, TRV, BRK.B, GILD, HON, UNP, BX, BLK, UL, XLF, XLK, EZU, IEUR, DEO, BUD, VZ, KMB, IBM.

I wrote this article myself, and it expresses my own opinions. I am not receiving compensation for it (other than from Seeking Alpha). I have no business relationship with any company whose stock is mentioned in this article.

Additional disclosure: Just incase I missed a long in the disclosure form, I am long every stock mentioned in the graphs posted in this article.

Editor’s Note: This article discusses one or more securities that do not trade on a major U.S. exchange. Please be aware of the risks associated with these stocks.

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The Evolution Of My Dividend Growth Portfolio – Seeking Alpha

Interspecies Hybrids Play a Vital Role in Evolution – Quanta Magazine

Controversies like this one underscore the possibility that the bad reputation of naturally occurring hybrids is not entirely justified. Historically, hybrids have often been associated with the sterile or unfit offspring of maladaptive crossings (such as the mule, born of a female horse and a male donkey). Naturalists have traditionally viewed hybridization in the wild as a kind of irrelevant, mostly rare, dead-end fluke. If hybrids arent viable or fertile or common, how could they have much influence on evolution? But as genomic studies provide new insights into how species evolve, biologists are now seeing that, surprisingly often, hybrids play a vital role in fortifying species and helping them take on useful genes from close relatives.

In short, maladaptive pairings dont tell the full story of interbreeding. The genetic transfer that takes place between organisms while their lineages are diverging has a hand in the emergence of adaptive traits and in the creation of new species altogether. According to Arnold, not only is it common for newly emerging species to reacquire genes through hybrid populations, but its probably the most common way evolution proceeds, whether youre talking about viruses, plants, bacteria or animals.

Most recently, signatures of hybridization have turned up in studies on the evolution of the jaguar. In a paper published last month in Science Advances, a team of researchers from institutions spanning seven countries examined the genomes of the five members of the Panthera genus, often called the big cats: lions, leopards, tigers, jaguars and snow leopards. The scientists sequenced the genomes of the jaguar and leopard for the first time and compared them with the already existing genomes for the other three species, finding more than 13,000 genes that were shared across all five. This information helped them construct a phylogenetic tree (in essence, a family tree for species) to describe how the different animals diverged from a common ancestor approximately 4.6 million years ago.

Some of these adaptations, however, may not have originated in the jaguar lineage at all. Eiziriks team found evidence of many crossings between the different Panthera species. In one case, two genes found in the jaguar pointed to a past hybridization with the lion, which would have occurred after their phylogenetic paths had forked. Both genes turned out to be involved in optic nerve formation; Eizirik speculated that the genes encoded an improvement in vision the jaguars needed or could exploit. For whatever reasons, natural selection favored the lions genes, which took the place of those the jaguar originally had for that trait.

Such hybridization illustrates why the Eizirik groups delineation of the Panthera evolutionary tree is so noteworthy. The bottom line is that this has all become more complex, Eizirik said. Species eventually do become separated, but its not as immediate as people would frequently say. He added, The genomes we studied reflected this mosaic of histories.

Although supporting data as detailed and as thoroughly analyzed as Eiziriks is rare, the underlying idea that hybridization contributes to species development is by no means new. Biologists have known since the 1930s that hybridization occurs frequently in plants (its documented in about 25 percent of flowering plant species in the U.K. alone) and plays an important role in their evolution. In fact, it was a pair of botanists who, in 1938, coined the phrase introgressive hybridization, or introgression, to describe the pattern of hybridization and gene flow they saw in their studies. Imagine members of two species lets call them A and B that cross to produce 50-50 hybrid offspring with equal shares of genes from each parent. Then picture those hybrids crossing back to breed with members of species A, and assume that their offspring do the same. Many generations later, nature is left with organisms from species A whose genomes have retained a few genes from species B. Studies have demonstrated that this process could yield entirely new plant species as well.

But animal species seemed more discrete, at least for a while. Most zoologists supported the biological species concept proposed in 1942 by the legendary biologist Ernst Mayr, who was one of the architects of the modern synthesis, the version of evolution theory that combined Darwins natural selection with the science of genetics. Mayrs biological species concept was based on reproductive isolation: A species was defined as a population that could not or did not breed with other populations. Even when exceptions to that rule started to emerge in the 1970s, many biologists considered hybridization to be too rare to be important in animals. We had a blinkered attitude, said James Mallet, an evolutionary biologist at Harvard University. Today, he added, saying that such hybridizations dont affect reconstructions of evolutionary history or that this wasnt useful in adaptive evolution thats no longer tenable.

This is especially true now that computational and genomic tools prove just how prolific introgression is even in our own species. Since 2009, studies have revealed that approximately 50,000 to 60,000 years ago, some modern humans spreading out of Africa interbred with Neanderthals; they later did so with another ancestral human group, the Denisovans, as well. The children in both cases went on to mate with other modern humans, passing the genes they acquired down to us. At present, researchers estimate that some populations have inherited 1 to 2 percent of their DNA from Neanderthals, and up to 6 percent of it from Denisovans fractions that amount to hundreds of genes.

In 2012, Mallet and his colleagues showed a large amount of gene flow between two hybridizing species of Heliconius butterfly. The following year, they determined that approximately 40 percent of the genes in one species had come from the other. Mallets team is now working with another pair of butterfly species that exchange even more of their genes: something like 98 percent, he said. Only the remaining 2 percent of the genome carries the information that separates the species and reflects their true evolutionary trajectory. A similar blurring of species lines has already been found in malaria-carrying mosquitoes of the Anopheles genus.

Other types of organisms, from fish and birds to wolves and sheep, experience their share of introgression, too. The boundaries between species are now known to be less rigid than previously thought, said Peter Grant, an evolutionary biologist at Princeton University who, along with his fellow Princeton biologist (and wife) Rosemary Grant, has been studying the evolution of Galpagos finches for decades. Phylogenetic reconstructions depict treelike patterns as if there is a clear barrier between species that arises instantaneously and is never breached. This may be misleading.

Arnold concurred. Its a web of life, he said, rather than a simple bifurcating tree of life. That also means its more necessary than ever before to examine the entire genome, and not just selected genes, to understand a species evolutionary relationships and generate the correct phylogeny. And even that might not be enough. It may well be, Mallet said, that some actual evolutionary patterns are still completely irrecoverable.

Genomic studies cant create a complete picture of the introgressive movements of genes. Whenever one species inherits genes from another, the outcome can be either deleterious, neutral or adaptive. Natural selection tends to weed out the first, although some of the genes we have inherited from Neanderthals, for example, may be involved in disorders such as diabetes, obesity or depression. Neutral introgressed regions drift, so its possible for them to remain in the genome for very long periods of time without having an observable effect.

But its the beneficial introgressions that particularly fascinate researchers. Take the Neanderthal and Denisovan DNA again: Those genes have allowed people to adapt to the harsh environs of places like the Tibetan plateau, protecting them against the harmful effects of high altitudes and low oxygen saturation, which in nonlocals can cause stroke, miscarriage and other health risks. Variants from interbreeding with archaic humans have also conferred immunity to certain infections and made skin and hair pigmentation more suitable for Eurasian climes.

Mallets butterflies, too, reflect evidence of adaptive hybridization, particularly with traits involved in mimicry and predator avoidance. Researchers had observed that although most Heliconius species had highly divergent wing coloration and patterning, some bore a striking resemblance to one another. The researchers believed that these species had independently converged on these traits, but it turns out thats only partially correct. Mallet and others have found that introgression was also responsible. The same goes for Galpagos finches: Pieces of their genomes that control for features including beak size and shape were shared through hybridization. Once again, parallel evolution cant explain everything.

For these effects to occur, the rate of hybridization can be and most likely is very small. For Mallets almost entirely hybridized butterflies, the occasional trickle of one hybrid mating every 1,000 normal matings is sufficient to completely homogenize genes between the species, he said. Thats pretty exciting.

As these patterns of introgression have become more and more predominant in the scientific literature, researchers have set out to uncover their evolutionary consequences. These go beyond the fact that speciation tends to be a much more gradual process than its often made out to be. Diversification, adaptation and adaptive evolution really do seem to be driven quite often by genes moving around, Arnold said.

The research done by Eizirik and his team makes a compelling case for this. Around the time when the gene introgressions they analyzed occurred, the populations of all five Panthera species are estimated to have declined, likely due to climate changes. The smaller a population is, the greater the probability that a harmful mutation will get affixed to its genome. Perhaps the gene flow found between the different species, then, rescued them from extinction, providing adaptive mutations and patching deleterious ones. This kind of infusion of genetic mutations is so large that it can cause really rapid evolution, Arnold said.

And the process doesnt end with speeding up evolution in a single species. Adaptive introgression can in turn contribute significantly to adaptive radiation, a process by which one species rapidly diversifies into a large variety of types, which then form new lineages that continue to adapt independently. The textbook case can be found in the great lakes of East Africa, which are home to hundreds upon hundreds of cichlid species, a type of fish that diversified in explosive bursts (on the evolutionary timescale) from common ancestors, largely in response to climatic and tectonic shifts in their environment. Today, cichlids vary widely in form, behavior and ecology thanks in large part to introgressive hybridization.

Biologists will need many more years to understand the full importance of hybridization to evolution. For example, Arnold wants to see further investigations like the ones that have been done on the finches in the Galpagos and the wolves of Yellowstone National Park: behavioral, metabolic and other analyses that will reveal how much of introgression is adaptive and how much is deleterious or neutral as well as whether adaptive introgression affects only particular kinds of genes, or if it acts in a more widespread manner.

Unfortunately, for conservationists and others challenged with managing the diversity of imperiled species, the absence of satisfactory answers poses more immediate problems. They must often weigh the value of protecting wild hybrid populations against the harm hybrids can do to established species, including the ones from which they emerged.

A case in point: In the 1950s, a pair of California bait dealers from the Salinas Valley, seeking to expand their business, hopped into a pickup truck and took off to central Texas and New Mexico. They brought back barred tiger salamanders, which could grow to more than double the size of Californias native tiger salamander. The new species quickly proved to be good for the local fishermen but bad for the local ecosystem: The introduced salamanders mated with the natives, creating a hybrid breed that could outcompete its parent species. Soon the California tiger salamander found itself in danger of being wiped out entirely, and it remains a threatened species today.

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Interspecies Hybrids Play a Vital Role in Evolution – Quanta Magazine

Justin Chon on YouTube’s evolution – Olean Times Herald

Justin Chon may have made it in Hollywood through a key role in the “Twilight” franchise, but he appreciates the “renegade” approach of YouTube. (Aug. 24)

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Justin Chon on YouTube’s evolution – Olean Times Herald

Gone Hunting: Shotgun shells have undergone an evolution for a resolution on lead shot issue – Greeley Tribune

I am a staunch believer in the Book of Genesis and what it teaches us about how we arrived at where we are today.

However, when it comes to shotgun ammunition, evolution is the key to successful hunting.

Thirty years ago, in 1987, the Federal government began phasing in its ban on toxic lead shot for waterfowl/migratory bird hunting. This ban spread nationwide in 1991.

The reasoning behind this ban was the thought that crippled birds that flew off, died and were then ingested by birds of prey such as our national symbol, the Bald Eagle. There was evidence to support this theory, with several instances of birds of prey found dead or dying from lead poisoning.

I know hunters that carry nothing but steel even when hunting upland/non-migratory birds just to avoid having to switch loads in the field.

Waterfowl hunters were sent scrambling for alternative ammunition. Even upland (pheasant/quail) hunters needed options if they were hunting on federal waterfowl production areas and national wildlife refuges.

The initial and often-used option to lead shot was steel shot. The results were not good. Unprepared for this new law, ammo manufacturers simply switched out steel for lead without changing much of anything else in the shell.

Steel shot is not nearly as heavy as lead shot and does not pack the wallop or shock when it contacts the target. Steel shot also patterns more tightly which reduces the “kill zone”. Hunters crippled more birds but didn’t kill them.

I can vividly remember hunting geese with my brother Jack in the cornfields north of Greeley back in the late 80s. The first morning flock of Canadian honkers were locked up, feet down and settling into our decoys. We emptied our shotguns on them.

It literally rained feathers on us as we watched that flock hurry into the sky and safety. Not one pellet penetrated enough to be lethal.

Ammo manufacturers tried alternative shot such as bismuth and tungsten, which were comparable to lead in weight and shocking power but not in price.

Manufacturers began to concentrate on making a better steel-shot shotgun shell. Evolution, trial and error, and test markets were used well, and finally, we have a better product.

It began with the guts of the shotshell. The wad that cradles the tiny pellets was re-tooled. It became a bit shorter to accommodate more pellets.

The primers that ignite the powder were redesigned to burn slower and reduce chamber pressure. The steel shot remained spherical but some manufacturers experimented with different shapes of the tiny BB’s. I likened this to the dimples on a golf ball. Ball manufacturers claim their dimple pattern is the best for straight flight or longer flight. The same claims were made by the shotshell makers. The results of this evolutionary period are shotgun shells that contain steel pellets that perform virtually as well as lead ammo.

My favorite lead ammo continues to be a Federal shotshell that contains 1 oz. of no. 4 lead pellets pushed by 3 drams of gunpowder at 1330 feet per second. I prefer this load for upland hunting because it has been my most consistently lethal load at all ranges and in any wind and weather conditions.

Federal, Remington, Fiocchi all make loads similar to what I have just described.

I know hunters that carry nothing but steel even when hunting upland/non-migratory birds just to avoid having to switch loads in the field.

A good example of an effective modern steel load is Federal’s Prairie Storm Steel. It comes in a 3-inch shell (requiring at least a 3-inch chamber in your shotgun) and launches number 3 or 4 steel shot at 1600 feet per second. Sixteen hundred feet per second is fast and should have enough wallop out at 40 yards or in the killing zone.

There are also two shapes of pellets or BB’s in the Prairie Storm shell. About half of the 170 pellets are spherical while the remaining pellets are spherical with a band (called Flitestoppers). They resemble the planet Saturn and help deliver a lethal punch.

I don’t hunt waterfowl much any more. I don’t like to kill something that I don’t like to eat. However, I do carry a box of steel shot along with me in my F-150 just in case I get the urge.

When you stop to think about it, steel shotgun shell evolution had to have a genesis, too.

Jim Vanek is a longtime hunter who lived in Greeley for many years. He can be reached at kimosabe14@msn.com.

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Gone Hunting: Shotgun shells have undergone an evolution for a resolution on lead shot issue – Greeley Tribune

Taylor Swift’s New Album Signals a Dark, Powerful Style She’s Never Shown Before – Glamour

Taylor Swift is a master of self-invention. It’s been said before, but she’s a success by design: behind the girl-next-door persona that’s so incredibly easy to relate to (yeah! Eff that guy and his precious truck, we’ll do better), there’s a whole machine of thought that goes into her image. Since she burst onto the scene in 2006, we’ve lived through multiple incarnations, watching her style evolve at every big turning point in her career.

Country princess, pop star, retro babe, fashun lover: everyone’s got their personal Swift era preference. Nowhere better is each image so succinctly summed up than the look that comes with a new album drop. So, with Reputation’s cover reveal and a rumored new single on the way, we’re taking a stroll back through her greatest beauty hitsand analyzing what this new era could signal.

PHOTO: Getty Images

Our first introduction to Swift as a Nashville teen, her country era was strongly, strongly boilerplate princess-themed. Innocence was the name of the game on her eponymously named album, alongside songs like “Teardrops on My Guitar” (DREW!), “Picture to Burn” (still a banger), and “Our Song.” With her naturally curly hair and love for the maximum amount of glitter on both her eyeshadow and dresses, it was very much a “this girl believes in fairytales and romance” moment, and one that both made her approachable to the middle school girl demo, and set her apart from the rest of the country music scene.

PHOTO: Getty Images

“You Belong With Me” hit in 2008, and who could forget Swift pulling a Parent Trap and playing both the girl next door and the villainous popular girl. Truly, this woman contains multitudesbut the greatest trick of all was selling the idea that Swift was just an average girl looking for love. The sparkles got toned down, and while her curls were still going strong, they started to move into a more styled, barrel curl look. Spanning from Fearless to Speak Now , these were the years of her image as a lovelorn lady out for her Nicholas Sparks story. “Mine,” “Dear John,” “If This Was a Movie,” “Better Than Revenge”there was drama, but Swift’s persona was always squarely on the right of it, with her curls and lipgloss there to back her up.

PHOTO: Ethan Miller

And with Red the curls exited stage right, in favor of her now-trademark red lip and sleek bangs. This was Swift with more vindication and agency: if you wrong her, you’re gonna get called out. Curls can have agency, but Swift’s transition to a totally smooth style read like she was tightening her grip on deciding who the world saw. There was still the romance in her lyricsand what’s more romantic than a red lip?but with Red’s cover showing her face half in the shadows, only her lips and a shiny lock of hair in the light, Swift painted a narrative of a girl who’d been burnt, but was surviving. The vibe was cardigans and Keds, with red lipstick and cat eye liner; a little kitschy, ’50s nostalgia-cute .

PHOTO: Getty

Ah, the age of “Blank Space,” “Shake It Off,” and “Bad Blood.” It was an aggressive time, matched by Swift’s turn to chic, femme fatale looks without a single hair out of place. Her red lips went darker, with 1989 ‘s cover revolving around her fractured, above the fray self: lips-down on the cover, nose-up on the album liner, and a faded, Polaroid-from-a-distance aesthetic. Truly, she hit an insane balance between approachable BFF (I’m just a girl baking cookies and taking roadtrips with Karlie Kloss ) and bombshell living above the rumors (those now-signature two-piece sets; ” It had to do with business “).

PHOTO: Getty Images

This ’twas not an era of much new music for Swift. Her only release was “I Don’t Want to Live Forever” with Zayn Malik for 50 Shades Darker . But personally, it was a huge. With an abundance of think pieces surrounding the Kim/Kanye fiasco , at this point, the world caught on to Swift’s immaculate image control. And so she transformed again.

The first signal came at Coachella , when she debuted a new platinum dye job (which came at Vogue ‘s persuasion ). Then at the Met Gala, she channeled Debbie Harry’s punk look with dark lips and a shaggier cut. This progressed into a few other decidedly less “safe” looks, including this unexpected rendezvous with contour and bubblegum pink gloss. That was in May 2016, and as you know, she’s been out of the spotlight pretty much since. (Her break from the red carpet, of course, was hardly a vacationduring her sexual assault trial earlier this month she paved the way for anyone fuzzy on consent with her concrete, unyielding testimony .)

Everything from here on is speculation, though we’ll surely be seeing plenty of Swift again soon enough. But what we can gather from her new album cover is that we’re in for the singer’s most powerful evolution (both personally and lyrically) yet. Significantly, the cover is black and whiteand with headlines covering half Swift’s face, fans are speculating that it implies we’ve only gotten half the story.

Her makeup is pared down and clean with the exception of a not just dark but jet-black lip, and her hair looks wet, which could allude to the concept of rebirth and renewal. Such can also be said that a snake represents the same since it sheds its skin (and it’d fit with her clean slate social media strategy). The conclusion would be that she’s had her persona (and thus, her style) built a certain way, and now the real her is coming out. It’s not commercial, bubblegum, or high-fashion approvedbut it’s mature and authentic, a look worn with the confidence of coming into your own.

Related Stories: -Taylor Swift’s New Shag Haircut Is All Kinds of Cool -Taylor Swift’s 10 Most Powerful Statements From Her Sexual Assault Trial Cross-Examination -Katy Perry and Taylor Swift’s Feud: A Timeline

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Taylor Swift’s New Album Signals a Dark, Powerful Style She’s Never Shown Before – Glamour

The Evolution Of Smart Speakers – Seeking Alpha

For a relatively nascent product category, smart speakers like Amazon (NASDAQ:AMZN) Echo and Google (NASDAQ:GOOG) (NASDAQ:GOOGL) Home are already seeing a huge influx of attention from both consumers and potential competitors eager to enter the market. Apple (NASDAQ:AAPL) has announced the HomePod and numerous other vendors have either unveiled or are heavily rumored to be working on versions of their own.

Harman Kardon (in conjunction with Microsoft (NASDAQ:MSFT)), GE Lighting and Lenovo (OTCPK:LNVGY) have announced products in the US, while Alibaba (NYSE:BABA), Xiaomi (Private:XI) and JD.com (NASDAQ:JD), among others, have said they will be bringing products out in China. In addition, Facebook (NASDAQ:FB) is rumored to be building a screen-equipped smart speaker called Gizmo.

One obvious question after hearing about all the new entrants is, how can they all survive? The short answer, of course, is they won’t. Nevertheless, expect to see a lot of jockeying, marketing and positioning over the next year or two because it’s still very early days in the world of AI-powered and personal assistant-driven smart speakers.

Yes, Amazon has built an impressive and commanding presence with the Echo line, but there are many limitations to Echos and all current smart speakers that frustrate existing users. Thankfully, technology improvements are coming that will enable competitors to differentiate themselves from others in ways which reduce the frustration and increase the satisfaction that consumers have with smart speakers.

Part of the work involves the overall architecture of the devices and how they interact with cloud-based services. For example, one of the critical capabilities that many users want is the ability to accurately recognize different individuals that speak to the device, so that responses can be customized for different members of a household. To achieve this as quickly and accurately as possible, it doesn’t make sense to try and send the audio signal to the cloud and then wait for the response. Even with superfast network connections, the inevitable delays make interactions with the device feel somewhat awkward.

The same problem exists when you try to move beyond the simple single query requests that most people are making to their smart speakers today. (Alexa, play music by horn bands, or Alexa, what is the capital of Iceland?) In order to have naturally flowing, multi-question or multi-statement conversations, the delays (or latency) have to be dramatically reduced.

The obvious answer to the problem is to do more of the recognition and response work locally on the device and not rely on a cloud-based network connection to do so. In fact, this is a great example of the larger trend of edge computing, where we are seeing devices or applications that use to rely solely on big data centers in the cloud start to do more of the computational work on their own.

That’s part of the reason you’re starting to see companies like Qualcomm (NASDAQ:QCOM) and Intel (NASDAQ:INTC), among others, develop chips that are designed to enable more powerful local computing work on devices like smart speakers. The ability to learn and then recognize different individuals, for example, is something that the DSP (digital signal processor) component of new chips from these vendors can do.

Another technological challenge facing current generation products is recognition accuracy. Everyone who has used a smart speaker or digital assistant on another device has had the experience of not being understood. Sometimes that’s due to how the question or command is phrased, but it’s often due to background noises, accents, intonation or other factors that essentially end up providing an imperfect audio signal to the cloud-based recognition engine. Again, more local audio signal processing can often improve the audio signal to be sent, thereby enhancing overall recognition.

Going further, most of the AI-based learning algorithms used to recognize and accurately respond to speech will likely need to be run in very large, compute-intensive cloud data centers. However, the idea of being able to start do pattern recognition of common phrases (a form of inferencing-the second key aspect of machine learning and AI) locally with the right kind of computing engines and hardware architectures is becoming increasingly possible. It may be a long time before all that kind of work can be done within smart speakers and other edge devices, but even doing some speech recognition on the device should enable higher accuracy and longer conversations. In short, a much better user experience.

As new entrants try to differentiate their products in an increasingly crowded space, the ability to offer some key tech-based improvements is going to be essential. Clearly there’s a great deal of momentum behind the smart speaker phenomenon, but it’s going to take these kind performance improvements to move them beyond idle curiosities and into truly useful, everyday kinds of tools.

Disclaimer: Some of the author’s clients are vendors in the tech industry.

Disclosure: None.

Editor’s Note: This article discusses one or more securities that do not trade on a major U.S. exchange. Please be aware of the risks associated with these stocks.

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The Evolution Of Smart Speakers – Seeking Alpha

Evolution | Definition of Evolution by Merriam-Webster

noun evolution e-v-l-shn, -v-

noun evolution e-v-l-shn, -v-

1 : the theory that the various kinds of existing animals and plants have come from kinds that existed in the past

2 : the process of development of an animal or a plant

noun evolution ev–l-shn also -v-

1: a process of change in a certain direction tumor evolution and progressionI. J. Fidler et al

2a: the historical development of a biological group (as a race or species) : phylogenyb: a theory that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations

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Evolution | Definition of Evolution by Merriam-Webster

evolution | scientific theory | Britannica.com

Alternative Title: descent

Evolution, theory in biology postulating that the various types of plants, animals, and other living things on Earth have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory.

The diversity of the living world is staggering. More than 2 million existing species of organisms have been named and described; many more remain to be discoveredfrom 10 million to 30 million, according to some estimates. What is impressive is not just the numbers but also the incredible heterogeneity in size, shape, and way of lifefrom lowly bacteria, measuring less than a thousandth of a millimetre in diameter, to stately sequoias, rising 100 metres (300 feet) above the ground and weighing several thousand tons; from bacteria living in hot springs at temperatures near the boiling point of water to fungi and algae thriving on the ice masses of Antarctica and in saline pools at 23 C (9 F); and from giant tube worms discovered living near hydrothermal vents on the dark ocean floor to spiders and larkspur plants existing on the slopes of Mount Everest more than 6,000 metres (19,700 feet) above sea level.

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heredity (genetics): Heredity and evolution

At the centre of the theory of evolution as proposed by Charles Darwin and Alfred Russell Wallace were the concepts of variation and natural selection. Hereditary variants were thought to arise naturally in populations, and then these were either selected for or against by the contemporary environmental conditions. In this way, subsequent generations either became enriched or impoverished for…

The virtually infinite variations on life are the fruit of the evolutionary process. All living creatures are related by descent from common ancestors. Humans and other mammals descend from shrewlike creatures that lived more than 150 million years ago; mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived 600 million years ago; and all plants and animals derive from bacteria-like microorganisms that originated more than 3 billion years ago. Biological evolution is a process of descent with modification. Lineages of organisms change through generations; diversity arises because the lineages that descend from common ancestors diverge through time.

The 19th-century English naturalist Charles Darwin argued that organisms come about by evolution, and he provided a scientific explanation, essentially correct but incomplete, of how evolution occurs and why it is that organisms have featuressuch as wings, eyes, and kidneysclearly structured to serve specific functions. Natural selection was the fundamental concept in his explanation. Natural selection occurs because individuals having more-useful traits, such as more-acute vision or swifter legs, survive better and produce more progeny than individuals with less-favourable traits. Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution. Beginning in the 1960s, a related scientific discipline, molecular biology, enormously advanced knowledge of biological evolution and made it possible to investigate detailed problems that had seemed completely out of reach only a short time previouslyfor example, how similar the genes of humans and chimpanzees might be (they differ in about 12 percent of the units that make up the genes).

This article discusses evolution as it applies generally to living things. For a discussion of human evolution, see the article human evolution. For a more complete treatment of a discipline that has proved essential to the study of evolution, see the articles genetics, human and heredity. Specific aspects of evolution are discussed in the articles coloration and mimicry. Applications of evolutionary theory to plant and animal breeding are discussed in the articles plant breeding and animal breeding. An overview of the evolution of life as a major characteristic of Earths history is given in community ecology: Evolution of the biosphere. A detailed discussion of the life and thought of Charles Darwin is found in the article Darwin, Charles.

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Mammal Mania

Darwin and other 19th-century biologists found compelling evidence for biological evolution in the comparative study of living organisms, in their geographic distribution, and in the fossil remains of extinct organisms. Since Darwins time, the evidence from these sources has become considerably stronger and more comprehensive, while biological disciplines that emerged more recentlygenetics, biochemistry, physiology, ecology, animal behaviour (ethology), and especially molecular biologyhave supplied powerful additional evidence and detailed confirmation. The amount of information about evolutionary history stored in the DNA and proteins of living things is virtually unlimited; scientists can reconstruct any detail of the evolutionary history of life by investing sufficient time and laboratory resources.

Evolutionists no longer are concerned with obtaining evidence to support the fact of evolution but rather are concerned with what sorts of knowledge can be obtained from different sources of evidence. The following sections identify the most productive of these sources and illustrate the types of information they have provided.

Paleontologists have recovered and studied the fossil remains of many thousands of organisms that lived in the past. This fossil record shows that many kinds of extinct organisms were very different in form from any now living. It also shows successions of organisms through time (see faunal succession, law of; geochronology: Determining the relationships of fossils with rock strata), manifesting their transition from one form to another.

When an organism dies, it is usually destroyed by other forms of life and by weathering processes. On rare occasions some body partsparticularly hard ones such as shells, teeth, or bonesare preserved by being buried in mud or protected in some other way from predators and weather. Eventually, they may become petrified and preserved indefinitely with the rocks in which they are embedded. Methods such as radiometric datingmeasuring the amounts of natural radioactive atoms that remain in certain minerals to determine the elapsed time since they were constitutedmake it possible to estimate the time period when the rocks, and the fossils associated with them, were formed.

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Radiometric dating indicates that Earth was formed about 4.5 billion years ago. The earliest fossils resemble microorganisms such as bacteria and cyanobacteria (blue-green algae); the oldest of these fossils appear in rocks 3.5 billion years old (see Precambrian time). The oldest known animal fossils, about 700 million years old, come from the so-called Ediacara fauna, small wormlike creatures with soft bodies. Numerous fossils belonging to many living phyla and exhibiting mineralized skeletons appear in rocks about 540 million years old. These organisms are different from organisms living now and from those living at intervening times. Some are so radically different that paleontologists have created new phyla in order to classify them. (See Cambrian Period.) The first vertebrates, animals with backbones, appeared about 400 million years ago; the first mammals, less than 200 million years ago. The history of life recorded by fossils presents compelling evidence of evolution.

The fossil record is incomplete. Of the small proportion of organisms preserved as fossils, only a tiny fraction have been recovered and studied by paleontologists. In some cases the succession of forms over time has been reconstructed in detail. One example is the evolution of the horse. The horse can be traced to an animal the size of a dog having several toes on each foot and teeth appropriate for browsing; this animal, called the dawn horse (genus Hyracotherium), lived more than 50 million years ago. The most recent form, the modern horse (Equus), is much larger in size, is one-toed, and has teeth appropriate for grazing. The transitional forms are well preserved as fossils, as are many other kinds of extinct horses that evolved in different directions and left no living descendants.

Using recovered fossils, paleontologists have reconstructed examples of radical evolutionary transitions in form and function. For example, the lower jaw of reptiles contains several bones, but that of mammals only one. The other bones in the reptile jaw unmistakably evolved into bones now found in the mammalian ear. At first, such a transition would seem unlikelyit is hard to imagine what function such bones could have had during their intermediate stages. Yet paleontologists discovered two transitional forms of mammal-like reptiles, called therapsids, that had a double jaw joint (i.e., two hinge points side by side)one joint consisting of the bones that persist in the mammalian jaw and the other composed of the quadrate and articular bones, which eventually became the hammer and anvil of the mammalian ear. (See also mammal: Skeleton.)

For skeptical contemporaries of Darwin, the missing linkthe absence of any known transitional form between apes and humanswas a battle cry, as it remained for uninformed people afterward. Not one but many creatures intermediate between living apes and humans have since been found as fossils. The oldest known fossil homininsi.e., primates belonging to the human lineage after it separated from lineages going to the apesare 6 million to 7 million years old, come from Africa, and are known as Sahelanthropus and Orrorin (or Praeanthropus), which were predominantly bipedal when on the ground but which had very small brains. Ardipithecus lived about 4.4 million years ago, also in Africa. Numerous fossil remains from diverse African origins are known of Australopithecus, a hominin that appeared between 3 million and 4 million years ago. Australopithecus had an upright human stance but a cranial capacity of less than 500 cc (equivalent to a brain weight of about 500 grams), comparable to that of a gorilla or a chimpanzee and about one-third that of humans. Its head displayed a mixture of ape and human characteristicsa low forehead and a long, apelike face but with teeth proportioned like those of humans. Other early hominins partly contemporaneous with Australopithecus include Kenyanthropus and Paranthropus; both had comparatively small brains, although some species of Paranthropus had larger bodies. Paranthropus represents a side branch in the hominin lineage that became extinct. Along with increased cranial capacity, other human characteristics have been found in Homo habilis, which lived about 1.5 million to 2 million years ago in Africa and had a cranial capacity of more than 600 cc (brain weight of 600 grams), and in H. erectus, which lived between 0.5 million and more than 1.5 million years ago, apparently ranged widely over Africa, Asia, and Europe, and had a cranial capacity of 800 to 1,100 cc (brain weight of 800 to 1,100 grams). The brain sizes of H. ergaster, H. antecessor, and H. heidelbergensis were roughly that of the brain of H. erectus, some of which species were partly contemporaneous, though they lived in different regions of the Eastern Hemisphere. (See also human evolution.)

The skeletons of turtles, horses, humans, birds, and bats are strikingly similar, in spite of the different ways of life of these animals and the diversity of their environments. The correspondence, bone by bone, can easily be seen not only in the limbs but also in every other part of the body. From a purely practical point of view, it is incomprehensible that a turtle should swim, a horse run, a person write, and a bird or a bat fly with forelimb structures built of the same bones. An engineer could design better limbs in each case. But if it is accepted that all of these skeletons inherited their structures from a common ancestor and became modified only as they adapted to different ways of life, the similarity of their structures makes sense.

Comparative anatomy investigates the homologies, or inherited similarities, among organisms in bone structure and in other parts of the body. The correspondence of structures is typically very close among some organismsthe different varieties of songbirds, for instancebut becomes less so as the organisms being compared are less closely related in their evolutionary history. The similarities are less between mammals and birds than they are among mammals, and they are still less between mammals and fishes. Similarities in structure, therefore, not only manifest evolution but also help to reconstruct the phylogeny, or evolutionary history, of organisms.

Comparative anatomy also reveals why most organismic structures are not perfect. Like the forelimbs of turtles, horses, humans, birds, and bats, an organisms body parts are less than perfectly adapted because they are modified from an inherited structure rather than designed from completely raw materials for a specific purpose. The imperfection of structures is evidence for evolution and against antievolutionist arguments that invoke intelligent design (see below Intelligent design and its critics).

Darwin and his followers found support for evolution in the study of embryology, the science that investigates the development of organisms from fertilized egg to time of birth or hatching. Vertebrates, from fishes through lizards to humans, develop in ways that are remarkably similar during early stages, but they become more and more differentiated as the embryos approach maturity. The similarities persist longer between organisms that are more closely related (e.g., humans and monkeys) than between those less closely related (humans and sharks). Common developmental patterns reflect evolutionary kinship. Lizards and humans share a developmental pattern inherited from their remote common ancestor; the inherited pattern of each was modified only as the separate descendant lineages evolved in different directions. The common embryonic stages of the two creatures reflect the constraints imposed by this common inheritance, which prevents changes that have not been necessitated by their diverging environments and ways of life.

The embryos of humans and other nonaquatic vertebrates exhibit gill slits even though they never breathe through gills. These slits are found in the embryos of all vertebrates because they share as common ancestors the fish in which these structures first evolved. Human embryos also exhibit by the fourth week of development a well-defined tail, which reaches maximum length at six weeks. Similar embryonic tails are found in other mammals, such as dogs, horses, and monkeys; in humans, however, the tail eventually shortens, persisting only as a rudiment in the adult coccyx.

A close evolutionary relationship between organisms that appear drastically different as adults can sometimes be recognized by their embryonic homologies. Barnacles, for example, are sedentary crustaceans with little apparent likeness to such free-swimming crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-swimming larval stage, the nauplius, which is unmistakably similar to that of other crustacean larvae.

Embryonic rudiments that never fully develop, such as the gill slits in humans, are common in all sorts of animals. Some, however, like the tail rudiment in humans, persist as adult vestiges, reflecting evolutionary ancestry. The most familiar rudimentary organ in humans is the vermiform appendix. This wormlike structure attaches to a short section of intestine called the cecum, which is located at the point where the large and small intestines join. The human vermiform appendix is a functionless vestige of a fully developed organ present in other mammals, such as the rabbit and other herbivores, where a large cecum and appendix store vegetable cellulose to enable its digestion with the help of bacteria. Vestiges are instances of imperfectionslike the imperfections seen in anatomical structuresthat argue against creation by design but are fully understandable as a result of evolution.

Darwin also saw a confirmation of evolution in the geographic distribution of plants and animals, and later knowledge has reinforced his observations. For example, there are about 1,500 known species of Drosophila vinegar flies in the world; nearly one-third of them live in Hawaii and nowhere else, although the total area of the archipelago is less than one-twentieth the area of California or Germany. Also in Hawaii are more than 1,000 species of snails and other land mollusks that exist nowhere else. This unusual diversity is easily explained by evolution. The islands of Hawaii are extremely isolated and have had few colonizersi.e, animals and plants that arrived there from elsewhere and established populations. Those species that did colonize the islands found many unoccupied ecological niches, local environments suited to sustaining them and lacking predators that would prevent them from multiplying. In response, these species rapidly diversified; this process of diversifying in order to fill ecological niches is called adaptive radiation.

Each of the worlds continents has its own distinctive collection of animals and plants. In Africa are rhinoceroses, hippopotamuses, lions, hyenas, giraffes, zebras, lemurs, monkeys with narrow noses and nonprehensile tails, chimpanzees, and gorillas. South America, which extends over much the same latitudes as Africa, has none of these animals; it instead has pumas, jaguars, tapir, llamas, raccoons, opossums, armadillos, and monkeys with broad noses and large prehensile tails.

These vagaries of biogeography are not due solely to the suitability of the different environments. There is no reason to believe that South American animals are not well suited to living in Africa or those of Africa to living in South America. The islands of Hawaii are no better suited than other Pacific islands for vinegar flies, nor are they less hospitable than other parts of the world for many absent organisms. In fact, although no large mammals are native to the Hawaiian islands, pigs and goats have multiplied there as wild animals since being introduced by humans. This absence of many species from a hospitable environment in which an extraordinary variety of other species flourish can be explained by the theory of evolution, which holds that species can exist and evolve only in geographic areas that were colonized by their ancestors.

The field of molecular biology provides the most detailed and convincing evidence available for biological evolution. In its unveiling of the nature of DNA and the workings of organisms at the level of enzymes and other protein molecules, it has shown that these molecules hold information about an organisms ancestry. This has made it possible to reconstruct evolutionary events that were previously unknown and to confirm and adjust the view of events already known. The precision with which these events can be reconstructed is one reason the evidence from molecular biology is so compelling. Another reason is that molecular evolution has shown all living organisms, from bacteria to humans, to be related by descent from common ancestors.

A remarkable uniformity exists in the molecular components of organismsin the nature of the components as well as in the ways in which they are assembled and used. In all bacteria, plants, animals, and humans, the DNA comprises a different sequence of the same four component nucleotides, and all the various proteins are synthesized from different combinations and sequences of the same 20 amino acids, although several hundred other amino acids do exist. The genetic code by which the information contained in the DNA of the cell nucleus is passed on to proteins is virtually everywhere the same. Similar metabolic pathwayssequences of biochemical reactions (see metabolism)are used by the most diverse organisms to produce energy and to make up the cell components.

This unity reveals the genetic continuity and common ancestry of all organisms. There is no other rational way to account for their molecular uniformity when numerous alternative structures are equally likely. The genetic code serves as an example. Each particular sequence of three nucleotides in the nuclear DNA acts as a pattern for the production of exactly the same amino acid in all organisms. This is no more necessary than it is for a language to use a particular combination of letters to represent a particular object. If it is found that certain sequences of lettersplanet, tree, womanare used with identical meanings in a number of different books, one can be sure that the languages used in those books are of common origin.

Genes and proteins are long molecules that contain information in the sequence of their components in much the same way as sentences of the English language contain information in the sequence of their letters and words. The sequences that make up the genes are passed on from parents to offspring and are identical except for occasional changes introduced by mutations. As an illustration, one may assume that two books are being compared. Both books are 200 pages long and contain the same number of chapters. Closer examination reveals that the two books are identical page for page and word for word, except that an occasional wordsay, one in 100is different. The two books cannot have been written independently; either one has been copied from the other, or both have been copied, directly or indirectly, from the same original book. Similarly, if each component nucleotide of DNA is represented by one letter, the complete sequence of nucleotides in the DNA of a higher organism would require several hundred books of hundreds of pages, with several thousand letters on each page. When the pages (or sequences of nucleotides) in these books (organisms) are examined one by one, the correspondence in the letters (nucleotides) gives unmistakable evidence of common origin.

The two arguments presented above are based on different grounds, although both attest to evolution. Using the alphabet analogy, the first argument says that languages that use the same dictionarythe same genetic code and the same 20 amino acidscannot be of independent origin. The second argument, concerning similarity in the sequence of nucleotides in the DNA (and thus the sequence of amino acids in the proteins), says that books with very similar texts cannot be of independent origin.

The evidence of evolution revealed by molecular biology goes even farther. The degree of similarity in the sequence of nucleotides or of amino acids can be precisely quantified. For example, in humans and chimpanzees, the protein molecule called cytochrome c, which serves a vital function in respiration within cells, consists of the same 104 amino acids in exactly the same order. It differs, however, from the cytochrome c of rhesus monkeys by 1 amino acid, from that of horses by 11 additional amino acids, and from that of tuna by 21 additional amino acids. The degree of similarity reflects the recency of common ancestry. Thus, the inferences from comparative anatomy and other disciplines concerning evolutionary history can be tested in molecular studies of DNA and proteins by examining their sequences of nucleotides and amino acids. (See below DNA and protein as informational macromolecules.)

The authority of this kind of test is overwhelming; each of the thousands of genes and thousands of proteins contained in an organism provides an independent test of that organisms evolutionary history. Not all possible tests have been performed, but many hundreds have been done, and not one has given evidence contrary to evolution. There is probably no other notion in any field of science that has been as extensively tested and as thoroughly corroborated as the evolutionary origin of living organisms.

All human cultures have developed their own explanations for the origin of the world and of human beings and other creatures. Traditional Judaism and Christianity explain the origin of living beings and their adaptations to their environmentswings, gills, hands, flowersas the handiwork of an omniscient God. The philosophers of ancient Greece had their own creation myths. Anaximander proposed that animals could be transformed from one kind into another, and Empedocles speculated that they were made up of various combinations of preexisting parts. Closer to modern evolutionary ideas were the proposals of early Church Fathers such as Gregory of Nazianzus and Augustine, both of whom maintained that not all species of plants and animals were created by God; rather, some had developed in historical times from Gods creations. Their motivation was not biological but religiousit would have been impossible to hold representatives of all species in a single vessel such as Noahs Ark; hence, some species must have come into existence only after the Flood.

The notion that organisms may change by natural processes was not investigated as a biological subject by Christian theologians of the Middle Ages, but it was, usually incidentally, considered as a possibility by many, including Albertus Magnus and his student Thomas Aquinas. Aquinas concluded, after detailed discussion, that the development of living creatures such as maggots and flies from nonliving matter such as decaying meat was not incompatible with Christian faith or philosophy. But he left it to others to determine whether this actually happened.

The idea of progress, particularly the belief in unbounded human progress, was central to the Enlightenment of the 18th century, particularly in France among such philosophers as the marquis de Condorcet and Denis Diderot and such scientists as Georges-Louis Leclerc, comte de Buffon. But belief in progress did not necessarily lead to the development of a theory of evolution. Pierre-Louis Moreau de Maupertuis proposed the spontaneous generation and extinction of organisms as part of his theory of origins, but he advanced no theory of evolutioni.e., the transformation of one species into another through knowable, natural causes. Buffon, one of the greatest naturalists of the time, explicitly consideredand rejectedthe possible descent of several species from a common ancestor. He postulated that organisms arise from organic molecules by spontaneous generation, so that there could be as many kinds of animals and plants as there are viable combinations of organic molecules.

The English physician Erasmus Darwin, grandfather of Charles Darwin, offered in his Zoonomia; or, The Laws of Organic Life (179496) some evolutionary speculations, but they were not further developed and had no real influence on subsequent theories. The Swedish botanist Carolus Linnaeus devised the hierarchical system of plant and animal classification that is still in use in a modernized form. Although he insisted on the fixity of species, his classification system eventually contributed much to the acceptance of the concept of common descent.

The great French naturalist Jean-Baptiste de Monet, chevalier de Lamarck, held the enlightened view of his age that living organisms represent a progression, with humans as the highest form. From this idea he proposed, in the early years of the 19th century, the first broad theory of evolution. Organisms evolve through eons of time from lower to higher forms, a process still going on, always culminating in human beings. As organisms become adapted to their environments through their habits, modifications occur. Use of an organ or structure reinforces it; disuse leads to obliteration. The characteristics acquired by use and disuse, according to this theory, would be inherited. This assumption, later called the inheritance of acquired characteristics (or Lamarckism), was thoroughly disproved in the 20th century. Although his theory did not stand up in the light of later knowledge, Lamarck made important contributions to the gradual acceptance of biological evolution and stimulated countless later studies.

The founder of the modern theory of evolution was Charles Darwin. The son and grandson of physicians, he enrolled as a medical student at the University of Edinburgh. After two years, however, he left to study at the University of Cambridge and prepare to become a clergyman. He was not an exceptional student, but he was deeply interested in natural history. On December 27, 1831, a few months after his graduation from Cambridge, he sailed as a naturalist aboard the HMS Beagle on a round-the-world trip that lasted until October 1836. Darwin was often able to disembark for extended trips ashore to collect natural specimens.

The discovery of fossil bones from large extinct mammals in Argentina and the observation of numerous species of finches in the Galapagos Islands were among the events credited with stimulating Darwins interest in how species originate. In 1859 he published On the Origin of Species by Means of Natural Selection, a treatise establishing the theory of evolution and, most important, the role of natural selection in determining its course. He published many other books as well, notably The Descent of Man and Selection in Relation to Sex (1871), which extends the theory of natural selection to human evolution.

Darwin must be seen as a great intellectual revolutionary who inaugurated a new era in the cultural history of humankind, an era that was the second and final stage of the Copernican revolution that had begun in the 16th and 17th centuries under the leadership of men such as Nicolaus Copernicus, Galileo, and Isaac Newton. The Copernican revolution marked the beginnings of modern science. Discoveries in astronomy and physics overturned traditional conceptions of the universe. Earth no longer was seen as the centre of the universe but was seen as a small planet revolving around one of myriad stars; the seasons and the rains that make crops grow, as well as destructive storms and other vagaries of weather, became understood as aspects of natural processes; the revolutions of the planets were now explained by simple laws that also accounted for the motion of projectiles on Earth.

The significance of these and other discoveries was that they led to a conception of the universe as a system of matter in motion governed by laws of nature. The workings of the universe no longer needed to be attributed to the ineffable will of a divine Creator; rather, they were brought into the realm of sciencean explanation of phenomena through natural laws. Physical phenomena such as tides, eclipses, and positions of the planets could now be predicted whenever the causes were adequately known. Darwin accumulated evidence showing that evolution had occurred, that diverse organisms share common ancestors, and that living beings have changed drastically over the course of Earths history. More important, however, he extended to the living world the idea of nature as a system of matter in motion governed by natural laws.

Before Darwin, the origin of Earths living things, with their marvelous contrivances for adaptation, had been attributed to the design of an omniscient God. He had created the fish in the waters, the birds in the air, and all sorts of animals and plants on the land. God had endowed these creatures with gills for breathing, wings for flying, and eyes for seeing, and he had coloured birds and flowers so that human beings could enjoy them and recognize Gods wisdom. Christian theologians, from Aquinas on, had argued that the presence of design, so evident in living beings, demonstrates the existence of a supreme Creator; the argument from design was Aquinass fifth way for proving the existence of God. In 19th-century England the eight Bridgewater Treatises were commissioned so that eminent scientists and philosophers would expand on the marvels of the natural world and thereby set forth the Power, wisdom, and goodness of God as manifested in the Creation.

The British theologian William Paley in his Natural Theology (1802) used natural history, physiology, and other contemporary knowledge to elaborate the argument from design. If a person should find a watch, even in an uninhabited desert, Paley contended, the harmony of its many parts would force him to conclude that it had been created by a skilled watchmaker; and, Paley went on, how much more intricate and perfect in design is the human eye, with its transparent lens, its retina placed at the precise distance for forming a distinct image, and its large nerve transmitting signals to the brain.

The argument from design seems to be forceful. A ladder is made for climbing, a knife for cutting, and a watch for telling time; their functional design leads to the conclusion that they have been fashioned by a carpenter, a smith, or a watchmaker. Similarly, the obvious functional design of animals and plants seems to denote the work of a Creator. It was Darwins genius that he provided a natural explanation for the organization and functional design of living beings. (For additional discussion of the argument from design and its revival in the 1990s, see below Intelligent design and its critics.)

Darwin accepted the facts of adaptationhands are for grasping, eyes for seeing, lungs for breathing. But he showed that the multiplicity of plants and animals, with their exquisite and varied adaptations, could be explained by a process of natural selection, without recourse to a Creator or any designer agent. This achievement would prove to have intellectual and cultural implications more profound and lasting than his multipronged evidence that convinced contemporaries of the fact of evolution.

Darwins theory of natural selection is summarized in the Origin of Species as follows:

As many more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.

Natural selection was proposed by Darwin primarily to account for the adaptive organization of living beings; it is a process that promotes or maintains adaptation. Evolutionary change through time and evolutionary diversification (multiplication of species) are not directly promoted by natural selection, but they often ensue as by-products of natural selection as it fosters adaptation to different environments.

The publication of the Origin of Species produced considerable public excitement. Scientists, politicians, clergymen, and notables of all kinds read and discussed the book, defending or deriding Darwins ideas. The most visible actor in the controversies immediately following publication was the English biologist T.H. Huxley, known as Darwins bulldog, who defended the theory of evolution with articulate and sometimes mordant words on public occasions as well as in numerous writings. Evolution by natural selection was indeed a favourite topic in society salons during the 1860s and beyond. But serious scientific controversies also arose, first in Britain and then on the Continent and in the United States.

One occasional participant in the discussion was the British naturalist Alfred Russel Wallace, who had hit upon the idea of natural selection independently and had sent a short manuscript about it to Darwin from the Malay Archipelago, where he was collecting specimens and writing. On July 1, 1858, one year before the publication of the Origin, a paper jointly authored by Wallace and Darwin was presented, in the absence of both, to the Linnean Society in Londonwith apparently little notice. Greater credit is duly given to Darwin than to Wallace for the idea of evolution by natural selection; Darwin developed the theory in considerably more detail, provided far more evidence for it, and was primarily responsible for its acceptance. Wallaces views differed from Darwins in several ways, most importantly in that Wallace did not think natural selection sufficient to account for the origin of human beings, which in his view required direct divine intervention.

A younger English contemporary of Darwin, with considerable influence during the latter part of the 19th and in the early 20th century, was Herbert Spencer. A philosopher rather than a biologist, he became an energetic proponent of evolutionary ideas, popularized a number of slogans, such as survival of the fittest (which was taken up by Darwin in later editions of the Origin), and engaged in social and metaphysical speculations. His ideas considerably damaged proper understanding and acceptance of the theory of evolution by natural selection. Darwin wrote of Spencers speculations:

His deductive manner of treating any subject is wholly opposed to my frame of mind.His fundamental generalizations (which have been compared in importance by some persons with Newtons laws!) which I dare say may be very valuable under a philosophical point of view, are of such a nature that they do not seem to me to be of any strictly scientific use.

Most pernicious was the crude extension by Spencer and others of the notion of the struggle for existence to human economic and social life that became known as social Darwinism (see below Scientific acceptance and extension to other disciplines).

The most serious difficulty facing Darwins evolutionary theory was the lack of an adequate theory of inheritance that would account for the preservation through the generations of the variations on which natural selection was supposed to act. Contemporary theories of blending inheritance proposed that offspring merely struck an average between the characteristics of their parents. But as Darwin became aware, blending inheritance (including his own theory of pangenesis, in which each organ and tissue of an organism throws off tiny contributions of itself that are collected in the sex organs and determine the configuration of the offspring) could not account for the conservation of variations, because differences between variant offspring would be halved each generation, rapidly reducing the original variation to the average of the preexisting characteristics.

The missing link in Darwins argument was provided by Mendelian genetics. About the time the Origin of Species was published, the Augustinian monk Gregor Mendel was starting a long series of experiments with peas in the garden of his monastery in Brnn, Austria-Hungary (now Brno, Czech Republic). These experiments and the analysis of their results are by any standard an example of masterly scientific method. Mendels paper, published in 1866 in the Proceedings of the Natural Science Society of Brnn, formulated the fundamental principles of the theory of heredity that is still current. His theory accounts for biological inheritance through particulate factors (now known as genes) inherited one from each parent, which do not mix or blend but segregate in the formation of the sex cells, or gametes.

Mendels discoveries remained unknown to Darwin, however, and, indeed, they did not become generally known until 1900, when they were simultaneously rediscovered by a number of scientists on the Continent. In the meantime, Darwinism in the latter part of the 19th century faced an alternative evolutionary theory known as neo-Lamarckism. This hypothesis shared with Lamarcks the importance of use and disuse in the development and obliteration of organs, and it added the notion that the environment acts directly on organic structures, which explained their adaptation to the way of life and environment of the organism. Adherents of this theory discarded natural selection as an explanation for adaptation to the environment.

Prominent among the defenders of natural selection was the German biologist August Weismann, who in the 1880s published his germ plasm theory. He distinguished two substances that make up an organism: the soma, which comprises most body parts and organs, and the germ plasm, which contains the cells that give rise to the gametes and hence to progeny. Early in the development of an egg, the germ plasm becomes segregated from the somatic cells that give rise to the rest of the body. This notion of a radical separation between germ plasm and somathat is, between the reproductive tissues and all other body tissuesprompted Weismann to assert that inheritance of acquired characteristics was impossible, and it opened the way for his championship of natural selection as the only major process that would account for biological evolution. Weismanns ideas became known after 1896 as neo-Darwinism.

The rediscovery in 1900 of Mendels theory of heredity, by the Dutch botanist and geneticist Hugo de Vries and others, led to an emphasis on the role of heredity in evolution. De Vries proposed a new theory of evolution known as mutationism, which essentially did away with natural selection as a major evolutionary process. According to de Vries (who was joined by other geneticists such as William Bateson in England), two kinds of variation take place in organisms. One is the ordinary variability observed among individuals of a species, which is of no lasting consequence in evolution because, according to de Vries, it could not lead to a transgression of the species border [i.e., to establishment of new species] even under conditions of the most stringent and continued selection. The other consists of the changes brought about by mutations, spontaneous alterations of genes that result in large modifications of the organism and give rise to new species: The new species thus originates suddenly, it is produced by the existing one without any visible preparation and without transition.

Mutationism was opposed by many naturalists and in particular by the so-called biometricians, led by the English statistician Karl Pearson, who defended Darwinian natural selection as the major cause of evolution through the cumulative effects of small, continuous, individual variations (which the biometricians assumed passed from one generation to the next without being limited by Mendels laws of inheritance [see Mendelism]).

The controversy between mutationists (also referred to at the time as Mendelians) and biometricians approached a resolution in the 1920s and 30s through the theoretical work of geneticists. These scientists used mathematical arguments to show, first, that continuous variation (in such characteristics as body size, number of eggs laid, and the like) could be explained by Mendels laws and, second, that natural selection acting cumulatively on small variations could yield major evolutionary changes in form and function. Distinguished members of this group of theoretical geneticists were R.A. Fisher and J.B.S. Haldane in Britain and Sewall Wright in the United States. Their work contributed to the downfall of mutationism and, most important, provided a theoretical framework for the integration of genetics into Darwins theory of natural selection. Yet their work had a limited impact on contemporary biologists for several reasonsit was formulated in a mathematical language that most biologists could not understand; it was almost exclusively theoretical, with little empirical corroboration; and it was limited in scope, largely omitting many issues, such as speciation (the process by which new species are formed), that were of great importance to evolutionists.

A major breakthrough came in 1937 with the publication of Genetics and the Origin of Species by Theodosius Dobzhansky, a Russian-born American naturalist and experimental geneticist. Dobzhanskys book advanced a reasonably comprehensive account of the evolutionary process in genetic terms, laced with experimental evidence supporting the theoretical argument. Genetics and the Origin of Species may be considered the most important landmark in the formulation of what came to be known as the synthetic theory of evolution, effectively combining Darwinian natural selection and Mendelian genetics. It had an enormous impact on naturalists and experimental biologists, who rapidly embraced the new understanding of the evolutionary process as one of genetic change in populations. Interest in evolutionary studies was greatly stimulated, and contributions to the theory soon began to follow, extending the synthesis of genetics and natural selection to a variety of biological fields.

The main writers who, together with Dobzhansky, may be considered the architects of the synthetic theory were the German-born American zoologist Ernst Mayr, the English zoologist Julian Huxley, the American paleontologist George Gaylord Simpson, and the American botanist George Ledyard Stebbins. These researchers contributed to a burst of evolutionary studies in the traditional biological disciplines and in some emerging onesnotably population genetics and, later, evolutionary ecology (see community ecology). By 1950 acceptance of Darwins theory of evolution by natural selection was universal among biologists, and the synthetic theory had become widely adopted.

The most important line of investigation after 1950 was the application of molecular biology to evolutionary studies. In 1953 the American geneticist James Watson and the British biophysicist Francis Crick deduced the molecular structure of DNA (deoxyribonucleic acid), the hereditary material contained in the chromosomes of every cells nucleus. The genetic information is encoded within the sequence of nucleotides that make up the chainlike DNA molecules. This information determines the sequence of amino acid building blocks of protein molecules, which include, among others, structural proteins such as collagen, respiratory proteins such as hemoglobin, and numerous enzymes responsible for the organisms fundamental life processes. Genetic information contained in the DNA can thus be investigated by examining the sequences of amino acids in the proteins.

In the mid-1960s laboratory techniques such as electrophoresis and selective assay of enzymes became available for the rapid and inexpensive study of differences among enzymes and other proteins. The application of these techniques to evolutionary problems made possible the pursuit of issues that earlier could not be investigatedfor example, exploring the extent of genetic variation in natural populations (which sets bounds on their evolutionary potential) and determining the amount of genetic change that occurs during the formation of new species.

Comparisons of the amino acid sequences of corresponding proteins in different species provided quantitatively precise measures of the divergence among species evolved from common ancestors, a considerable improvement over the typically qualitative evaluations obtained by comparative anatomy and other evolutionary subdisciplines. In 1968 the Japanese geneticist Motoo Kimura proposed the neutrality theory of molecular evolution, which assumes that, at the level of the sequences of nucleotides in DNA and of amino acids in proteins, many changes are adaptively neutral; they have little or no effect on the molecules function and thus on an organisms fitness within its environment. If the neutrality theory is correct, there should be a molecular clock of evolution; that is, the degree to which amino acid or nucleotide sequences diverge between species should provide a reliable estimate of the time since the species diverged. This would make it possible to reconstruct an evolutionary history that would reveal the order of branching of different lineages, such as those leading to humans, chimpanzees, and orangutans, as well as the time in the past when the lineages split from one another. During the 1970s and 80s it gradually became clear that the molecular clock is not exact; nevertheless, into the early 21st century it continued to provide the most reliable evidence for reconstructing evolutionary history. (See below The molecular clock of evolution and The neutrality theory of molecular evolution.)

The laboratory techniques of DNA cloning and sequencing have provided a new and powerful means of investigating evolution at the molecular level. The fruits of this technology began to accumulate during the 1980s following the development of automated DNA-sequencing machines and the invention of the polymerase chain reaction (PCR), a simple and inexpensive technique that obtains, in a few hours, billions or trillions of copies of a specific DNA sequence or gene. Major research efforts such as the Human Genome Project further improved the technology for obtaining long DNA sequences rapidly and inexpensively. By the first few years of the 21st century, the full DNA sequencei.e., the full genetic complement, or genomehad been obtained for more than 20 higher organisms, including human beings, the house mouse (Mus musculus), the rat Rattus norvegicus, the vinegar fly Drosophila melanogaster, the mosquito Anopheles gambiae, the nematode worm Caenorhabditis elegans, the malaria parasite Plasmodium falciparum, the mustard weed Arabidopsis thaliana, and the yeast Saccharomyces cerevisiae, as well as for numerous microorganisms. Additional research during this time explored alternative mechanisms of inheritance, including epigenetic modification (the chemical modification of specific genes or gene-associated proteins), that could explain an organisms ability to transmit traits developed during its lifetime to its offspring.

The Earth sciences also experienced, in the second half of the 20th century, a conceptual revolution with considerable consequence to the study of evolution. The theory of plate tectonics, which was formulated in the late 1960s, revealed that the configuration and position of the continents and oceans are dynamic, rather than static, features of Earth. Oceans grow and shrink, while continents break into fragments or coalesce into larger masses. The continents move across Earths surface at rates of a few centimetres a year, and over millions of years of geologic history this movement profoundly alters the face of the planet, causing major climatic changes along the way. These previously unsuspected massive modifications of Earths past environments are, of necessity, reflected in the evolutionary history of life. Biogeography, the evolutionary study of plant and animal distribution, has been revolutionized by the knowledge, for example, that Africa and South America were part of a single landmass some 200 million years ago and that the Indian subcontinent was not connected with Asia until geologically recent times.

Ecology, the study of the interactions of organisms with their environments, has evolved from descriptive studiesnatural historyinto a vigorous biological discipline with a strong mathematical component, both in the development of theoretical models and in the collection and analysis of quantitative data. Evolutionary ecology (see community ecology) is an active field of evolutionary biology; another is evolutionary ethology, the study of the evolution of animal behaviour. Sociobiology, the evolutionary study of social behaviour, is perhaps the most active subfield of ethology. It is also the most controversial, because of its extension to human societies.

The theory of evolution makes statements about three different, though related, issues: (1) the fact of evolutionthat is, that organisms are related by common descent; (2) evolutionary historythe details of when lineages split from one another and of the changes that occurred in each lineage; and (3) the mechanisms or processes by which evolutionary change occurs.

The first issue is the most fundamental and the one established with utmost certainty. Darwin gathered much evidence in its support, but evidence has accumulated continuously ever since, derived from all biological disciplines. The evolutionary origin of organisms is today a scientific conclusion established with the kind of certainty attributable to such scientific concepts as the roundness of Earth, the motions of the planets, and the molecular composition of matter. This degree of certainty beyond reasonable doubt is what is implied when biologists say that evolution is a fact; the evolutionary origin of organisms is accepted by virtually every biologist.

But the theory of evolution goes far beyond the general affirmation that organisms evolve. The second and third issuesseeking to ascertain evolutionary relationships between particular organisms and the events of evolutionary history, as well as to explain how and why evolution takes placeare matters of active scientific investigation. Some conclusions are well established. One, for example, is that the chimpanzee and the gorilla are more closely related to humans than is any of those three species to the baboon or other monkeys. Another conclusion is that natural selection, the process postulated by Darwin, explains the configuration of such adaptive features as the human eye and the wings of birds. Many matters are less certain, others are conjectural, and still otherssuch as the characteristics of the first living things and when they came aboutremain completely unknown.

Since Darwin, the theory of evolution has gradually extended its influence to other biological disciplines, from physiology to ecology and from biochemistry to systematics. All biological knowledge now includes the phenomenon of evolution. In the words of Theodosius Dobzhansky, Nothing in biology makes sense except in the light of evolution.

The term evolution and the general concept of change through time also have penetrated into scientific language well beyond biology and even into common language. Astrophysicists speak of the evolution of the solar system or of the universe; geologists, of the evolution of Earths interior; psychologists, of the evolution of the mind; anthropologists, of the evolution of cultures; art historians, of the evolution of architectural styles; and couturiers, of the evolution of fashion. These and other disciplines use the word with only the slightest commonality of meaningthe notion of gradual, and perhaps directional, change over the course of time.

Toward the end of the 20th century, specific concepts and processes borrowed from biological evolution and living systems were incorporated into computational research, beginning with the work of the American mathematician John Holland and others. One outcome of this endeavour was the development of methods for automatically generating computer-based systems that are proficient at given tasks. These systems have a wide variety of potential uses, such as solving practical computational problems, providing machines with the ability to learn from experience, and modeling processes in fields as diverse as ecology, immunology, economics, and even biological evolution itself.

To generate computer programs that represent proficient solutions to a problem under study, the computer scientist creates a set of step-by-step procedures, called a genetic algorithm or, more broadly, an evolutionary algorithm, that incorporates analogies of genetic processesfor instance, heredity, mutation, and recombinationas well as of evolutionary processes such as natural selection in the presence of specified environments. The algorithm is designed typically to simulate the biological evolution of a population of individual computer programs through successive generations to improve their fitness for carrying out a designated task. Each program in an initial population receives a fitness score that measures how well it performs in a specific environmentfor example, how efficiently it sorts a list of numbers or allocates the floor space in a new factory design. Only those with the highest scores are selected to reproduce, to contribute hereditary materiali.e., computer codeto the following generation of programs. The rules of reproduction may involve such elements as recombination (strings of code from the best programs are shuffled and combined into the programs of the next generation) and mutation (bits of code in a few of the new programs are changed at random). The evolutionary algorithm then evaluates each program in the new generation for fitness, winnows out the poorer performers, and allows reproduction to take place once again, with the cycle repeating itself as often as desired. Evolutionary algorithms are simplistic compared with biological evolution, but they have provided robust and powerful mechanisms for finding solutions to all sorts of problems in economics, industrial production, and the distribution of goods and services. (See also artificial intelligence: Evolutionary computing.)

Darwins notion of natural selection also has been extended to areas of human discourse outside the scientific setting, particularly in the fields of sociopolitical theory and economics. The extension can be only metaphoric, because in Darwins intended meaning natural selection applies only to hereditary variations in entities endowed with biological reproductionthat is, to living organisms. That natural selection is a natural process in the living world has been taken by some as a justification for ruthless competition and for survival of the fittest in the struggle for economic advantage or for political hegemony. Social Darwinism was an influential social philosophy in some circles through the late 19th and early 20th centuries, when it was used as a rationalization for racism, colonialism, and social stratification. At the other end of the political spectrum, Marxist theorists have resorted to evolution by natural selection as an explanation for humankinds political history.

Darwinism understood as a process that favours the strong and successful and eliminates the weak and failing has been used to justify alternative and, in some respects, quite diametric economic theories (see economics). These theories share in common the premise that the valuation of all market products depends on a Darwinian process. Specific market commodities are evaluated in terms of the degree to which they conform to specific valuations emanating from the consumers. On the one hand, some of these economic theories are consistent with theories of evolutionary psychology that see preferences as determined largely genetically; as such, they hold that the reactions of markets can be predicted in terms of largely fixed human attributes. The dominant neo-Keynesian (see economics: Keynesian economics) and monetarist schools of economics make predictions of the macroscopic behaviour of economies (see macroeconomics) based the interrelationship of a few variables; money supply, rate of inflation, and rate of unemployment jointly determine the rate of economic growth. On the other hand, some minority economists, such as the 20th-century Austrian-born British theorist F.A. Hayek and his followers, predicate the Darwinian process on individual preferences that are mostly underdetermined and change in erratic or unpredictable ways. According to them, old ways of producing goods and services are continuously replaced by new inventions and behaviours. These theorists affirm that what drives the economy is the ingenuity of individuals and corporations and their ability to bring new and better products to the market.

The theory of evolution has been seen by some people as incompatible with religious beliefs, particularly those of Christianity. The first chapters of the biblical book of Genesis describe Gods creation of the world, the plants, the animals, and human beings. A literal interpretation of Genesis seems incompatible with the gradual evolution of humans and other organisms by natural processes. Independently of the biblical narrative, the Christian beliefs in the immortality of the soul and in humans as created in the image of God have appeared to many as contrary to the evolutionary origin of humans from nonhuman animals.

Religiously motivated attacks started during Darwins lifetime. In 1874 Charles Hodge, an American Protestant theologian, published What Is Darwinism?, one of the most articulate assaults on evolutionary theory. Hodge perceived Darwins theory as the most thoroughly naturalistic that can be imagined and far more atheistic than that of his predecessor Lamarck. He argued that the design of the human eye evinces that it has been planned by the Creator, like the design of a watch evinces a watchmaker. He concluded that the denial of design in nature is actually the denial of God.

Other Protestant theologians saw a solution to the difficulty through the argument that God operates through intermediate causes. The origin and motion of the planets could be explained by the law of gravity and other natural processes without denying Gods creation and providence. Similarly, evolution could be seen as the natural process through which God brought living beings into existence and developed them according to his plan. Thus, A.H. Strong, the president of Rochester Theological Seminary in New York state, wrote in his Systematic Theology (1885): We grant the principle of evolution, but we regard it as only the method of divine intelligence. The brutish ancestry of human beings was not incompatible with their excelling status as creatures in the image of God. Strong drew an analogy with Christs miraculous conversion of water into wine: The wine in the miracle was not water because water had been used in the making of it, nor is man a brute because the brute has made some contributions to its creation. Arguments for and against Darwins theory came from Roman Catholic theologians as well.

Gradually, well into the 20th century, evolution by natural selection came to be accepted by the majority of Christian writers. Pope Pius XII in his encyclical Humani generis (1950; Of the Human Race) acknowledged that biological evolution was compatible with the Christian faith, although he argued that Gods intervention was necessary for the creation of the human soul. Pope John Paul II, in an address to the Pontifical Academy of Sciences on October 22, 1996, deplored interpreting the Bibles texts as scientific statements rather than religious teachings, adding:

New scientific knowledge has led us to realize that the theory of evolution is no longer a mere hypothesis. It is indeed remarkable that this theory has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge. The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of this theory.

Similar views were expressed by other mainstream Christian denominations. The General Assembly of the United Presbyterian Church in 1982 adopted a resolution stating that Biblical scholars and theological schoolsfind that the scientific theory of evolution does not conflict with their interpretation of the origins of life found in Biblical literature. The Lutheran World Federation in 1965 affirmed that evolutions assumptions are as much around us as the air we breathe and no more escapable. At the same time theologys affirmations are being made as responsibly as ever. In this sense both science and religion are here to stay, andneed to remain in a healthful tension of respect toward one another. Similar statements have been advanced by Jewish authorities and those of other major religions. In 1984 the 95th Annual Convention of the Central Conference of American Rabbis adopted a resolution stating: Whereas the principles and concepts of biological evolution are basic to understanding sciencewe call upon science teachers and local school authorities in all states to demand quality textbooks that are based on modern, scientific knowledge and that exclude scientific creationism.

Opposing these views were Christian denominations that continued to hold a literal interpretation of the Bible. A succinct expression of this interpretation is found in the Statement of Belief of the Creation Research Society, founded in 1963 as a professional organization of trained scientists and interested laypersons who are firmly committed to scientific special creation (see creationism):

The Bible is the Written Word of God, and because it is inspired throughout, all of its assertions are historically and scientifically true in the original autographs. To the student of nature this means that the account of origins in Genesis is a factual presentation of simple historical truths.

Many Bible scholars and theologians have long rejected a literal interpretation as untenable, however, because the Bible contains incompatible statements. The very beginning of the book of Genesis presents two different creation narratives. Extending through chapter 1 and the first verses of chapter 2 is the familiar six-day narrative, in which God creates human beingsboth male and femalein his own image on the sixth day, after creating light, Earth, firmament, fish, fowl, and cattle. But in verse 4 of chapter 2 a different narrative starts, in which God creates a male human, then plants a garden and creates the animals, and only then proceeds to take a rib from the man to make a woman.

Biblical scholars point out that the Bible is inerrant with respect to religious truth, not in matters that are of no significance to salvation. Augustine, considered by many the greatest Christian theologian, wrote in the early 5th century in his De Genesi ad litteram (Literal Commentary on Genesis):

It is also frequently asked what our belief must be about the form and shape of heaven, according to Sacred Scripture. Many scholars engage in lengthy discussions on these matters, but the sacred writers with their deeper wisdom have omitted them. Such subjects are of no profit for those who seek beatitude. And what is worse, they take up very precious time that ought to be given to what is spiritually beneficial. What concern is it of mine whether heaven is like a sphere and Earth is enclosed by it and suspended in the middle of the universe, or whether heaven is like a disk and the Earth is above it and hovering to one side.

Augustine adds later in the same chapter: In the matter of the shape of heaven, the sacred writers did not wish to teach men facts that could be of no avail for their salvation. Augustine is saying that the book of Genesis is not an elementary book of astronomy. It is a book about religion, and it is not the purpose of its religious authors to settle questions about the shape of the universe that are of no relevance whatsoever to how to seek salvation.

In the same vein, John Paul II said in 1981:

The Bible itself speaks to us of the origin of the universe and its make-up, not in order to provide us with a scientific treatise but in order to state the correct relationships of man with God and with the universe. Sacred scripture wishes simply to declare that the world was created by God, and in order to teach this truth it expresses itself in the terms of the cosmology in use at the time of the writer.Any other teaching about the origin and make-up of the universe is alien to the intentions of the Bible, which does not wish to teach how the heavens were made but how one goes to heaven.

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evolution | scientific theory | Britannica.com

Evolution (2001) – IMDb

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When a meteorite falls to Earth two college professors, Dr. Ira Kane and Prof. Harry Phineas Block, are assigned the job of checking the site out. At the site, they discover organisms not of this planet. Soon the site is taken over by the government, forcing Ira and Harry to the side. As the new life-forms begin to evolve and start to get more and more dangerous, it’s up to the two professors to save the planet. Written by FilmFanUK

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Evolution (2001) – IMDb

Welcome to Evolution 101!

EVOLUTION 101

Introduction

Patterns

Mechanisms

Microevolution

Speciation

Macroevolution

The big issues

Welcome to Evolution 101! by the Understanding Evolution team

What is evolution and how does it work? Evolution 101 provides the nuts-and-bolts on the patterns and mechanisms of evolution. You can explore the following sections:

See the full table of contents here.

Evolution 101 For Teachers in Spanish

Evolution 101 in Turkish

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Welcome to Evolution 101!

Evolution (2016) – Rotten Tomatoes

10-year-old Nicolas lives with his mother on a remote island, in a village inhabited solely by women and young boys. In a hospital overlooking the ocean, all the boys are subjected to a strange medical treatment. Only Nicolas questions what is happening around him. He senses that his mother is lying to him, and is determined to find out what she does with the other women at night, on the beach. What he discovers is the beginning of a nightmare into which he is helplessly drawn. But in Stella, a young nurse at the hospital, Nicolas finds an unexpected ally.

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Evolution (2016) – Rotten Tomatoes

Evolution | New Scientist

Biologically simple they may be, but planarian flatworms have evolved two completely different ways to detect light and one doesnt involve their heads

Water bears, or tardigrades, can survive long periods without any water discovering how they do it could lead to new ways to store vaccines in desiccated form

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Evolution | New Scientist


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