Is natural selection still a major force in human evolution? As far back as high school biology, weve been taught to think the answer must be yes. But is it really true?

Charles Darwin published his groundbreaking On the Origin of Species back in 1859 around the same timethatanother Charles Charles Dickens was making a name for himself writing about social conditions in England. Dickens stories emerge from a period in which only 50 percent of British childrensurvived to adulthood a numbernot so different from animals in the jungle. In that vein, Darwin was right when he saidnatural selection was operating on humans full force.

Yet the forces that came into play in the 1850s are far different from those we experience today, prompting some high profile biologists to suggest thatour advanced medical capabilitieshave, in effect, blunted natural selection. In a 2013 Radio Times interview,science communicator David Attenborough describedit this way:

We are the only species to have put a halt to natural selection, of its own free will, as it were. We stopped natural selection as soon as we started being able to rear 9599 percent of our babies that are born.

Lowerchild mortalityrates arelargely due to vaccines, water purification, modern medical care and other technological advancements thathave allowed thehuman population toapproach the 7.5 billion mark.Why does this matter for natural selection? Its a simple matter that dying before producing offspring is the most effective filter for agene pool. Lesser traits are weeded out, while stronger ones are passed on.

Charles Darwin

And since natural selection is the most notable evolutionary force, there is a question about whether any significant human evolution will continue far into the future. Theres no clear answer yet, but compiling various proposals and hypotheses leads to a handful of future scenarios:

Scenario I: No major changes on the horizon

This is the boring scenario, so well get it out of the way first. While death before reproduction is an effective tool for culling out undesirable traits, there is a flip side to the equation.In the Stone Age, being less intelligent than ones peers would put an individual at higher risk of premature death by animal attack, for instance. But natural selection alsopromoted thedevelopment of valuable traits. Our smarter ancestorscould hunt better andfind more food leading to the development of farming and a host of other advancesthat enabled them to stay alive and reproduce.

But then a strange thing happened. Human society developed a sense of ethics and justice that led us to protect the weak. Today, we heal the sick. Infant mortality is low. Andchildren of low intelligence are put into special education classes. As a result,manyindividuals who would have been weeded out in the Stone Age are growing up to pass on their genes.

Finally, human populations are no longer genetically isolated. Along with natural selection, reproductive isolation such asfounder effects and population bottlenecksare major evolutionary processes that have shaped humanity. Buttoday, there is substantialgene flow as people from different continents frequently join to mate.The so-called races are blending, so humanity is evolving in that sense. But it is happening so quickly that within a couple of generations there wont be much left to blend. The planetary gene pool will be mixed about, leaving little room forhuman physical characteristics tochange in any significant way.

Scenario II: Natural selection continues

The main argument here is that currently were in a peaceful time, biologically speaking. Yet we could be on the verge of disease pandemics causing a Darwinian selective sieve.The jet-set age the very factor underlying the gene flow thats blending human races also can be a driving factor for the spread of a pandemic. The notorious influenza epidemic of 1918 came right at the end of World War I, claiming more lives than battlefield injuries.Today, we are much better at monitoring infectious disease threats the containment of the2014 Ebola virus outbreakin West Africa is one example but we also have a growing human population. Furthermore, there is some concern that antibiotic resistance could outpace the development of new anti-microbial drugs.

Alongside premature death, evolution is also powered by sexual selection. This means that although we support the survival of nearly every newborn to reproductive age, those who are better fit in terms of intelligence, ability to generate income, and physical attractiveness, could be more likely to attract mates who share those features.

Scenario III: Evolution shifts to off-world human colonies

In scenario I, we noted the absence ofreproductive isolation in modern times. Butthere is serious talk about sending humans to colonize other worlds. This could involve building freespace colonies (miniworlds constructed from asteroid material and shaped to rotate to provide gravity), floating cities in the atmosphere of Venus, or homes on the surface or below ground on various worlds. The most popular idea onepromoted by Elon Musk who hopes relocate thousands of humans within decades is to colonize Mars. Unless the colonists are placed completely underground, the Martian radiation environment could have a significant selective effect on human genetics.This is not because it would kill off colonists themselves, but it couldrender many of them sterile, or at least put significant selective pressure onreproductive cells.

Since we dont yet understandhow human embryos would fare in fractional gravity gravitational pull at the Martian surface is only 38 percent that of Earth theres a possibility of selective pressure in this regard too.

All of this is without assuming any isolation, because, of course humans would be arriving regularly from Earth, thereby adding to the Martian gene pool. On the other hand, should humans successfully colonize planets of other star systems, some biologists think that the reproductive isolation could be complete enough for notable evolutionary divergence away from Earthbound human populations.

Scenario IV: Transhumanism will drive evolution

Were already seeing humans using gene therapy to alter their genes. Transhumanists seek to change themselves through a range oftechnologies, includingrobotics, bionics, computer mind uploading,artificial wombs andgenome editing. These technologies are potentiallystrong enough to give humans the power to essentially take over their own evolution.

With a desire to improve both human performance and appearance,the transhumanist factor makes it hard to predict where this could go.Theartificial womb, for example, could remove constraints on fetal head circumference. Its not impossible to thinkwe could seehumans sporting heads like the science fiction alien, ET. The TV science fiction cloning thriller Orphan Black included a transhumanist character with a tail. Will there be people like that in real life? Or should we merely expect body additions that are practical, such as wings for colonists on low-gravity planets?

Importantly, the four scenarios outlined above are idealized. Each has its merits, and so all can occur. It could be thathuman evolution will continue,based on a combination of each of these scenarios.

David Warmflash is an astrobiologist, physician, and science writer. Follow @CosmicEvolution to read what he is saying on Twitter.

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