Photo: Ernst Mayr, by University of Konstanz / CC BY (https://creativecommons.org/licenses/by/2.5).
Editors note:Dr. Shedingeris a Professor of Religion at Luther College in Decorah, Iowa. He is the author of a recent book critiquing Darwinian triumphalism,The Mystery of Evolutionary Mechanisms. See also the earlier entries in this series:
The phenomenon of convergent evolution is well documented in the literature of evolutionary biology. In StrickbergersEvolution, Brian K. Hall and Benedikt Hallgrimsson make frequent reference to it. But what does it mean? In the 19th century, St. George Jackson Mivart made convergence a central aspect of his criticism of theOrigin of Species. If the variations on which natural selection acts are produced randomly, Mivart reasoned, one would expect different kinds of variations to arise in geographically isolated populations, leading to evolutionary divergence, not convergence. But the latter is what we see. StrickbergersEvolution, while acknowledging the reality of convergence, tries mightily to ignore the non-Darwinian character of this phenomenon, leading to some striking inconsistencies.
In a chapter outlining evidence for evolution we read:
The evolution of different organisms, or parts of organisms, in similar directions convergent evolution (convergence) indicates that selection for similar features in different evolutionary lineages can, and often does, lead to functionally similar anatomical structures.
But how and why do these similar features arise in different lineages so that natural selection can preserve them? Hall and Hallgrimsson do not say. They even note that air-breathing evolved 67 times, but ignore the larger implications of how such a complex series of similar mutations could occur repeatedly if the mutations are produced without reference to the needs of the organism, or randomly.
This problem becomes even more acute when Hall and Hallgrimsson write:
As in parallelism, convergence derives from the exposure of different lineages to similar environmental factors evincing similar selective forces. Common adaptive features thus can be attained in each group through independent genetic changes.
But selection does not produce the independent genetic changes. It only preserves them once they arise. The phenomenon of convergence thus raises real questions about just how independent those genetic changes are. Also, by focusing so much on the environment as driving evolutionary change, do Hall and Hallgrimsson realize they sound a lot more like Lamarck than they do Darwin?
Nevertheless, in a startling reversal on the very next page, Hall and Hallgrimsson contradict everything they have just said in a discussion of the work of Sandra Mitchell, a philosopher of science who proposed a scheme of hierarchical levels of complexity in organisms. The third level of her hierarchy is called evolved complexity, the principle that similar adaptive challenges have resulted in a diversity of forms of organisms, not a single form. But I thought similar adaptive challenges drove evolution to produce similar adaptive forms! Hall and Hallgrimsson ignore the contradiction.
In fact, just two pages later, seemingly unaware of Mitchells work whichtheycited, they write:
When similar organisms are exposed to similar environments in different localities, evolution can produce strikingly convergent or parallel features, whether a structure, function, or behavior.
Mitchell is correct that Darwins theory predicts evolutionary divergence. Unfortunately for Darwin, evolutionary convergence is often what we observe.
Still, Hall and Hallgrimsson press on undeterred by this gap between theory and observation. In a discussion of plant evolution we read:
Not surprisingly, these beautiful adaptations reflectconvergenceas selection under similar environmental conditions produced similar plant phenotypes in different lineages residing in different geographical localities.
George Ledyard Stebbins, the biologist who brought botany into the fold of the modern evolutionary synthesis in the 1940s, also wrestled with the meaning of convergence among plants. He saw convergence as evidence for some type of guiding force in plant evolution either natural selection or orthogenesis. As an orthodox Darwinian, Stebbins discarded orthogenesis as a possibility and opted for natural selection by default. But observing how plants with primitive features continue to exist alongside plants with more evolved features, rather than the latter replacing the former, Stebbins wrote:
The differentiation of orders and families of flowering plants through the action of natural selection under present conditions is well-nigh impossible to imagine.
I couldnt agree more. And this is the reason the phenomenon of convergence still stands as a powerful challenge to Darwinian evolution, just as Mivart understood as early as the 1870s.
More recently, paleontologist Simon Conway Morris has made convergent evolution the focus of his work. In a striking passage Conway Morris writes:
Wherever one looks it seems as if evolution has very little choice. If, then, we can discover the details of the metaphysical map across which life must navigate to the very few available solutions, then are we not on the threshold of a predictive biology? This view of life cuts cleanly across one of the central areas of neo-Darwinian thinking, an area that insists on the randomness of evolution and the unpredictability of its outcomes.
Such an idea is, of course, anathema to the biological establishment. According to Conway Morris, the best way to enrage an evolutionary biologist is to sidle up and suggest that evolution has a remarkable directionality. If you are lucky, he quips, all youll need is a clean handkerchief to dab the spots of spittle, but sometimes the response is closer to foaming.
Remarkably, even Ernst Mayr (pictured above) was forced to tacitly acknowledge the challenge to Darwinism posed by convergence. Earlier in his career, Mayr agreed with Francois Jacob that a tinkerer made a better analogy for the action of natural selection than an engineer. Engineering was just too teleological a comparison for a process said to have no foresight or directionality. But in his final bookWhat Evolution Is, written in his nineties, Mayr positively marveled at the phenomenon of convergence:
Convergence illustrates beautifully how selection is able to make use of the intrinsic variability of organisms to engineer adapted types for almost any kind of environmental niche.
So much for the tinkerer bumbling about in his garage. Natural selection, according to Mayr, does work more like the intelligent engineer after all.
StrickbergersEvolutionrepeatedly acknowledges the widespread nature of the phenomenon of convergent evolution. But by consistently ignoring the profound challenge convergence poses to Darwinian natural selection, the textbook denies students the opportunity to grapple with big and important questions about the history of life on Earth, questions that got the attention of Mivart in the 1870s, Conway Morris more recently, and that even did not escape the notice of Ernst Mayr. This greatly undermines the very educational process a textbook should be designed to foster.