Evolutionary biologist Michael Lynch has published a paper in PNAS this month in which he takes to task scientists in general, and evolutionary biologists in particular, for interpreting “virtually every aspect of biodiversity in adaptive terms.” His language is aggressive: he throws around words like “untenable” and likens the tendency of biologists to dramatize the power of natural selection to the invocation of an intelligent designer. At the end, he writes, “This tone of dissent is not meant to be disrespectful.” Given that he has specifically criticized some of his colleagues, it’s inevitable that some will be offended. But I love this paper, I’m thrilled it’s out there, and I hope it’s making a big splash.
For those interested in my wee little opinion, here’s why:
Theodosius Dobzhansky wrote, “Nothing in biology makes sense except in light of evolution.” Indeed. In general, scientists agree on this. At most biology seminars, in most lecture halls in most universities, the importance of the evolutionary history of the subject under discussion is not contested. It may be overlooked, misunderstood, or understudied, but it is generally not contested. And what about outside the discipline of biology? Same thing. Anthropology and psychology, for example, often invoke evolutionary explanations to describe human experiences. In other words, within academia, evolution is not controversial. But—wait for it—that’s the problem. I’m not about to congratulate modern academic discourse for accepting the scientific establishment of evolutionary biology, even if the topic is controversial in popular culture. Instead, I’m frustrated with and critical of the overuse of evolutionary explanations where they don’t belong.
The problem is that everyone likes evolution. (Everyone except those who loathe it, of course.) The concept of evolution has extraordinary explanatory power, because evolution answers the why question. Natural selection is the evolutionary process that generates adaptations. Therefore, natural selection is the evolutionary process most likely to be invoked to answer any “why?” question of interest. Why did we survive while the Neanderthals went extinct? Because we had a special beneficial mutation. Why isn’t hemochromatosis more rare? Because our hemochromatosis-prone ancestors survived the plagues of Europe. Why do people have religion? Because tool-making was successful. These are just a few examples of overzealous invocations of natural selection that have got me all bent out of shape, and I’ve only been blogging for a few months. But full disclosure, for readers who are not biologists: this criticism is not original. For example, Stephen Jay Gould and Richard Lewontin wrote a scathing diatribe against oversubscription to the “adaptationist programme” back in the 1970s, which I have already wielded in a critical post here. To put it simply, the issue is that it’s not okay to describe the existence of some biological phenomenon as an adaptation, present or past, just because you can dream up a use for it. In their article, Gould and Lewontin describe how and when it’s appropriate to consider the role of natural selection in shaping the subject at hand. And now Michael Lynch has addressed this topic again, nearly 30 years later. The exciting thing is that Lynch’s criticism is specific and incisive, because it delineates the problem in quantifiable, don’t-argue-with-the-numbers terms of population genetics.
What’s population genetics? Well, that’s part of the problem, too. Population genetics is a quantitative field that examines how gene frequencies change over time, in populations, under different conditions. It’s math. It’s building small, mathematical models of imaginary populations with prescribed parameters as a way of understanding how evolution can work. Since this field has been around for a while, and since some of the most important wet-lab technologies evolutionary biologists use today are recent inventions, it could be said that our understanding of population dynamics, based on population genetics theory, outstrips our ability to test these concepts empirically. So pop gen theory should inform experimental research, but it doesn’t, not always. Not all evolutionary biologists, let alone general biologists, are trained in population genetics. As Lynch writes, “The field of population genetics is technically demanding, and it is well known that most biologists abhor all things mathematical.” Well, maybe, maybe not. But the point is made—population genetics is overlooked, and that’s a big problem, because the very foundations of evolutionary theory are evidenced by this discipline.
Population genetics outlines four forces in evolution: natural selection, genetic drift, mutation and gene flow. Natural selection is a function of organismal fitness; more fit individuals leave more offspring, and the next generation is better adapted to the environment. But the remaining three forces are nonadaptive because they are not a function of fitness. So it follows that the degree to which natural selection has played a role in the evolution of your favorite subject is a function of the power of natural selection relative to the power of the other three forces. And guess what? Population genetics is really, really good at evaluating just this kind of dynamic. Central to Lynch’s argument are the relationships between effective population size, selection and drift. For example, Lynch challenges the claim that the increase in eukaryotic genome complexity over time is a function of natural selection. First, he points out that the larger a gene is, the more vulnerable it is to disruption, since most mutations are deleterious. Therefore, to be established in a population, a modification that makes a gene larger and more complex—with a greater “mutational hazard”—must either have a strong, immediate benefit, or… the population must be so small as to make selection unimportant. Selection is powerful in large populations, but weak in small ones; this may be the most important, and basic, tenet of population genetics. And guess what? Parameter estimates for organisms of increasing complexity show a sharply decreasing relationship to population size. So much so, in fact, that Lynch claims that “the paths open to evolutionary exploration are fundamentally different between unicellular and multicellular species for reasons completely unassociated with organism complexity.” It’s a simple point, well-articulated and defended with baby population genetics theory. Lynch writes about quite a bit more in this paper, but for me this line of reasoning is the most powerful. I’ve definitely been persuaded to schedule a bit more population genetics reading into my own education!