Monday, March 09, 2009

Did biologists really think that human evolution stopped?

John Hawks has responded to my posting on the Discover article. In that posting [Are Humans Still Evolving?] I criticized Hawks and his colleagues for claiming that the consensus view among scientists is that human evolution is over.

Recall that the opening sentence of the article is ...
For decades the consensus view—among the public as well as the world’s preeminent biologists—has been that human evolution is over.
John replies with: Did biologists really think that human evolution stopped?.
Yet despite the abundant evidence that human biologists have opposed the idea of recent human evolution, I still think that McAuliffe's opening sentence does construct a "straw man" argument. Many prominent examples don't prove that there has been a decades-long consensus that human evolution stopped. And our research is not about human evolution merely continuing -- we think it actually accelerated. Evidence that some biologists thought that human evolution stopped is interesting. But the reality is that almost no one has thought that human evolution accelerated.
So, John and I agree that McAuliffe's article was misleading when she suggested that the consensus view was that human evolution had stopped. It was misleading in spite of the fact that one can find quotations from some prominent scientists who might have held this view.

My position is that the consensus view is often that found in the leading textbooks. The textbooks on evolution always discuss recent examples of human evolution; such as lactose tolerance, skin color, and shifts in the frequency of blood type alleles. Most of them spend time discussing races and diversity—usually with the goal of dispelling false concepts of race, but always with the assumption that humans have, and are, evolving.

Take Evolution by Barton et al. (2007), for example. They have sections on the evolution of humans by random genetic drift and natural selection. One section on page 775 is headed: Natural Selection Has Shaped, and Is Shaping, Human Evolution. The standard examples are explained.

Barton et al. discuss what happens when negative selection is relaxed due to medical advances and they point out, correctly, that this will lead to the accumulation of formerly deleterious alleles by drift. They even address the very issue that Hawks writes about (p. 775).
It is unclear how effective natural selection has been in our recent history. On the one hand, our relatively low effective population size (Ne ~ 104) makes it impossible for us to avoid accumulating mildly deleterious mutations, and there is evidence that more such mutations have accumulated along our lineage than along that of our sister species, the chimpanzee. However, it could be that our low effective population size actually reflects the effects of a large number of selective sweeps, and therefore the success of natural selection. (Recall that Ne is really a measure of the inverse rate of genetic drift. It can be reduced by both low population size and selective sweeps.) When a favorable mutation is swept to fixation, it may carry with it more weakly deleterious alleles that happen to be tightly linked to the original mutation. Thus, we can make two radically different interpretations of the observed low genetic diversity within our species: on the one hand, that it reflects a low population size in the past, implying inefficient selection, or on the other, that it is a side effect of intense adaptive selection.
Since this is what evolutionary biologists are teaching undergraduates, I think it's fair to say that the consensus among evolutionary biologists is that humans are evolving. Indeed, most evolutionary biologists know that it is impossible to stop evolution.

I guess we can conclude that the author of the Discover article didn't get her straw man version of human evolution from John Hawks. Where, then, did she get it? Maybe it was from Henry Harpending, John's former postdoc advisor and collaborator.

Harpending has just published a book with Gregory Cochran, another collaborator on the "acceleration" paper. The opening chapter of The 10,000 Year Explosion is Overview: Conventional Wisdom.
Scientists have long believed that the "great leap forward," some 40,000 t0 50,000 years ago in Europe, marked the advent of cultural evolution and the end of significant biological evolution in humans. At this time, the theory goes, humans developed culture, as shown by the sophisticated new tools, art, and forms of personal decoration that emerged in the Upper Paleolithic. Culture then freed the human race from the pressures of natural selection: We made clothes rather than growing fur and built better weapons rather than becoming stronger.

The argument that the advent of behavioral modernity somehow froze human evolution is dependent on the notion of a static environment. In other words, if a population—of humans, wolves, crabgrass, you name it—experiences a stable environment for a long time, it will eventually become genetically well matched to that environment. Simple genetic changes then do little to improve individual fitness, because the species is close to an optimum.
I assume John Hawks will agree that this "conventional wisdom" is a straw man. Perhaps Gregory Cochran and Henry Harpending are referring to a subset of scientists when they talk about arrested human evolution. Perhaps they are only referring to adaptationists, or maybe just adaptationist anthropologists?

My own view is that humans are evolving (duh!) and that the evidence for accelerated evolution is unconvincing at this time, but intriguing. I think this is the consensus view among evolutionary biologists. If I were trying to educate the general public that's what I would tell them.

I would not tell them that a revolution in our thinking about human evolution is in progress.


4 comments :

  1. I wonder just how many profs are using that Barton et. al book to teach undergrads.

    I suggested to my evolution prof last year that he consider using it in place of the 7th edition Evolutionary Analysis assigned for the class, but he seemed to think that Barton's textbook was a bit too math/molecular oriented for undergrads to comprehend. Of course, that was just his opinion, but considering that many of the people in my class last year could barely do basic hardy-weinberg problems, I too doubt they'd readily absorb Barton et. al's book.

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  2. @Chris

    A former post-doc in my lab described Dr. Barton as a very nice guy who described himself as a mathematician first, and biologist second. I've read papers of his that begin and end with equations. I'm not sure how the textbook is, but perhaps your prof. is basing his judgment on Dr. Barton's technical work?

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  3. The other authors are Derek Briggs, Jonathan Eisen, David Goldstein, and Nipam Patel.

    You may recognize Jonathan Eisen's name. He has a blog called The Tree of Life.

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  4. We are by the way, commencing work on a second edition .. which will for better or worse, have a bit less math. And in addition, we are probably going to make a significantly shorter version which will have even less math. We wanted to challenge people but alas may have challenged them a bit too much ...

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