Selection? Let us compute

170px-Charles_Darwin_by_Julia_Margaret_Cameron_2Over at The New York Review of books H. Allen Orr has put up a reaction to A troublesome inheritance. It’s very similar to Jerry Coyne’s take, the part about science (e.g., population structure being non-trivial) is deemed acceptable, but speculations in the second half of the book are not as appreciated. This is not surprising, and seems typical for working population geneticists (though do note that R. A. Fisher’s A Genetical Theory of Natural Selection has quite a bit of sociological speculation in the second half). But I have to say that I disagree with Orr when he says that “it seems hard to maintain that educated people deny that DNA sequences differ subtly among continents.” Jerry Coyne has a follow-up post where he praises Orr’s review, but adds:

This is what I also claimed, and of course got slammed by the race-denialists who are motivated largely by politics. To a biologist, races are simply genetically differentiated populations, and human populations are genetically differentiated. Although it’s a subjective exercise to say how many races there are, human genetic differentiation seems to cluster largely by continent, as you’d expect if that differentiation evolved in allopatry.


You can see what Coyne is talking about in the comments of his blog, for example:

“The idea that human populations are genetically identical, and “races” are purely social constructs, reflecting nothing about genetic differences, is simply wrong." [quoting Coyne -Razib]

This is completely false. I don’t see how any analysis of genetic differences will produce a ‘black’ race that combines Africans, Sinhalese and Australian Aborigines, nor that will justify the ‘one drop’ rule.

Can you believe that Jerry Coyne actually has to respond to this sort of thing? The whole point of formal means of clustering populations is to avoid these a priori social constructions. Gross phenotypes like skin color seem only moderately informative of population history. But a great number of educated people talk about human variation with the taxonomic sophistication of an 11 year old from the Jim Crow south.

In any case, I want to highlight a second area where Orr has a mild slap at Wade, and that’s on the science. Others have touched on this, but a key issue is that Nicholas Wade is covering a beat which is changing by the month, so obviously a lot of the science has now been superseded. Orr states:

Wade’s survey of human population genomics is lively and generally serviceable. It is not, however, without error. He exaggerates, for example, the percentage of the human genome that shows evidence of recent natural selection. The correct figure from the study he cites is 8 percent, not 14, and even this lower figure is soft and open to some alternative explanation…

What’s the truth here? This is a very live area of science. Last summer a preprint was posted on arXiv, Genome wide signals of pervasive positive selection in human evolution. The title makes the conclusion pretty clear. It’s now been published in Genome Research. The authors argue that background selection is confounded with regions of positive selection, in such a manner that the latter is obscured. I blogged it when it came out, if you want to dig deeper. At this point all the controversies about selectionism vs. neutralism really are irrelevant, as there’s enough data to go around that you can actually concretely test hypotheses. In regards to humans my own position is now leaning more toward greater, rather than less, selection, despite the small effective size of our species. That’s because I suspect that we’ve only scratched the surface when it comes to ‘soft selection’ on standing variation….

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