Thursday, May 29, 2008

Genetic orthodoxy?   posted by p-ter @ 5/29/2008 07:16:00 PM

John Hawks, in a post on scientists who dispute the acceleration hypothesis (acceleration deniers?), makes reference to "the Stanford school of genetic orthodoxy". So what is this?

Essentially, he's referring to the current paradigm (I'm as much of a fan of hyperbole as anyone else, but paradigm is clearly the more appropriate word here) in the field of population genetics about the peopling of the world. The story goes like this: a small set of individuals from an ancestral population in Africa moved somewhere in the Middle East, and grew. Then from there, a small set of individuals moved nearby in each direction and settled. Ditto for those populations, and so on. These "serial bottlenecks" kept occurring until the entire world was populated, replacing the individuals that were there before them.

The observation that solidified this paradigm comes from this paper, which showed an impressive negative correlation between distance from East Africa and genetic diversity, consistent with each population containing a subset of the diversity of the populations it came from. Since then, that sort of approach has been used in a number of similar applications, including this nice one on the peopling of the Americas.

Further support for this paradigm comes from more recent work modeling human demography--it's simply not true that this out-of-Africa hypothesis is enforced like an orthodoxy. See, for example this paper entitled "Statistical evaluation of alternative models of human evolution" (lest you think that alternative models of human evolution aren't being evaluated), which concludes for a single origin of humans in Africa. This doesn't test the "serial bottleneck" model, but does address the multiregional hypothesis, which I think is the major point for Hawks. Or consider a more recent paper, which attempts (with moderate success) to infer the colonization history of the world. The results favor out-of-Africa, as well as serial bottlenecks (though theses bottleneck, it must be noted, were essentially built into their model).

Now, new data may alter some of these models somewhat--David Reich and other claim here (in a News and Views article) that they see evidence for multiple waves of migration from Africa in PCA analysis, though it remains to be seen how those results hold up.

I'm not sure what Hawks thinks of these papers--for all I know, they're making the multiregional hypothesis into a statistical straw man that is easily demolished, but the point remains that the consolidation of these observations into a paradigm is not entirely without reason. The statistical methods and genetic data are available to challenge it, and skeptics (I know many) are more than welcome to try their hand.