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March 07, 2005

Geography predicts human genetic diversity


Geography predicts neutral genetic diversity of human populations
is a paper in Current Biology which supports a "Recent African Origin" (RAO) model for H. sapiens. I noted a month ago that Richard Dawkins, an evolutionary biology consensus bellwether, if not an outstanding researcher himself, has promoted the alternative (and often under-publicized) Out of Africa again & again model in The Ancestor's Tale, which posits periodic migrations out of the Africa that washed over Eurasia as well as non-trivial levels of hybridization with local homonid populations in various regions. This paper falls in the camp of the current "Standard Model" which emphasizes one migration and replacement. Unfortunately, neither the abstract above nor the press release is very detailed or specific, though in the latter there is the assertion that "The loss of genetic diversity along colonization routes is smooth, with no obvious genetic discontinuity, thus suggesting that humans cannot be accurately classified in discrete ethnic groups or races on a genetic basis."1 Neither was the supplemental data particularly illuminating as to the loci they used in their model. Someone with full access might enlighten me on that last particular point.

1 - The frequencies of various HLA alleles may also change at a constant rate in a clinal fashion so that "objective" or typological classification may pose difficulties, but, that does not falsify the reality that a likelihood of match between two individuals is proportional to geographic distance between them in terms of origin, and that match can be very important indeed. Though the clines may show no discontinuities, population densities often do, so in the service of pragmatism one might simply assign "boundaries" between groups where the density of individuals on the ground is the lowest so as to do the least damange when classifications have serious consequences. I'm saying a classification scheme must be judged in the context of ends to which that scheme is being devised. "Accuracy" is in some ways an illusion because the human body is simply a collective of cells encoded by bits of DNA which may have disparate phylogenetic histories-race may well be a reification, but that need not be an insult. I am one who leans toward precision as opposed to accuracy as the prime value, because accuracy can only be judged by its fruits in this particular case. Many of these "race exists vs. does not exist" debates are quibblings over very similar data sets where a few words like "significant" are recalibrated or shaded in various directions.

Posted by razib at 11:18 PM