Over at PLOS there is an eminently readable paper titled Population History and Natural Selection Shape Patterns of Genetic Variation in 132 Genes. The authors sequenced DNA from about 60 individuals, 30 African Americans and 30 European Americans, and found the latter showed evidence of recent adaptation, whether that be mediated via positive selection sweeps or balancing selection (the former would tend to homogenize the polymorphisms so that the advantageous variant would dominate in frequency while the latter would display intermediate frequencies so as to maximize heterozygous genotypes). This jives with a recent paper that finds evidence for widespread selection sweeps outside of Africa among humans, or the possibility that lightening of Eurasian skin is due to the release of functional constraints when humans left their natal continent (in other words, instead of strong constraints toward one variant a thousand flowers bloom, possibly due to genetic drift or sexual selection).
A quick search of this blog will reveal a lot of papers linked to that talk about neutral markers which trace phylogenies and genetic distances between populations. This is great for tracking population movements, but doesn’t tell us much about what forces reshaped the populations once they moved into a new habitat. In Journey of Man Spencer Wells alludes to a change in modern humans when they entered the novel environment of Central Asia. Wells alludes to the fact that the human toolkit changed during our species Central Asian sojourn, but he never elaborates in detail whether the changes were driven by cultural evolution or a genetic change, though it seems plausible that both occurred as they often drive each other (I tend to think that cultural changes reshape the genetics of a population a lot more than people realize).
As a specific example, the authors of the above paper target in particular two genes that have been implicated in the rate limiting step of calcium absorption metabolic pathway. Both genes show evidence of selection sweeps in Europeans, that is, there was very strong selection on the locus for a particular variant which is disproportionately represented, at least when compared to the null hypothesis of neutrality (random walking variations of polymorphisms). The authors suggest that this is related to the fact that Europeans also have a great capcity for lactase metabolization as adults (lactose tolerance). Lactose tolerance is obviously an adaptation driven by the domestication of cattle, a cultural feature that is contingent upon environmental factors.
The authors note that African Americans do not show evidence of a selection sweep on that locus. African Americans also tend not to exhibit a high lactose tolerance. Now, what I am curious about, do lactose tolerant Nilotic Africans also show evidence of a selection sweep on the calcium absorption regulating genes? There is research that suggests that adult lactose tolerance in Africans is controlled by a different allele than among Europeans, so perhaps a different variant will be found.
Related: Henry Harpending on neutral vs. functional genome as a reflection of a group’s history.
Posted by razib at 08:16 PM