Regular readers of this weblog know that there are some quick “back of the envelope” prediction equations that one can appeal to to get a rough sense of how quickly evolution can proceed. For example, the time until fixation of a neutral (no selection + or -) mutant is 4Ne generations, where Ne is the effective breeding population. On a quantitative polygenic trait the response to selection, R is proportional to h2, the heritability, multiplied by the selection coefficient, S (R = h2*S being the classic empirical breeder’s equation). Nevertheless, sometimes it is important to get an empirical feel for how quickly selection, mutation and drift can operate together (along with migration) to shape, exhaust and replenish variation.
With that in mind, I was talking to a physical anthropologist the other day and he mentioned a curious fact, Amerindians are generally built as if they were a Arctic people, no matter the latitude. That is, there is a tendency for northern peoples in Eurasia to be stockier and possess shorter legs and longer trunks than southern people (though they tend to be more massive). In the Americas this pattern is recapitulated, but, the tropical peoples tend to be far stockier and more similar to their northern equivalents than in the Old World. The inference would be that selection has not had enough time to reshape the body proportions of the peoples of the Americas toward their “latitude appropriate” ratios. But, we also know a little bit about the peopling of the New World, and no matter when you believe Amerindians arrived, it seems plausible that the modern population seems to have gone through a population bottleneck via Berengia about 10,000 years ago. In other words the native genetic background of the ancestral population would also have been relatively depauperate of the variation which selection needs to work its wonders. Take home message? 10,000 years isn’t enough time for mutation to replenish the variation that would result in the normal differences in body proportions of H. sapiens sapiens as a function of latitude.
Of course, this is a canned scenario where variation was reduced because of the bottleneck, both demographic and environmental, that occurred during the Amerindian’s circumpolar trek. But, it is gives us a general sense of the power (or lack of) at least one parameter, mutation. Of course, regular readers also know that I’ve suggested rapid selection over the last 10,000 years, what gives? Remember, to some extent the Amerindians are an assay of the mutation parameter, they were isolated from the World Island (Eurasia + Africa) where migration could serve to spread favorable variations from deme to deme.
Reference: Hall et. al., Pleistocene migration routes into the Americas: Human biological adaptations and environmental constraints, Evolutionary Anthropology: Issues, News, and Reviews Volume 13, Issue 4 , Pages 132 – 144
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