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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But there are morphological differences among Amerindians due to where they live -- for example, the Blackfoots of the Canadian prairie are, in line with Bergman's Law, very big, while tropical Amerindians tend to be much smaller. And the Amerindians of the Altiplano have big chests so they can breathe the thin air, while their Amazonian neighbors do not.
"relatively depauperate of the variation which selection needs to work it's wonders.
Why is it that a bottleneck would have such a profound effect? Are fresh allelic variants in small populations rare occurances? What can a population do to increase genetic diversity that selection needs to work on? Does large increase in the homogenous population number help much, or do you really need the introduction of alleles from different pops?
"That is, there is a tendency for northern peoples in Eurasia to be stockier and possess shorter legs and longer trunks than southern people"
I've also noticed something relevant to this, I think. What I've noticed is that northern Eurasian peoples very often look, well, "Eurasian", i.e., of mixed NE Asian & European ancestry. The allometric observations about the peoples of Eurasia could be due to admixture as much as to adaptation, no?
"10,000 years isn't enough time for mutation to replenish the variation .. ..."
I guess that the above would be the answer to my questions. And I'm guessing that the process might be all the slower in very small pop. sizes, as the probability for arising mutations might be less in that case? I really need to try to brush up on, & learn more of, the basics here.