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Health

Genetic thugs in the house!

Gene ExpressionBy Razib KhanApril 19, 2006 9:07 PM

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I've been a bitanal about genetic drift over the past few days. The reason is simple, replace "random genetic drift" with "sampling error", and note how ridiculous some of the things scientists will say to journalists when they come a callin' sound all of a sudden. "Hm...no, I haven't done research in this area, but it seems like sampling error could generate that sort of pattern." "Well, it could be sampling error, they can't prove it isn't." I'm not saying that random genetic drift isn't a good null hypothesis, I'm just saying that it is a deux ex machina that sounds good when you can't think of anything else, operationally a skyhook. Some readers brought up some interesting points, but let me reiterate that my objection was to the idea that founder effect upon a small original population could fix alternative morphs, and, that these phenotypes would then be separated by operationally impermeable sexual barriers. A reader brought up Jews, but the best evidence suggests that Ashkenazi Jews experienced 1% introgression from the surrounding population per generation, so that they are about 1/2 European genomically at this point. And this is a small minority population, I do not deny that intragroup variance exists and could be structured along ethnic/class lines, I am simply skeptical that powerful spatial and temporal forces would result in perpetuation of initial substructure. To be less verbose, what the anthropologist alluded to seems to make sense in the context of a species which begins to settle an island archipelago, but last I checked the New World was stitched together by broad expanses of land. Alleles are leaky and will soak the field unless an adaptive fitness barrier gets in their way. In any case, I want to finish with two things. First, I have an a priori genetic reason why I'm skeptical of the "Two Wave" replacement theory posited by the authors of the paper mentioned in the previous posts, but you'll have to keep checking because I don't have time to post it right now. And second, this paper is great at showing how manipulating parameters can give you all sorts of funky results which flip conventional wisdom on its head. Here's the good part:

Using models developed for population extinction and recolonization, we show that a large census size consistent with the multiregional model can be reconciled with an effective population size of 10,000, but genetic variation among demes must be high, reflecting low interdeme migration rates and a colonization process that involves a small number of colonists or kin-structured colonization. Ethnographic and archeological evidence is insufficient to determine whether such demographic conditions existed among Pleistocene human populations, and further work needs to be done. More realistic models that incorporate isolation by distance and heterogeneity in extinction rates and effective deme sizes also need to be developed. However, if true, a process of population extinction and recolonization has interesting implications for human demographic history.

No shit? References:Local extinction and recolonization, species effective population size, and modern human origins, Eller E, Hawks J, Relethford JH, 2004.

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