Just a quick review about some issues that I assumed implicitly in my post where I took issue with genetic drift as a force for population variation. It isn't like genetic drift can't result in variation...but the researcher seemed to be pointing to founder effect which would homogenize alternative populations and "fix" them into alternative states. For founder effect to really work you need to reduce the effective population size and squeeze genetic polymorphism out of the gene pool. Consider the equation for decline in heterozygosity1:
Ht = (1 - 1/(2N))tH0
Where H0 is the intial heterozygosity and t is measured in generations. It seems clear that as N approaches infinite there won't be a change, random genetic drift will not sift out the variation from the population because the sampling process is too weak of a force. In contrast, a low N will result in rapid reduction in heterozygosity, ergo, genetic uniformity. Consider:
t = 100 generations
H0 = 0.5 (half the population is heterozygous)
Where N = 20, H20 ~ 0.04, that is, 96% of individuals will now be homozygous.
In contrast, where N = 200, H20 ~ 0.39, so only 61% of individuals will be homozygous in the 20th generation.
20 generations is probably going to be around 400 human years. In any case, you get the general gist, for founder effects to be powerful you need to sample a small number of founders from the original population. This drives the "N" down as the effective population is reduced.2
The model that the anthropologist proposed assume small founder populations. But, it then assumed that these populations remained genetically distinct. How plausible is this? Not very in my opinion, unless you take into account selection and assortative mating.
Long term equilibrium gene frequencies under drift & migration (between populations) is 1/(4Nm + 1), where N is effective population again, and m is the proportion of migrants within a given generation. The key to note is that if you run this equation through any iteration you'll see that only about 1 migrant per generation is needed to maintain equilibrium, and that this equilibrium can be attained very quickly through genetic exchange. Small populations wouldn't need many migrants because they are small. If large populations entered the New World, well, they would exhibit more internal genetic variation and less influence from drift.
I suspect you could cobble together a model of drift and subsequent interpopulation barriers...but it doesn't seem to pass the test of parsimony. The only reason I sketch this out minimally is that it might behoove the public to know that a lot of scientists are talking out of their asses when they point to drift. it is basically hand-waving.
1 - Heterozygosity would be the frequency of individuals with alternative alleles at a given locus. For example, someone with a "brown eyed" allele and a "blue eyed" allele on the locus that controls for eye color.
2 - Long term effective population is equivalent to the harmonic mean, so it is closer to the small end of the overall range as a function of time.
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First things first, I totally didn't read the article. So, if this is idiotic, then I'm not ignorant just lazy.
I don't understand why assortitive mating is off the table in the discussion of drift. It seems to me that assoritive mating would be essential for drift to have any effect in this case. In the disasrotitive case where individuals preferred to mate with others most dissimilar from themselves, only one migrant per generation would be required as you suggested because they would instantly be "mated" so-to-speak. However, in the other extreme, assoritive mating would prevent any gene flow even in co-spatial populations and any number of migrants would not prevent drift (assuming the populations remained sufficiently small). Therefore it seems that the amount of assortitive-ness is essential in understanding the potential for drift.
I don't understand why assortitive mating is off the table in the discussion of drift.
it isn't, but i don't see it as a powerful enough force to prevent morphological convergence if said convergence was neutral. it isn't an either/or. also, assortative mating is, in my book, a form of selection (e.g., sexual selection). ask yourself how many populations you know where assortative mating has maintained genetic differences over thousands of years?
ask yourself how many populations you know where assortative mating has maintained genetic differences over thousands of years
I thought there were well documented genetic differences within some Jewish populations due to the lack of intermarriage with outsiders? That would seem to fit the bill.
I thought there were well documented genetic differences within some Jewish populations due to the lack of intermarriage with outsiders? That would seem to fit the bill.
all long term jewish populations exhibit introgression from what i can tell. eg., european jewish groups look different, but not that different from surrounding populations. ditto for bene israel in india or sephardic jews or yemeni jews. the introgression is ofen female mediated. see here.
but, the key point i would like to bring attention to is this:
a) jews are a tiny minority within a given spatial location
b) this sort of assortative mating increases substructure with a spatial boundary, it doesn't create them across it
in other words, i think assortative mating could do fancy tricks, but it wouldn't keep people spatially separated, only induce a caste society.