Saturday, April 22, 2006

I was wrong about skin color in Europeans and East Asians   posted by Razib @ 4/22/2006 07:32:00 PM

In December I posted a hypothesis about skin color evolution in East Asians and Europeans in light of new research which suggested that Europeans were fixed for a particular locus which explained a great deal of the variation between themselves and Africans. The facts are like so:

1) There are probably around 5 loci of major effect that explain skin color variation in human beings (older pedigree analysis as well as new genomic data seems to be focusing on a number in this range).

2) One of the major loci, MC1R, is highly polymorphic in Europeans, constrained in dark-skinned populations to the ancestral consensus sequence (Africans, to a lesser extent in South Asians), and being driven toward fixation on a derived allele in East Asians.

3) A new locus in Europeans seems fixed and is responsible for the skin color depigmentation, but in both East Asians and Africans this locus is in an ancestral condition.

From these data I hypothesized that East Asians and Europeans were genetically isolated so they reached the same fitness optimums (assuming light skin has selective benefits at high latitudes) via different genetic architectures. In Europeans MC1R is diversified (either through frequency dependent selection or relaxation) because SLC24A5 handles the phenotype. In East Asians SLC24A5 is in an ancestral state, so MC1R has been coopted via the derived allele to generate the same end via different means.

I think I was wrong, and the evil Finns are to blame...they exhibit a rather high frequency of the MC1R allele which confers light skin in East Asians, but they're in Europe! The idea that I threw out there is really contingent on powerful genetic barriers, beneficial alleles have a way of sweeping everywhere that they confer fitness all things being equal. Lactose tolerance is spread throughout Eurasia, and not because of massive population movements, while various malarial resistance alleles are pretty common across regions just where malaria is endemic. Perhaps there was a massive ice sheet in the middle of Eurasia, and Finns are the byproduct of an admixture event, but my understanding is that MC1R has been under positive selection for around ~10,000 years in East Asians. That's pretty recent, and I don't think that the genetic barriers across Eurasia were that strong for that period, they should been able to "borrow" SLC24A5 instead of throwing up their own trick de novo. Other shit is going down...and I'm sure that pleiotropy has its dirty hand involved here.

But I was wrong, though I did give it a good college try :)

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