In Genome-wide detection and characterization of positive selection in human populations there’s an interesting part which intrigues me:
The EDAR polymorphism is notable because it is highly differentiated between the Asian and other continental populations…and also within Asian populations (in the top 1% of SNPs differentiated between the Japanese and Chinese HapMap samples). Genotyping of the EDAR polymorphism in the CEPH…global diversity panel…shows that it is at high but varying frequency throughout Asia and the Americas (for example, 100% in Pima Indians and in parts of China, and 73% in Japan)…Studying populations like the Japanese, in which the allele is still segregating, may provide clues to its biological significance.
EDAR has a central role in generation of the primary hair follicle
What’s going on? My blind and fact-free guess is this polymorphism has something to do with hair form. Eye-balling the map I see that Cambodians are balanced for ancestral and derived, and they’re the Southeast Asian population which is “Mongoloid” that stereotypically has the curliest hair. As for the ~73% value in Japan, that makes sense if you accept the finding that Japanese are hybrid population between Yayoi rice farmers from the East Asian mainland and Jomon natives to a ratio of 3:1, with the latter exhibiting the ancestral allele and the former the derived. The Jomon were probably a collection of people of whom the Ainu of Hokkaido are/were the last distinct remnants in the historical period. The Ainu of course have less stereotypically East Asian features despite their genetic relatedness to other groups in Northeast Asia. One of their characteristics is a tendency toward wavy hair.
(note that this doesn’t mean that I think there was selection for thick very straight hair. I assume it is likely a byproduct effect)