|MadSci Network: Genetics|
I've been doing my best to research this excellent question, and it does look like there's some scientific evidence supporting J.J. Brannon's assertion that "The children of low-melanin northern humans had a better supply of vitamin D" from the Franklin Inst. Online Anatomy Forum, to which you refer. With respect to your question as to why Eskimos do not express this same "blonde haired/blue eyed/light skinned" phenotype, it may be possible that the Eskimos compensated for their reduced exposure to sunlight through a comparatively increased consumption of fish, since fish liver/oil is the only major, natural, dietary source of vitamin D.
I think that your 2nd question regarding the physical characteristics of Caucasian vs. non-Caucasian populations also has some bearing upon this observed distinction. The Eskimos are descended from the Asiatic tribes of East Asia, whereas the Scandinavians are descended from the Caucasian tribes of Western Asia. For reasons that are not clear, individuals of Caucasian ancestry display considerably greater phenotypic variability than those of Asian ancestry. For example, dark hair and eyes are much more common in those of Asian compared to Caucasian descent. Consequently, the probability for genetic selection of a blonde haired/blue eyed/light skinned, Asiatic Eskimo subpopulation would be very low compared to that of Caucasian Scandinavians. So a high proportion of fish in the Eskimo diet may have contributed to their survival in the northern latitudes, given their reduced ability to adapt genetically.
It may also be possible that small groups within the population of Scandinavian immigrants who carried this general phenotype were attracted to &/or isolated in this particular region due to some unknown forces of that time. This process constitutes an established form of genetic drift known as the founder principle. In support of this hypothesis, the following Evolution lecture notes, from the Univ. of Texas, describing the Evolution of Bottlenecked Populations cite a study that found genetic evidence for such "founder effects" in the Scandinavian populations, although they do not link these effects directly to this phenotype. This potential founder effect, along with the selective advantage of a better supply of vitamin D, could have contributed to a more rapid selective penetrance of this phenotype throughout the Scandinavian population.
Finally, I really would like to have found some statistics to quantify the actual frequency of the "blonde haired/blue eyed/light skinned phenotype" in those of Caucasian compared to Asian descent, which would have also addressed your 2nd question more directly, but I didn't want to delay responding any longer. I did find reports that raised some questions as to whether or not the blonde/red hair & blue eyes seen in the Berbers of Northern Africa may have originated through interbreeding with "blonde haired/blue eyed/light skinned" Europeans, rather than have been an ancestral trait of the Berber population.
In any case, I hope that this explanation is reasonably understandable and will at least provide you with another perspective on human evolutionary & population genetics. Thanks a lot for the excellent question,
Jeff Buzby, Ph.D.
CHOC Research Institute
MadSci Genetics Network
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