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November 19, 2003

It's better on fire

OK, I was recently pointed to a few articles on redheads. I've seen them in the press before, but didn't notice the mini-bumper-crop recently.

First, a few articles on the evolutionary significance of redheads. Here is an article that talks about the connection between pale skin and Vitamin D synthesis. This is the standard adaptationist explanation, a perspective recently being edged aside by sexual selection hypotheses (which are of course, "sexier"), for instance, this article says redheads are sexier. Ufortunately, these theories of sexual selection can sometimes be hard to corroborate insofar as different surveys give various responses and you have to siphon cultural influences out of the mix-and ultimately sexual selection does have a functional purpose.

I would like to go back to the adaptationist hypothesis insofar as it relates to vitamin D. Recently new evidence suggests that ancient Britons abandoned their marine/fishing lifestyle as soon as they encountered the Neolithic option, about 5,000 years ago. Claudia on the message board pointed out that the Inuit can have dark skin because of the high percentage of marine foods in their diet (which have Vitamin D), despite their circumpolar location. The above articles note red hair has the highest levels of expression in both Ireland and Scotland (with high water marks of around 10%). And yet, look at the proximity to the ocean of these two lands. Especially in Scotland where the rugged uplands probably made a hunter-gatherer lifestyle difficult the bounty of salmon and other marine resources would likely have been enticing. Therefore, I wonder if Vitamin D deficiency would have been that great of a problem. On the other hand, with the transition to agriculture and pastoralism 5,000 years ago in these cloudy lands and the sudden withdrawl of fish from the diet, a strong selective pressure for redheads might have arisen quite suddenly (red hair being correlated with the light skin that allows vitamin D synthesis).

OK, that was all conjecture, but I think it is something to think about. But what about the thesis that redheads will disappear? Well, the genes won't, but since the expression of this phenotype occurs at such a low percentage even in regions where it is common, I can believe that it is declining in numbers as a visible phenotype as people from various parts of the British Isles intermarry at a greater rate (red hair is caused by a combination of several alleles on the MC1R locus, and so recombination with people without red hair would result in the diffusion of these alleles throughout a more numerous population where expression of the trait would decrease-see Hardy-Weinberg Equilibrium). It might even be that the great emigration of Scottish and Irish peoples during the 17th to 19th centuries to the European settler colonies resulted in a sharp drop in the percentage of peoples with red hair in the world. Not only would the descendents of Scotts and Irish intermarry with Italians, English, French and Poles, but, more likely, they would intermarry with individuals from other areas of Scotland or Ireland where red hair was not as common. For instance, the physical anthropologist Carlton Coon noted that the prevelance of dark hair increases as one moves south and west through Ireland, and in the Irish diaspora regional differences between Catholic Irish would decline as geographic distance would not longer be an issue. My bigger point is that population substructure, a level of inbreeding, exists on a smaller scale than nation or folk. In fact, prior to 10,000 years ago, when all humans lived in small bands, I suspect population substructure was high enough that many peculiar recessive phenotypes expressed themselves in inbred clans. For instance, I see pictures of people with reddish or blondish hair sometimes when looking through old issues of National Geographic in the context of Melanesia (this does not include the blonde aboriginals of Australia)-these peoples with their peculiarities were edged aside in most of southeast Asia by the black haired folk that radiated from the environs of modern southern China....

Posted by razib at 04:21 PM