Thursday, January 31, 2008

Skin color is a deceptive character   posted by Razib @ 1/31/2008 01:05:00 AM

The figure to the left is from Signatures of Positive Selection in Genes Associated with Human Skin Pigmentation as Revealed from Analyses of Single Nucleotide Polymorphisms. I thought of this chart when considering the idea that the phenotypic races that we see around us might be relatively new; perhaps an artifact of recent human evolution. Look at "Oceania," those are Bougainville Islanders, from off the coast of Papua New Guinea. In the CEPH-HGDP populations the "South Asians" are from the much lighter skinned northwest fringe of the subcontinent; otherwise, I suspect you would be seeing the South Asian group moving toward the location of the Bougainville Islanders. This is not a surprising finding, earlier studies implied that very dark-skinned populations tended to exhibit a "consensus sequence" due to functional constraint; there's a reason humans are dark-skinned around the equator, and there's only one way to do it. But here's an important point: Bougainville Islanders are closer to East Eurasians than they are to other world populations in terms of ancestry. In other words, the dark-skin and the genes which confer that trait that results in an affinity between Melanesians and Africans in appearance is not a function of relatively recent common descent, but of local adaptation. Similarly, extreme dark-skinned South Asian groups are generally closer to Europeans in terms of ancestry than light-skinned East Asians.

This is all pretty common sense when you think about it. But with that said skin color is a very salient trait. The skin is our biggest organ, it's a large part of what others see. Therefore, there is a natural human tendency to classify in colors. If you read the reports from Chinese delegations who were sent to investigate Cambodia they describe the natives as "black." Similarly, according to Mary Lefkowitz the ancient Greeks observed that there were the blacks of Ethiopia and those of Southern India. They also noted that both the Egyptians and North Indians were brown-skinned people ("wheat colored"). But, perhaps importantly, they often distinguished the various peoples by other characteristics (e.g., Ethiopians and Indian hair form). So on the one hand you have an nod to the importance of skin color as a criterion of perception & categorization, and on the other hand an acknowledgment that populations differ in more than color. But in the United States there are peculiar social conditions which result in problematic conflations.

As everyone knows, to be very dark-skinned in the United States was identical to being of one race for a greater part of our history. Certainly there was a small Native American population, but they could be discarded from the shaping of social norms because of their low numbers. To have dark-skin was to be of African ancestry. Though there were certainly other distinguishing characteristics between those of African and European ancestry, skin color was the most visible and noticeable. It was used as the main discriminatory trait because that was all that necessary. This still persists in our folk culture when people talk about individuals "being discriminated against because of the color of their skin." Skin color connotes a racial identity. And yet you have groups like South Asians, who overlap with African Americans in complexion, but are not really"black" as we understand it. Steve Sailer has been noting for years the implicit value system highlighted by the reality that the very dark-skinned Vijay Singh is not identified as a black golfer, while the lighter-skinned (and only 1/4 African in ancestry) Tiger Woods is. Of course it doesn't work this way all the time, and South Asians are often identified as black, at least upon first impression. But the more confusing situations can also occur because of the nature of American categorizations. So tight is the correlation of non-white and "black" in the minds of some people that really peculiar characterizations can ensue. For example, in high school I had an acquaintance who would refer to myself & a Cambodian girl as black. That was understandable, we both had brown-skin. But, one day he referred to a Chinese friend of mine as black. This friend was not a dark-skinned, she had a brunette white complexion (not olive). When I queried my acquaintance about the fact that this "black" individual was probably lighter skinned than at least 1/3 of our other classmates (all of whom were white), he simply insisted that she was a "Chinese black." That was about as far as I got, obviously he couldn't express the inchoate associations within his mind between racial identity and skin color. In his world, there were whites and blacks. If someone wasn't white, that entailed that they were black.

As is rather clear from the content on this weblog we are getting a good fix on the genetics of pigmentation. Not only do we know the patterns of inheritance via classical pedigree analysis, but we now have a good grasp on which regions of the genome control world-wide variation in melanin content of the skin, eye and hair. We are even beginning to understand when selection began to occur on the loci which control this variation. We have some working hypotheses of why skin color is under functional constraint, and what sort of changes might drive adaptive evolution. But all this is sometimes harder to discuss because the typical American has so many social and psychological associations between skin color and group identity. It isn't just another trait, like bristles on the back of a Drosophila, no, it is the token of one of the most significant sociological phenomena which characterize American society today. Steve will have quite a bit to blog about into the foreseeable future.

Note: I suspect that the transposition of genomic knowledge to folk wisdom is easier in societies such as Brazil or India where extant phenotypic variation on this trait exhibits a larger range, much of it within families. Race and color are still very important issues, but the joints around which the perceptions are carved are more flexible and numerous.

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