Skin color evolution in Europeans and Social skin color vs Disease in Puerto Ricans

Science last year (thanks commenter!) had a news story about the evolution of European skin color:

Researchers have disagreed for decades about an issue that is only skin-deep: How quickly did the first modern humans who swept into Europe acquire pale skin? Now a new report on the evolution of a gene for skin color suggests that Europeans lightened up quite recently, perhaps only 6000 to 12,000 years ago. This contradicts a long-standing hypothesis that modern humans in Europe grew paler about 40,000 years ago, as soon as they migrated into northern latitudes. Under darker skies, pale skin absorbs more sunlight than dark skin, allowing ultraviolet rays to produce more vitamin D for bone growth and calcium absorption. "The [evolution of] light skin occurred long after the arrival of modern humans in Europe," molecular anthropologist Heather Norton of the University of Arizona, Tucson, said in her talk.

This seems to be in agreement with accelerating recent selection in the human genome. The Science story is referring to the AAPA 2007 meeting. More from the Science story regarding the SLC24A5 gene:

The genetic origin of the spectrum of human skin colors has been one of the big puzzles of biology. Researchers made a major breakthrough in 2005 by discovering a gene, SLC24A5, that apparently causes pale skin in many Europeans, but not in Asians. A team led by geneticist Keith Cheng of Pennsylvania State University (PSU) College of Medicine in Hershey found two variants of the gene that differed by just one amino acid. Nearly all Africans and East Asians had one allele, whereas 98% of the 120 Europeans they studied had the other (Science, 28 October 2005, p. 601).

This is a wonderful confirmation of Cavalli-Sforza's prediction about recent selection for skin color:

Either way, the implication is that our European ancestors were brown-skinned for tens of thousands of years--a suggestion made 30 years ago by Stanford University geneticist L. Luca Cavalli-Sforza. He argued that the early immigrants to Europe, who were hunter-gatherers, herders, and fishers, survived on ready-made sources of vitamin D in their diet. But when farming spread in the past 6000 years, he argued, Europeans had fewer sources of vitamin D in their food and needed to absorb more sunlight to produce the vitamin in their skin. Cultural factors such as heavier clothing might also have favored increased absorption of sunlight on the few exposed areas of skin, such as hands and faces, says paleoanthropologist Nina Jablonski of PSU in State College.

Perhaps it was the larger population sizes made possible by farming that made it possible for the adaptive mutation to arise in one individual, or the mutation pre-existed in early agriculturalists.

I was looking through the book of abstracts (pdf) of this year's AAPA 2008, and another skin color-related abstract caught my attention:

Social classification, skin color, and genetic ancestry: a bio-cultural analysis of health disparities.

A.L. Non, C.C. Gravlee, C.J. Mulligan. Dept. of Anthropology, University of Florida.

Consistent disparities in health are well documented across racially defined groups for many complex diseases. Researchers have proposed both genetic and socio-cultural hypotheses to explain these disparities, but few studies combine both genetic and sociocultural data to test competing hypotheses directly. Here we address this problem in the context of debate about hypertension in populations of African descent. Some researchers suggest that people of greater African ancestry are genetically predisposed to develop high blood pressure. In support of this hypothesis, some researchers point to preliminary evidence of a modest association between blood pressure and genetic estimates of African ancestry. However, the association between ancestry and disease phenotypes may be due to residual confounding with environmental stressors such as discrimination and poverty. Our study, based on fieldwork in southeastern Puerto Rico, is the first to test these alternatives. We incorporate genetic measures of ancestry, genotyping of candidate genes, and an ethnographically derived measure of social classification that estimates how individuals’ color is perceived in everyday social interactions. We find that social classification of color, but not genetic ancestry, is associated with both systolic and diastolic blood pressure through an interaction with socioeconomic status. These findings emphasize the importance of gathering bociocultural data in studies on health and disease, rather than relying on skin color or genetic ancestry as a proxy for potential disease status.

Remember that while skin color is correlated with ancestry, the correlation is not perfect, i.e., a person with more Caucasoid than Negroid ancestry could in fact be darker than a person with more Negroid than Caucasoid ancestry (*). On top of that, the social perception of skin color may blur the picture even more, since skin color is not value-neutral in most multiracial societies. Thus, "social skin color" is two levels removed from "ancestry". What this abstract suggests is that "social skin color", rather than "ancestry" is the culprit for the disease discrepancies. Hopefully a paper on this will follow.

(*) "More" within reason, e.g., a 100% Caucasoid will be almost always lighter than a 100% Negroid.