Skin color & sexual dimorphism?

Some new data to throw into the argument about the origin of light skin (it seems that dark skin obviously arose when we lost our fur, seeing as functional constraint is strong in dark-skinned populations and unexposed skin in our nearest primate relatives is pink). From Dienekes:

Women have lighter skin than men do across a wide range of populations, even on the unexposed skin of the upper inner arm, possibly because of sexual selection by men for lighter-skinned women. If this hypothesis is true, human skin color should become more sexually dimorphic with increasing distance from the equator, since sexual selection for lighter skin in women would be less constrained by natural selection for darker skin in both sexes. Yet when Madrigal and Kelly (2006) analyzed skin reflectance data from 53 different samples, they found that the most dimorphic human populations were actually those of medium skin color at medium latitudes.

Dienekes presents some values, and suggests that “In these data points it looks to me that Iranians and Kurds have the highest sexual dimorphism.” I don’t know what to make of that. Recall that sexual dimorphism often arises rather slowly as a genetically coded trait because obviously if you select for one sex at one end of the population range (sexual selection usually operates via male reproductive skew in most populations), their opposite sex offspring should also skew in that direction. A more complex genetic architecture which takes into account modifier genes on sex chromosomes (which will differ across sexes), or modulation via expression of sex hormones (which are dependent upon loci on those sex chromosomes ultimately, like SRY), seems necessary. Also, I was skimming through Nina Jablonski’s Skin:A Natural History and on page 89 she states: “…one consistent observation is that women have lighter skin color than men. This is true for all indigenous peoples, even for those who are very dark-skinned, among whom such differences are not readily visible….” Jablonski’s own hypothesis is presented in her paper The Evolution of Human Skin Color:

…the lighter skin pigmentation of females is needed to permit relatively greater UV light penetration of the integument for previtamin D3 synthesis. The extra calcium needs of females during pregnancy and lactation are met by increasing plasma concentrations of 1.25-dihydroxyvitamin D, which in turn enhances calcium absorption in the intestine….

Pregancy and lactation are critical periods which determine fitness. By focusing on this Jablonski gets a pretty good yield in terms of differential fecundity. She does not dismiss the importance of sexual selection as a secondary or subsequent factor in generating or heightening dimorphism. For the general interpopulation variation in skin color Jablonski focuses upon the balance between the need to prevent the breakdown of folate (due to UV) and produce enough vitamin D (also due to UV). She points to the Inuit as exceptions that prove the rule, insofar as their dark-skin is comprehensible because their diet is rich in vitamin D.

From the genomic perspective we know that the architecture varies by location for similar phenotypic outcomes in regards to skin color. Even if the locus where a derived allele emerges is the same across two populations to generate the same phenotype (or contribute to the overall effect), that allele is often different, suggesting an independent mutational event. I would not be surprised if varied selective forces end up shaping human skin color variation. Though the correlation between UV and skin color is pretty clear, that may simply be the first principle component, with other factors rounding out the variation….

Update: Just an additional thought: a lot of the genomic data suggests recent selective sweeps on some of the genes for light skin (e.g., a variation of MC1R in China, the SLC45A2 derived allele in Europe, etc.). I think this is a big weakness in the model proposed by Jablonski, after all, it isn’t like humans just showed up in northern Eurasia within the last 10,000 years. So what gives? I suspect that the lack of variety in the diets of agricultural peoples is an important factor. In other words, the more varied diet of hunter-gatherers (or late Ice Age big game hunters) didn’t necessitate skin lightening to increase Vitamin D synthesis. Or, there are other selective pressures which we don’t know about.

The seeds of fundamentalism?

In the discussion thread for the Ayaan Hirsi Ali post there was some mooting of the nature of Islamic fundamentalism. I think this story is illustrative of the issues at work that might surprise:

…Khan told him he first became attracted to radical Islam because the tradition he grew up with was forcing him into an arranged marriage. The radical Imams were offering him a way out.

“A lot of guys I know, actually, have become radicalized, or initially took the first steps towards learning more about Islam and their way of life as a result of them being tried to being forced to marry someone they don’t want to marry,” Butt tells Simon.

Of course “traditional” South Asian youth don’t object to arranged marriages, it is those who are inculcated with “Western” values who tend to find them abhorrent in conception. The fact is that the folkways that immigrants from Third World countries bring with them aren’t really appropriate for their new cultures, and so their children have to find their own way (I know whereof I speak!). Fundamentalist Islam rooted in the Salafist movement is a safety valve for many of these first generation immigrants because it is a modern creation. I don’t want to get into the details of the origins of the Salafi movement, but the reality is that some of its early thinkers were actually quite liberal. The manner it which it mutated makes that seem a bizarre possibility, but it is as it is. My own personal experience is that my more “fundamentalist” relatives are amongst the most open to rejection cultural tradition, so long as that rejection can be grounded in Islamic principles. The malleability of such principles are, I believe, the root of the mutagenic nature of an ideology which presents itself as timeless, and yet is very much a sign of the times. Most immigrant youth do not have the orientation to become atheists whose individualistic self-absorption transcend deep group ties. That’s why the emergence of more “liberal” Islams is essential. Some of that process is going on now as Muslims rework the meaning of their religion in a Western cultural context. But part of the dynamic also has to be from without, just as Western culture forced Jews to accommodate outside of the ghetto, and America denied the Roman Catholic Church any status but that of just another denomination amongst many, so we must get our heads out of the multicultural sand and delegitimatize the sense of entitlement that many “community leaders” of a reactionary bent have in the Muslim community (this is more true for our friends across the pond).

Culture vs. genes

From page 56 of Ancient Greece:

…Sometimes the colonists [that is, Greek founders of colonial settlements outside of Greece] enjoyed a friendly welcome from the local inhabitants where they settled; sometimes they had to fight to win land for their new community. Since colonizing expeditions were apparently usually all male, wives from the colonists had to be found among the locals, either through peaceful negotiation for violent kidnappings.

This is the second time I’ve seen a reference to the non-Greek women who were present at the founding of Greek colonial cities (from Naples to Syracuse in the West to Byzantium in the East). Pretty soon I’ll have to dig into the scholarly literature. My interest is stimulated by the fact that I don’t know of any aspects of these Greek colonies which were culturally non-Greek (though “in the know” can enlighten me!). That is, it seems that the fathers determined the culture of their children. Of course, later colonial males would have arrived, diluting the impact of the initial hybrid generations, but it seems quantitatively this would pale in comparison to the demographic expansion that the founders would have engaged in. Makes me wonder about the Rape of the Sabine Women. Another factor is of course our perception of the ancient world comes through texts filtered through the preconceptions of the authors, a lot of variation “on the ground” is being elided as common motifs are highlighted.

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The importance of labs?

Steve Gimbel has a post up where he expresses skepticism of the utility of lab sections. Janet, Chad & Chad and RPM all offered responses. All that needs to be said from the various angles that I would have touched upon has been said, so I won’t add much more, except to recall my discussion over at the literary blog The Valve about the testimony of Steve Fuller during the Dover trial. For those of you who don’t know, Fuller is a scholar of science (that is, he studies science as opposed to being a scientist) who has suggested that Intelligent Design is a worthy research program, and is willing to testify to that effect. I noted that Fuller has no undergraduate science background at all (in contrast to Gimbel), so that undermines some of his credibility to speak as an expert. Though I don’t think you always to have to “live” something to understand it, sometimes experience is the best course one can take, and in the case of scholars of science having some undergraduate background (even as a minor) is not particularly difficult. One may, or may not, agree with my contention, but the author of the post responded:

Razib, I disagree very strongly with Fuller’s position about this–to the point of mystification–but it’s parochial to suggest that more time taking multiple-choice tests and dissecting things would have affected his later thinking. It’s just completely irrelevant to the argument he’s making.

To dismiss a science eduction as “multiple-choice tests and dissecting things” is, I think, a serious trivialization of what I was speaking of. Science is a culture, and if you are an anthropologist of science, broadly speaking, you must know that culture. Similarly, if one’s goal is to get students to understand the importance and nature of science, what better way then immerse them in the tedium and minutiae, and frankly, often irrelevance, of day to day empirical work (even if they are operationally “cook book” experiments).

Day 3 of hot sauce – Hottest Fuckin’ Sauce

devilIII.jpgAs some of you might have noticed, I was not impressed with Hot Sauce #2. For day 3, I had pepper-crusted beef, bacon, and arugula sandwiches and spicy mustard with the Hottest Fuckin’ Sauce. As you can tell by the picture to the left where I’m pouring, yes, pouring, the sauce over my sandwich, this isn’t that hot of a concoction. The ingredients listed are: Habanero Peppers, Water, African Oleoresin, Scotch Bonnet Peppers, Salt, Onion, Vegetable Oil, Acetic Acid, Garlic and Xanthan Gum. I don’t know what did it, but this might not be as spicy as Dave’s Original “Flavorless” Insanity Sauce, but there is a richness to the taste which impressed me. Not to be cliche, but this hot sauce brought out the flavor in the food, as opposed to muffling it. In some ways Hottest Fuckin’ Sauce reminds me a lot of the home-made habanero sauce I’ve had before. This a definite 8 out of 10.