Skin color & sexual dimorphism?

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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.

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10 Comments

  1. I think Jablonski has hit the nail on the head. 
     
    Males have been selected to recognize those females who have better reproductive success potential. 
     
    Who would have thought?

  2. I could be wrong, but I have made the casual observation that the face is usually less pigmented than other parts of the body. At least in northern Europeans, the facial skin does not appear to tan as easily as the arms, legs or back. 
     
    I had previously viewed this as an adaptation to wearing clothing in a cold climate, where in order to absorb sunlight to produce vitamin D, natural selection favored lighter facial skin since this was more likely exposed than other areas. 
     
    Now I wonder after reading this article, was the condition brought about by simply a preference for fair skin in a sexual mate since the face is the most readily observed feature of a heavily clothed person, 
     
    If there is an aesthetic preference for lighter skin, was the preference influenced by an instinctive sense that fairer skinned mates would be likely healthier and produce more viable offspring in low UV environments? 
     
    I have no reason to assume fair skin was considered aesthetically more appealing than darker skin color by our ancestors. As a data point of one, my being blond and blue-eyed doesn?t stop me from finding dark skinned, dark haired woman very attractive. 
     
    As a consequence of occupying high latitude, overcast environments reduced skin pigment was rapidly selected for to minimize Vitamin D deficiency. It seems to me this has little to do with aesthetic preference unless the aesthetic preference was also selected for to assure the fairer skinned mate was more likely chosen.

  3. but I have made the casual observation that the face is usually less pigmented than other parts of the body. 
     
    no. page 71 of skin
     
    generally, the face and appendages are well-endowed, while the trunks is less so. melanocytes are most highly concentrated in the groin…. 
     
     
    If there is an aesthetic preference for lighter skin, was the preference influenced by an instinctive sense that fairer skinned mates would be likely healthier and produce more viable offspring in low UV environments?
     
     
    this has always seemed plausible to me. the only issue is that it in northern regions there really aren’t dark skinned people around anymore not to be attracted to, so you’d have to make a ‘pleistocene mind’ sort of argument i’d think.

  4. In North America there is a known prediliction (albeit rare) for black infants to get rickets during the winter months, due to a lack of sun exposure.

  5. How bizarre. So the reason there are more darkheads in the population is because they are more reproductive? 
     
    If a blonde is fairest of them all, shouldnt there be more blondes?  
     
    Give me a choice between a hipless, fair-skinned, faceless Chinese girl and a swarthy, dark haired hip swaying Latina, hey guess who Id pick to mate with? 
     
    Lets stop imagining whatever happened in the past, lets take a poll NOW. Anybody?

  6. In North America there is a known prediliction (albeit rare) for black infants to get rickets during the winter months, due to a lack of sun exposure. 
     
    same issue with brownz in britain. especially ladies that walk around in black garbage bags all day.

  7. 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. 
     
    It can be explained easily if sexual dimorphism acts on only a subset of the total number of skin-color genes. In very light-skinned people those genes are non-functioning, so there is nothing to act on. In very dark-skinned people the effect of those genes could have less of an impact on phenotype because of non-sexually-dimorphic genes.

  8.  
    It can be explained easily if sexual dimorphism acts on only a subset of the total number of skin-color genes.
     
     
    sure. my point is that sex. dim. is a slow process, so i am skeptical that it could have emerged in the mid latitudes and stabilized easily. perhaps what you are getting at is that it only manifests in the mid latitudes cuz of the variation on the loci….

  9. perhaps what you are getting at is that it only manifests in the mid latitudes cuz of the variation on the loci…. 
     
    Yes. If I understand you correctly, it’s there in very dark skinned people, just not visible.

  10. Give me a choice between a hipless, fair-skinned, faceless Chinese girl and a swarthy, dark haired hip swaying Latina, hey guess who Id pick to mate with? 
     
    You don’t have to ask me.

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