I ran into this the recently I thought it was an interesting find:
quote:"A biological issue that has not been satisfactorily resolved is the role of melanin in skin and other animal tissues. A hypothesis is outlined here to account for the evolution of black skin and the ubiquity of melanin in vertebrate tissues. Evidence is presented that melanization of skin and other tissues forms an important component of the innate immune defense system. A major function of melanocytes, melanosomes and melanin in skin is to inhibit the proliferation of bacterial, fungal and other parasitic infections of the dermis and epidermis. This function can potentially explain (a) the latitudinal gradient in melanization of human skin; (b) the fact that melanocyte and melanization patterns among different parts of the vertebrate body do not reflect exposure to radiation; (c) provide a theoretical framework for recent empirical findings concerning the antimicrobial activity of melanocytes and melanosomes and their regulation by known mediators of inflammatory responses."
Mackintosh, James (2001). The antimicrobial properties of melanocytes, melanosomes and melanin and the evolution of black skin. Journal of Theoritical Biology 211(2): 101-113.
The idea is that melanization of the skin is actually a responce to paraisitization and that variance along latitude is due to parasite load decreasing as you move north and that variance longitudinal might be due to climate shifts that increase or decrease the parasite load.
The question of latitude can be explained by the parasite model because temperature decreases as you move from the equator and the colder climates have less parasite activity. Changes in skin tone due to latitude do not conclusively prove either the UV model or the parasite model. The main thing that the parasite model attempts to explain is variation of skin tone across the same latitude. It makes a specific prediction that is different from the UV model. The parasite model says that dry arid climates with less parasites will produce people of lighter skin tone then areas of more humid and higher rain fall (this is just one example) along the same latitude. The UV model would predict that skin tone would not change along the same latitude or would even be darker in the more arid environments. I don't think the evidence is really conclusive one way or another.
As far as the role the melanin plays, it seems to be responsible for aiding the immune system in some degree as well as blocking UV light. The two hypotheses maybe acting in conjunction with each other.
I am involved in several studies here at the University of New Mexico involving mate preferences. In the most simplistic case males prefer women who have strong signals of estroginization, women (this is cycle dependent though) have a preference for strong testosteronization in men. One of the effects of strong estroginization is a lightening of the skin and hair. Men prefer lighter skin and hair tones with in the normal range of their culture. So men living in environments with darker skinned people will have a preference for a darker skinned women then in say, Norway, but the preference will be for the lighter skin with in his subset. This is the only evidence that I know of in regards to sexual selection for skin tones. Perhaps once the natural selection acting for darker skin was lessoned the sexual selection could push it lighter and lighter. But this would only work one way (from dark to light).
Here are some relevant quotes from this paper (Manning, JT ; Bundred, PE ; Mather, FM. Second to fourth digit ratio, sexual selection, and skin colour EVOLUTION AND HUMAN BEHAVIOR; JAN 2004; v.25, no.1, p.38-50) that I thought might help:
quote:“We suggest that it is sexual selection (through male–male competition, which favours testosteronized men in polygynous societies and mate choice for light-skinned oestrogenized women in monogamous societies) which is the primary selection pressure that determines the skin's sensitivity to ultraviolet (UV) light and hence skin colour itself. Polygyny favours the evolution of high prenatal testosterone and this leads to a susceptibility to sunburn and skin infections. Monogamy favours high prenatal oestrogen that is protective against sunburn and skin infections. Very black skin evolves in populations with high UV and polygyny. Very light skin is associated with low intensities of UV and monogamy.”
quote:“In all human groups, males tend to be darker skinned than females This sexual dimorphism is likely to arise from the differences in prenatal and adult oestrogen found in females and males. Women's skin lightens at puberty whereas men's skin becomes darker, and there is evidence from twin studies that this sexual differentiation is under genetic control. Oestrogen may in fact increase the production of melanin, but the effect is not strong and is only apparent at high concentrations. The sex difference in skin colour may arise from early organisational effects of oestrogen, and indirectly from the increase in female subcutaneous fat seen at puberty because within this layer androgens are converted to oestrogens.”
quote:“One strong correlate of polygyny is high pathogen load. Therefore, a function of melanin as a barrier against skin pathogens may be an important aspect of the evolution of dark skin. The case for a role of melanin as an inhibitor of proliferation of bacterial and fungal infections in the dermis and epidermis has been convincingly made by [Mackintosh, 2001]. He points out that the distribution of melanin in different tissues of the body does not strongly suggest a photo-protection function. Thus, melanocytes can be plentiful in areas, which are not often exposed to UV such as the skin of the genitalia, the throat, nasal and auditory passages, and internal membranes such as the peritoneum and brain tissues.”
Also this is from Aoki, 2002. K. Aoki, Sexual selection as a cause of human skin colour variation: Darwin's hypothesis revisited. Annals of Human Biology 29 (2002), pp. 589–608.
quote:“The dark skin of tropical peoples is likely to be an adaptation to the strong ultraviolet (UV) radiation near the equator, perhaps protecting against sunburn or degradation of folate. By contrast, the adaptive value of light skin is questionable. In particular, the relevance of vitamin D deficiency rickets as a selective factor has been cogently criticized. Population genetic studies on the melanocortin 1 receptor (MC1R) gene (one of the genes responsible for normal human skin colour variation) also cast doubt on the role of positive natural selection in the evolution of light skin. Natural selection may favour dark skin everywhere, though to a lesser extent at higher latitudes. Darwin believed that racial differences in skin colour were caused by sexual selection. Available evidence suggests that in each society a lighter-than-average skin colour is preferred in a sexual partner. Such a preference would generate sexual selection for light skin that counteracts natural selection for dark skin. The observed latitudinal gradient in skin colour may result from the balance between natural and sexual selection.”
It seems that there are multiple things operating here all at the same time. Which is not surprising to me in the least. The explanation of skin color can not be completely explained by any one selective force. UV protection, parasite load, sexual selection (both intra and inter), proximate hormonal influences, ect. All play a significant part.
A trait doesnt have to be sex-linked genetically to be sexualy dimorphic. I quoted some research in the post above that showed relative lightness and darkness of skin is linked to hormones. Testosterone is linked to darker skin and estrogin to lighter skin. There also appears to be a sexual preference of men for the lighter skin and hair tones for women with in ones group.
The studies that I have read all say that on average women are lighter then men cross culturally. If estrogin, as it appears to be, is truly linked to lighter skin tones then it makes perfect since. Her at the Univeristy of New Mexico a lot of studies have been done and are being done right now about attractivness ratings and assesments. Men prefer women with strong estrogin signals.
Aoki, 2002 cited above talks about how the theory of vitamin D and rickets acting as a natural selective pressure for lighter skin isn't with out its criticisms. Most serious metastudies on the topic find strong evidence for a wide range of proxmiate and ultimate causes. UV protection is included in these but so is pathogen resistence, immuno system aiding, sexual selection, dominate mating systems in the society and a range of early enviormental cues both in the womb and once the child is born. There are VERY few things in evolution that can be summed up nice and tidy with a singal selective agent. Skin color isn't one of them.
The research that I have seen suggest that there may be strong natural selection pressures for dark skin everywhere. In more northern climates the natrual selection pressure maybe reduced to the point that sexual selection could drive the population to lighter skin tones. If there is strong selective pressure on women for lighter skin tones and, as you have said, there is no sex linked genes wouldn't the men in such a population also proceed to get lighter? I have not seen any evidence that there is sexual preference for DARKER men, but also have seen no convincing evidence that there is preference for lighter men.
The only sexual selection pressure that seems related to darkening up people is in highly polygamous societies which push for massive testosterone due to intragender mate competition. If the natural selection pressures for darker skin is reduced and there is minimal male-male competition, I don't see why sexual selection on females for lighter skin wouldn't lead to whole populations becoming lighter.
quote:Yes, that makes sense, but it also contradicts Brennakimi who maintains that there is sexual preference for darker men among females. Regardless, I'll take your word over hers unless she can produce evidence to support this.
There have been some studies that suggest perhaps there is a preference of women for lighter colors in men as well. This is related back to the idea of neotony. That humans seem to prefer characteristics in mates that are usually associated with juvinial traits in other primates. But the problem with female preference is that it is far from constant. There is a major shift between what a women likes and dislikes across their cycle. One change that applies here would be during peak fertility points there is a preference for more testosteronized men, but during extended sexuality a preference for much more estroginzed men. So any study that wanted to look at female preference for skin color would have to take cycle effects into account, and I havent seen any such study. The only one saw a slight preference for lighter skin tones but there was no account taken for cycle effects. Untill that is done I remain agnostic one way or another.
But there does appear to be SOME evidence for male-male competition leading to darker skin. This is more to do with higher testosterone being favored in more polygamous systems.
quote:But assuming random sorting during meiosis it wouldn't matter, if one sex had a preference, & the other didn't either way, then there would be a directional pressure in favour of that sex across the population as a whole.
So if the women were recieving sexual selection pressure to become lighter, and there was no longer a strong natural selection pressure to maintain darker skin you would expect them to become lighter and lighter. If there was a genetic component beyond sensitivity to estrogin levels then the sons as well as the daughters would be lighter, so perhaps the whole population would move towards lighter skin.