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Author | Topic: Skin colors and latitude | |||||||||||||||||||||||
Parsimonious_Razor Inactive Member |
I ran into this the recently I thought it was an interesting find:
quote: 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.
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Parsimonious_Razor Inactive Member |
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).
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Parsimonious_Razor Inactive Member |
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: quote: quote: 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: 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.
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Parsimonious_Razor Inactive Member |
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.
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Parsimonious_Razor Inactive Member |
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.
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Parsimonious_Razor Inactive Member |
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.
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Parsimonious_Razor Inactive Member |
quote: 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: 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.
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