Skin colors and latitude

Bren, Caucasians lost their melanism for a purpose, it interfered with vitamin D production in the skin, I've already talked about this, as evidenced by higher than average incidences of rickets in Scandinavian black children.Now, we have the knowledge that high UV is harmful & blocked by melanin. We also know that UV & sunlight levels in general are higher as you approach the equator. We also know that humans manufacture vitamin D in their skins & this is associated with light. Ergo, low melanin levels are adaptations to maintain vitamin D production, where there is lessened selective pressure for high level melanism due to low UV levels. High melanin levels are an adaptation to high UV. The different environments provide selective pressures that go in opposite directions as regards melanism.Now, what do you have to support the sexual selection theory of melanism? Because they ran around half naked in tropical desert sunshine, perhaps? And I wouldn't describe a country in the tropics that strays extremely close to the equator as being close to the pole! This is evidence for me, not you.Mark

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There are 10 kinds of people in this world; those that understand binary, & those that don't

Also, aesthetic choice doesn't quite cut it as an explanation IMO. Because it is not clear why, out of the blue, an aesthetic that had presumably previously been perfectly happy with people in various browns suddenly decided to prefer paler versions.

go read jared diamond- the third chimpanzee and leave me alone.

Bren, No, I'm not arguing against Jared, I'm arguing against you. I have presented evidence that supports melanism level being related to sunlight levels & therefore latitude. You have asserted without evidence that sexual selection is involved. I don't deny that it may have a bearing, but I'm certainly not going to accept it on your say so sans evidence.Mark

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There are 10 kinds of people in this world; those that understand binary, & those that don't

ok how's this. lighter skinned black women are becoming more and more prevalent while darker skinned black women are becoming rarer. darker skinned black men are becoming more and more prevalent and lighter skinned black men rarer. why? because that is what is (and has been for a long time) viewed as attractive. so it is sexually selected for (by choosing partners esp. that the alternative do not reproduce as often... well this is the general idea.). this has no relationship to latitude.i think you and i may be arguing different ideas entirely (this is not new...) you may be arguing the purpose and reason for an adaptation and i may be arguing why it was allowed to continue. basically. if no one had found the first dark or light person attractive, they would not have reproduced and the trait would have been lost.

Bren, I'm not aware that this is true at all. As far as I'm aware skin colour is not sex-linked chromosomally, therefore what you suggest can't be true. Think about it, parents don't produce lighter females & darker males. The premise being, if you read your text carefully, that females are likely to be lighter, males darker. The offspring are likely to be inbetween. The next generation gets the same mixing.I think you are confusing, if you excuse the potentially racist overtone (not intended, but descriptive), the "international chocolate" colour of many "blacks", of whom many would be lucky to trace 1/8th of their recent ancestry to Africa. This has nothing to do with sexual selection since males are just as likely to be "mixed race" as females.Can you name an indigenous population where the females are lighter than males? The very notion belies an ignorance of how genotype affects phenotype, & particularly how such geno/phenotypes are linked to sex chromosomes based upon rather obvious observations of human population in general.In my last post I asked you to support your sexual selection hypothesis, you haven't done so, in fact you have proposed something contradicted by genetics & not supported by it.Mark

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There are 10 kinds of people in this world; those that understand binary, & those that don't

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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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.

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So that's why the girls I knew when I was a teenager spent so much time working on their tans and why women go to tanning booths and buy sunless tanning products. Or is it the other way around?There is a preference for lighter skin tones in Asia and skin lightening products are big there but IMO these differences are primarily cultural.IMO the aforementioned paper by Jablonski and Chaplin makes a good case for the protection argument basing it more on protection of sweat glands and prevention of folic acid breakdown than on protection from sunburn. I have just obtained the paper on antimicrobial affects of melanin and haven't had time to read it yet.I am not sure if it has been discussed on this thread but I think that I have previously posted that skin color variations are primarily controlled by the interaction of two hormones Apha melanocyte stimulating hormone and agouti signalling protein with the Melanocortin 1 (MC1R) receptor. This interaction controls the amount of the dark eumelanin containing pigment granules (eumelanosomes) and reddish pheomelanin containing pigment granules (pheomelanosomes) that are produced and transferred from the melanocytes to the keratinocytes. MC1R is highly polymorphic in humans with at least 65 known alleles with nonsynonomous changes at last count (Reese, Am J Hum Genet. 2004 Nov;75(5):739-51 ) and many of the varients are associated with differences in skin and hair color and sun sensitivity. A less well known factor in skin color is the tendancy of the melanosomes in dark skin to be indvidually distributed and bound in clusters in lighter skin. Our work indicates that this is controlled by the keratinocyts and not the melanocytes (Minwalla et al. J Invest Dermatol. 2001 Aug;117(2):341-7) so MC1R polymorphism is probably not the only genetic variable affecting skin color even though it is a major one.Randy
PS Anyone interested in how sunless tanners work can see my little FAQ answer in the August issue of Scientific American. (or you can read it online We're Sorry - Scientific AmericanThis message has been edited by Randy, 11-17-2004 11:00 PM

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ok how's this. lighter skinned black women are becoming more and more prevalent while darker skinned black women are becoming rarer. darker skinned black men are becoming more and more prevalent and lighter skinned black men rarer. why? because that is what is (and has been for a long time) viewed as attractive. so it is sexually selected for (by choosing partners esp. that the alternative do not reproduce as often... well this is the general idea.). this has no relationship to latitude.

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Now a CULTURAL aesthetic influence I could buy. First question though, where is this process supposed to be occurring? How fast? Is it a real phenomenon, or just something local, or just a misaprehension?In the west, a white aesthetic predominates. It seems to me that women, the major recipients of "attractiveness programming", respond to this by duplication. What this means fior non-white women is skin-lightening acids, hair straightening etc. Equally, black women selected to appear on TV or other mass media - that is, selected to be attractive by the prevailing white aesthetic - are those that carry the most caucasian features. Beyonce is an excellent case in point, barely a trace of African descent in her bone structure. This applies even in the case of, say, rap music videos, in which there are very few actually african-looking women, merely caucasians with a dusky tone.

i'm not talking about popular culture in america. i'm talking about partner selection among particular groups. look at black couples... generally, the woman has lighter skin and the man has darker. this is not a new trend brought on by hollywood aesthetic. look at old african-american short stories and novels. amos fortune: free man describes the sister as being very beautiful because of her carmel skin.

it's not sex linked. it's sexual selection... conscious choice of parter characteristics.that is my point entirely. there is no indigenous group of darker males and lighter females... these traits are being selected for by conscious personal choice with whom to mate.

As I understand it this is a very recent phenomenon driven by the advent of colour film in Hollywood in the thirties (movie stars lived mostly in California - they developed tans - people want to be like their idols kind of a deal). This column in USA Today attributes the original fad to Coco Chanel in the twenties.I haven't got time to dig any evidence up at the moment but I think you'll find that historically the ideal in Europe at least was for the skin to be as white as possible. This was certainly true in Elizabethan times in England where women used to apply a paste containing mercury to whiten their complection. And so is the preference for darker skin tones in Caucasians. I suspect both are recent developments.

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Confused ? You will be...

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:

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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.

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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.

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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.

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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.

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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.

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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.

Bren, Then why did you say black females are getting lighter & males darker? Mark

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There are 10 kinds of people in this world; those that understand binary, & those that don't

My mother will attest to this: her grandmother would scrub her down with hydrogen peroxide when she went to Missouri to visit. "Child, you look like a gypsy!"