Human Races

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In the basic sense of natural selection, yes, probably. Africans have near-universally dark skin because it's so sunny where they are, and too hot for clothing. Therefore persons with extremely light skin did not leave as many children as dark-skinned folk.

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Not quite true - partially . Skin color is a balance between two factors - damage from UV light, and vitamin D synthesis. Temperature isn't really a significant factor. Melanin, being a strongly light-absorbing pigment, protects the cells from UV damage, but limits vitamin D synthesis. In equatorial environments, there is plenty of light, so most of it needs to be absorbed. In polar environments, there is little light, so most of it needs to be allowed into the skin. Tanning is the body's response to cellular damage from light: your body reacts by having melanocytes produce more melanin. As a consequence, there is no safe form of tanning (apart from "fake" tanning methods) - the tanning itself is a response to damage occuring.What we see with races is really an early stage of cladogenesis, but we're still very closely related on the scheme of things... I wouldn't even put us at the subspecies level yet. Now, with air travel and an increasingly interconnected global environment, I think the odds of humanity splitting into multiple species are about nil, until we get major relativistic distances between human cultures.------------------
"Illuminant light,
illuminate me."[This message has been edited by Rei, 10-19-2003]

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There are cases where the circumstantial evidence is good enough that it's pretty persuasive; the persistence of lactose tolerance in populations that have historically practiced dairy-farming is a good example. I can't think of any that are markers for racial groups, however.

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Let's just ignore disease adaptations (like sickle cell) for now; probably the most frequently used racial identifier - skin color - does have regional dependence. Skin color is based on a balance between two issues: genetic damage from UV light (having extra melanin absorbs the light, and reduces the risk of damage), and vitamin-D synthesis (which melanin inhibits). In regions with more annual light intensity, darker skin is adapted for; in regions with less annual light intensity, lighter skin is.I agree, though, that there is no distinct point where one can make a fair delimitation; it's a gradient of allele frequency, which varies depending on which gene you're talking about.------------------
"Illuminant light,
illuminate me."