didn't get to see that post?). What I didn't go over was the fact that, in general, the distribution of different skin colors is distributed clinally. And by that, I mean that the distribution of more concentrated amounts of melanin is generally closer to the equator, and the farther away from the equator, the less melanin there is in skin/hair/eyes. Now, this is a general trend, but there are plenty of exceptions, however there does seem to be some natural selection at work to explain why this is patterned this way.
So, the ancestor of all humans (Homo sapiens) who arose around 150,000 years ago (again, give or take on the date) had a lot of melanin in their skin. When they started to migrate into other parts of the world, lighter skin color started to either be selector for, or was a shift that was neutral and in general not selected against, so it just kind of happened (there's been a lot of debate about this). Anyhow, there are lots of theories regarding how these changes occurred and the selective forces behind them. Here are a few:
1. Most people have heard about the Skin Cancer Hypothesis. Because having more melanin in the skin protects from UV exposure and therefore damage and skin cancer (a major killer), dark skin is selected for in regions closer to the equator where we have more direct UV exposure.
2. The Nutrient Photolysis Hypothesis is another important one. Vitamin B folate is extremely important in the body, especially for pregnant women (those horse pill vitamins pregnant women take have lots of folate in them). Folate is also sensitive to UV exposure, so more melanin in the skin protects the folate within the body.
|Don't google images of frostbite|
without a strong stomach!
There are actually more theories/hypotheses regarding how skin colors has been operated on by natural selection, where having a specific amount of melanin in the skin--a little or a lot--has been advantageous to an individual and therefore they have spread their genes to more of the population. However, this post has seriously gotten super long!
Questions, sing it out in the comments! And please, please go vote!