Science Sundays: Natural Selection and Pigmentation

By: Meradeth Houston on 10:14 AM

Welcome back to Science Sundays! Before I launch into to today's topic I'm going to make a shameless plea for votes on my book's cover :) If you have two seconds, go to the You Gotta Read book cover contest here, and vote for #5, Colors Like Memories. Pretty please??

Okay, so today we're going to talk about the distribution of skin color across the globe. Last week we discussed how we ended up with so much variation in coloration due to continuous traits (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.

Human skin color has undergone some interesting changes over the past few million years, ever since we split with chimps around 6 million years ago (give or take a million years--the date is pretty tentative). At that time our hominin ancestors had light skin covered by dense body hair that protected them from the sun. However about 1.5 million years ago, due to environmental reasons, we were in the sun more and needed to cool off easier (probably through sweating to cool ourselves, though there are a lot of theories on exactly how this happened). Anyhow, we developed dark skin color in response.

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.

3. The Vitamin D Hypothesis works the opposite as the first two, in that it selects for lighter skin. Vitamin production is catalyzed in the body through UV exposure, and without enough vitamin D, we can't absorb calcium from our diet, so we start to take calcium from our bones. This results in rickets, which is pretty nasty in that is bows the long bones in the body, and can reduce the birth canal region of the pelvis by 30% or so (um, ouch!). So, with less melanin in areas where there is less UV exposure, like away from the equator, it is possible to absorb more UV and therefore make enough vitamin D. (Random side note: women who are culturally kept indoors a lot often don't get enough sunlight and suffer from rickets too.)

Don't google images of frostbite
without a strong stomach!

4. The Cold Hypothesis. There is a lot of evidence that melanin is really sensitive to cold, and can lead to greater amounts of frostbite or other problems. Again, this would select for less melanin in the skin in regions where it's colder, just to be able to survive the cold easier. (I won't go into the studies that have been done to look into this, but let's just say melanin and liquid nitrogen don't mix well...unless you want a really interesting pattern on your pet.)

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!