My freshmen students and I have just spent a semester reading through Nina Jablonski’s book Skin: A Natural History in which she lays out a hypothesis for the evolution of human skin color variation based on natural selection, a.k.a. Darwinian evolution.
[I think it's a fantastic book for introducing anthropology to freshmen (written by a dear friend who I happen to also greatly admire), so I built a class around it.]
Where there is intense UV radiation (the tropics) people adapted to its destructive powers by evolving natural sunscreen, that is, lots of melanin in their skin.
Conversely, in areas where there is relatively little UV (away from the tropics, going towards the poles), people lost pigmentation in order to maximize the sun’s stimulation of Vitamin D synthesis in the skin (something that melanin inhibits).
Your skin color is about the UV environment of your ancestors. Thus, Seal and Heidi Klum are explained.
As all adaptive scenarios need be, these phenotypes are linked to reproductive success. Highly melanized skin is the primitive condition in humans, that our common ancestor in Sub-Saharan Africa evolved post-fur loss to prevent UV radiation from destroying folate—a process that can lead to death and birth defects of offspring. Once humans began dispersing around the globe, the ones to live in low UV environs evolved poorly melanized skin in order to allow enough vitamin D to be synthesized by a mother so that her fetus could form properly and then eventually grow up to reproduce successfully too. We're specifically talking about the development of the skeleton, since vitamin D is necessary for calcium to do its thing.
That women are lighter than men around the globe supports this notion that allowing UV to penetrate the skin during pregnancy is important.
Perhaps the strongest support for this hypothesis is the stunning map of the globe that Jablonski and Chaplin put together. With some exceptions, global distribution of UV intensity is positively correlated with the amount of melanin in indigenous humans so they were able to construct pretty accurate predictions of human skin color around the world based on UV.
Seal's ancestors are from Africa, while Heidi's are from Scandanavia.
[Of course, before that, Heidi's ancestors, like Seal's and yours and mine and everyone else's, lived in Africa.]
It’s an elegant explanation for the evolution of human skin color variation, and one that has gained a lot of support. But the vitamin D aspect of the story is definitely not a hypothesis preferred by all.
And then of course, as reported here on the MT recently, even a mega-study on vitamin D can't tell us for sure what levels are required to stave off health problems, or even what those health risks are!
But it's difficult to go into the details and nuance of these issues about skin color variation and vitamin D while introducing evolution to students. For many of my students, this is the first time that they’ve learned about evolution in a scholarly setting and we perform activities to illustrate the differences between Lamarckian evolution and Darwinian selection. Of course we also discuss all the known evolutionary forces—mutation, gene flow, drift, and selection—not just selection.
Few students are able to digest all of this the first time they learn it. And regardless of the explanations for why that may be (i.e. instruction quality, lack of effort, difficulty with the concepts, too much bias and misunderstanding brought into the classroom, etc…), it takes longer than a semester to understand how natural selection works and how it does not work.
For many students, the moment that they grasp natural selection, they begin to see the world through selection-colored spectacles. Everything has a reason, much like Dr. Pangloss's philosophy in Voltaire’s Candide. And it’s not just physical features... behaviors become adaptive by default as well.
It’s fine if these ideas are understood to be hypotheses, accounting for the complexities of the genes and physiological processes that lie behind the traits, and accounting for the limitations to testing them. But all too often students blindly assume that natural selection explains EVERYTHING.
[And this leads down the slippery slope to Social Darwinism so it's not something to take lightly.]
Now, even though the adaptationist perspective is rampant, that’s not at all the pattern that emerges when students interpret and explain human skin color.
They do the exact opposite! They take natural selection and adaptation out of half of the story!
Here’s (my paraphrasing of) how many of my freshmen students answer when they are asked to explain the Darwinian folate/Vitamin D hypothesis offered by Jablonski in her book:
Natural Selection explains melanized skin in the tropics because it acts as a natural sunscreen to protect against harmful UV. However, for the non-melanized people in regions with little UV exposure… well, they don't have much melanin because “they don’t need it.”
Depending on how you interpret that (aside from the possibilities that I'm not doing my job well enough, that they're not listening in class, or that they're not doing the reading), the students are invoking genetic drift, neutral theory, or Lamarckian principles! And Darwin is totally out.
I doubt many are aware of the theoretical significance of their answers. But by erroneously explaining a Darwinian concept, they're offering us a window into their intuition.
- If losing it is beneficial, then those who have lost it will out-survive and out-reproduce others and future generations will have more have-nots than haves. If it’s crucial to survival and reproduction, then the loss will become fixed in the population as an adaptation. - Darwinian adaptation through natural (or sexual) selection
- If you don’t need it then you can lose it without issue. Reproductive success does not depend on whether or not you have the trait or not, so chance alone will determine how prevalent haves or have-nots will be in any given generation. Relaxed selection plus chance can ultimately lead to the elimination of alleles all together! Furthermore, there is a constant and low mutation rate and while selection is weeding out deleterious mutations in some genes, or while it is favoring adaptive mutations in some genes, the mutations in genes for traits that do not affect evolutionary fitness can accumulate. As long as these mutations are not harmful and purged by selection, they can disrupt the gene and either damage the trait or cause complete loss. - Genetic drift and neutral theory (both with relaxed selection)
- If you don’t use it, you’ll lose it, meaning that the trait can fade within a lifetime if it's neglected. That neglected trait is passed on to future generations which will continue to experience its decline if they also stop using it, and if that’s widespread throughout the population, then that trait disappears. (It’s assumed that if a trait is not used that it’s not "needed," which is why the casual wording of this scenario can be similar to #2.) - Lamarckian evolution
The first two scenarios, #1 and #2, are widely accepted as biological phenomena, so they are valid hypotheses for the loss of pigmentation in people who live far from the tropics. The third scenario is seen by the scientific community as a misconceived foil to “real” evolution, having fascinating historical interest and useful pedagogical appeal, but that is all.
Okay then, how can we interpret what I've called this "intuitive" response by my students?
First of all, tanning is certainly enabling this muddling of Darwin. That skin color changes in response to stimulation by UV (and hormones!) and is not static during life makes it complex and matches it to UV in a non-evolutionary way, a way that they're used to assuming. A way that people around the world assume to be! Some of my Kenyan friends think that my skin would look like theirs if only I stopped wearing sunscreen lotion while I visit Kenya.
And second, if students think of melanin as natural sunscreen, then it's probably easy for them to take that metaphor too far and liken it, conceptually, to sunscreen lotion.
You apply sunscreen lotion when you need it and you don't apply it when you don't need it. You need it on the Equator, yet you don't need it as much far from the Equator. This feeds back into their evolutionary story: Melanized skin evolves to be where it's needed and it evolves away where it's not needed. This is, I think, the intuitive rationale behind my students' answers. Relaxed selection, neutral theory, and genetic drift provide the backing scientific power. Plus, pigmentation loss in other animals is overwhelmingly explained this way.
But, additionally, "need" can be one way to casually express the concept of Lamarckian "striving." Are my students really Lamarckists when they say that white people are that way because their ancestors didn't need much melanin? It's hard to say.
But I also can't help but wonder, What if their confusion is not just due to their theoretical naivete? What if a totally Darwinian explanation for human skin color variation is hard to understand because it just isn't the best explanation?
Maybe my next class should be dedicated to testing and fleshing out how we could test the adaptive hypothesis for human pigmentation loss versus the alternatives.
But even if we did know the real scenario, there would still be lingering questions...
Maybe it's a horrible under appreciation of deep time and convergent evolution...Maybe it's a gross underestimation of mutation and genetic drift...
But why do so many animals in UV-limited habitats (caves and sea floor) lose pigment? Is it just one of the few visible traits that a constant accumulation of mutations can safely derail under the watchful eye of selection?
And what about epigenetics and pigmentation? Hmm?
Oh, I don't know, maybe Stephen Colbert had it right: Pale skin is best for hiding in a snowbank.
* The Darwinian explanation for skin color variation that I describe here (called "Darwinian" because it's about natural selection acting on melanin differently in different environments), is NOT the same as Darwin's which he discusses in Descent of Man.