Genes for skin colour identified

This post was chosen as an Editor's Selection for ResearchBlogging.orgThere is racial variation within Homo sapiens. That’s a simple fact but its been the source of unending controversy, prejudice and human misery. The issue becomes all the more prickly when evolution is brought into the picture, with many attempting to argue several horrific racist acts had their roots in evolutionary ideas regarding the inferiority of certain groups.

Of course, attempting to justify racism with evolution is a fundamentally flawed idea. This is partly due to the fact that evolution notes no groups are objectively inferior or superior, just differently adapted. Cheetahs run fast yet we are smarter, who is truly superior?

Mostly, however, racist evolutionary arguments are flawed due to the fact that the biggest source of racial variation within humans appears to be skin colour! Not strength, intelligence or any other trait one might used to justify claims of inferiority but skin tone.

“My off-white colouring is superior!”

Skin tone varies because different groups of people have a different amount of a dark pigment called melanin. Once produced, melanin moves to the top level of skin cells where it protects their DNA from the UV radiation in sunlight. However, since sunlight is also a part of vitamin D production, too much melanin can result in a vitamin D deficiency.

Thus a balance must be struck: too much melanin and you can’t make enough vitamin D, too little and you’re skin is damaged by UV radiation. As such having an appropriate skin tone is an important step in recent human evolution.

Since different parts of the world receive different amounts of sunlight, this balance changes across the globe. From this emerges a selection pressure: the less sunlight one is exposed too, the more advantageous it will be to produce less melanin. So mutations which decrease the amount of melanin production will be favoured.

Perfectly evolved for crappy weather.

But what are these mutations? One would expect them to be positively selected (since they’re conferring an advantage). They should also be correlated with the skin tone we’re examining since they’re responsible for the skin tone we’re examining.

Previous research had identified mutations in SLC24A5 and SLC45A2 (genes involved in the transport of melanin from where it is produced to the epidermis) which drastically reduces the amount of melanin that reaches the top level of skin cells, resulting in paler colouring. Further, they have been positively selected for in Europeans and is correlated with their lighter skin tone, providing further evidence they are the genes we’re looking for.

Jedi mind tricks don’t work on science!

Of course, European white isn’t the only skin colour variant in existence. So recent research has attempted to find the mutations which led to another tone – Indigenous American.

There are about 70 or so potential genes responsible for this colouring so the scientists first sought to eliminate those which couldn’t be responsible. So they ran a variety of statistical tests to find out which genes weren’t being positively selected for since, as you should remember, the responsible genes should be.

At this point I should confess I am not really trained in genetics, statistics or genetic statistics. So I can only take their word for it that the statistics they used are valid and the conclusions they draw supported. If someone has information to the contrary I’ll listen.

Of the 70 genes tested only 14 showed evidence they were being positively selected. Of these, only 4 contained mutations which correlated with skin tone.

These genes were EGFR – which is responsible for the number of keratinocytes, which influences the number of melanocytes (cells which produce melanin – and OPRM1 – which also influences keratinocytes – as well as variants SLC24A5 and SLC45A2, mutations of which are responsible for the European skin tone.

This graph shows something important. I think.

It’s very rare a paper doesn’t end without a call for further research since that means the researchers will receive further funding. The result of this is that scientists will typically try and find as many flaws in their conclusion as they can to justify a follow-up study. Whilst this is normally good, the research is sometimes of such a high quality that these objections are bordering on absurd, as is the case here.

But “bordering” is the operative word in that phrase. They aren’t quite yet absurd, so they do raise some valid issues. For example, the other 70 genes might be positively selected for but their statistics couldn’t detect that. Or the genes they did find correlated with skin tone actually coded for another trait which was correlated with skin tone.

However, those are pretty minor objections and so one is safe in concluding that we have found 4 genes, mutations of which are responsible for European and Indigenous American skin tones, important developments in recent human evolution.

Provided I can trust their statistics 😉

Quillen EE, Bauchet M, Bigham AW, Delgado-Burbano ME, Faust FX, Klimentidis YC, Mao X, Stoneking M, & Shriver MD (2011). OPRM1 and EGFR contribute to skin pigmentation differences between Indigenous Americans and Europeans. Human genetics PMID: 22198722

17 thoughts on “Genes for skin colour identified

  1. Great Post. Science is amoral, moral superiority isn’t a scientifically valid concept.
    Also I like the Humor on the Graphs.
    I’ve noticed that people from Southwestern Europe (like me) tan more easily, than those from Northern Europe, any idea why this may be?

    • From a cursory googling it would seem to be that a tan is the result of exisiting melanin being altered and more melanin being produced, so if you had extra melanin by default or produced more in reaction to sunlight it would result in an easier tan.

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  3. My understnading is that darker skin color protects folic acid in a warm sunny climate. Folic acid is destroyed by sunlight. Low folic acid levels cause neurological defects in foetuses. The most famous is spina bifida. Such babies in the past did not survive and reproduce. Low Vit D levels also affect survivability to adulthood and reproduction. Skin cancer is a later in life disease and does not affect reproduction.

      • So the Mediterranean climate would select for the ability to tan easily, to prevent Low Vitamin D and rickets.
        Keep up the good work Mr. Benton.

        • Tanning seems to be a good way of balancing the two demands, with the skin lightning/darkening in response to the amount of sunlight. As such, it seems plausible that increased tanning ability would be selected for in an area with highly variable sunlight – such as the Mediterranean – although I do not know if this conclusion is true or not.

          Plausibility =/= validity, after all.

      • I commented that low Vit D reduces survivability. Light skin is to remove blocking of Vit D production in a lower UV light environment.

        Balancing Vit D production against folic acid destruction is a role of meleanin.

        The Mediterranean latitude would select for easy tanning to better balance Vit D production against folic acid destruction.

        The further north one goes the less uv that reaches the ground. Artic peoples get their Vit D from marine mammal blubber.

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  5. It does seem highly plausible.
    The production of Melanin would only occur when it is needed, saving the costs of producing it year round.
    I just read that some Northern Europeans are unable to tan, so the production of melanin appears to be costly enough to have been selected against when it’s not needed.
    What we need to show is that there is greater seasonal variability of UV rays in say Spain than in Ireland.
    This topic appears worthy of appearing in a journal, would you care to co-author one?

    • I would have thought this was well known already…

      Don’t forget that skin colour also reflects our ability to retain and lose heat. nearer the equator people have evolved to have darker skin partially so they lose heat quicker. Nearer the poles heat obviously needs to be retained.

      Tanning might just be an adaptation to help with heat retention as well. Whereas those who can’t tan, who live near the poles, probably can’t because most people would never have travelled by foot from norway to ecuador. So tanning would never have been advantageous..

  6. You assert “there is racial variation within Homo sapiens” and call this a “simple fact.” It is anything but. There is indeed variation within Homo sapiens, all kinds of it. Calling this variation “racial” doesn’t tell us much, and none of the evolutionary anthropologists or biologists I know use the adjective “race.” It is uninformative. Humans vary in all kinds of interesting ways, often in geographic clines, and we can categorize this variation using scientific concepts rather than social-political ones based on skin pigmentation.

    • Ultimately though there are these social-political concepts that define people according to skin colour and so, in a discussion of skin colour, I believe it is not unreasonable to mention them; if only to say how futile discriminating based upon them is.

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