Something else

Razib has talked about this – here’s what I think.

The various sweeping alleles that have made Europeans and North Asians have light skin  were not favored because they helped you garner extra vitamin D, at least not mostly. This is apparent from the allelic structure.  There is a single sweeping variant for SLC24A5: if paleness was the point, many partial loss-of-function alleles would be favored, rather like what we see with G6PD deficiency (a malaria defense).  But there is only one: so loss of function is not the point (or at least not the sole point): that particular variant has some other advantage, a big one.  The fact that it’s favored up on the Ethiopian plateau is another sign that it isn’t driven by vitamin D. There is only one blue-eye mutation (OCA2), only one main blond mutation (KITLG). The SLC24A5 mutation has by far the largest effect on skin color – if it isn’t being selected for paleness,  the  sweeping alleles of other coloration genes aren’t either.  Probably not even MC1R – it has many common loss-of-function mutations, which implies that loss of function is favored – but probably not because of the paleness. Something else.

If you’re close enough to the equator, you need dark skin, and the genes in these pathways are constrained – not free to change. But if they are free to change, then a change that improves fitness through a pleiotropic effect is now free to spread, since the resulting paleness is bearable. In many cases that would require a specific change, not just loss of function  – so we see an allelic structure unlike that of G6PD.

The same is likely the case with the various sweeping skin-color alleles in East Asia (although they are less colorful).

So we need to figure out what useful thing  these mutations actually do.  Dropping the vitamin D hypothesis raises lots of questions.

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101 Responses to Something else

  1. Diane Ritter says:

    So if you need dark skin close to the equator, why would pale skin be favored on the Ethiopian plateau?

  2. pyrrhus says:

    The only theory that seems likely to me is that these mutations enhance sexual appeal, perhaps exploiting a “bug” in the human brain…..

    • gcochran9 says:

      Probably not. For example, blue eyes: pretty much recessive as far as eye color goes. It’s almost impossible to get a sweep with a recessive trait, and it takes forever. But if that mutation has some other effect that is A. not blue eyes B. not recessive and C. advantageous, it can sweep.

      • AnonymousCoward says:

        @Cochran: what do you think of Peter Frost’s often-repeated hypothesis? IE, that hyperborean-context female competition for comparatively scarce males has made “beauty” (to put it bluntly) more advantageous to ice people than to sun people?

        In the case of eye color, I think it’s fair to say that blue is not so much recessive, as weakly co-dominant. A woman with green/golden eyes (blue/brown heterozygous) is different from a woman with dark brown eyes (brown/brown homozygous).

    • Boris Bartlog says:

      That’s Peter Frost’s theory. In my opinion this was an interesting idea before we got all our current information about the time depth and full extent of the relevant mutations, but it doesn’t seem like a very defensible hypothesis any more.

    • shartiste says:

      Sexual dimorphism in skin color supports this hypothesis.

    • I don’t know that theories as to why we lightened up are necessary at this point because 1) they are speculations that cannot be proven one way or another and 2) new information is coming out all the time dramatically changing what we know. One very recent report has prehistoric farmers from northern Greece with considerably darker skin somewhere in the time fame of 5500 to 7500 years before present. This is sketchy info at this point, no conclusions can yet be made, it comes from three individual whom are simply described as being darker. But it points to what is coming, real solid information answering questions we can now only speculate about.

      I found the link provided by Sprfls on the domestication syndrome to be found farther down in this thread to be real interesting. Not because I think domestication of ourselves gave us lighter skin but because it illustrates just how weirdly and unpredictably evolution works. For example why in the world would foxes and dogs both get floppy ears and spotted coats from being breed for domesticity. Pushing this back to the subject of this thread it sure looks like SLC24a5 is doing something else besides lightening skin or Ethiopians would not have it to the degree they do.

  3. I don’t this exactly requires dropping Vit D from the hypotheses. As you say, skin color is constrained at the equator, so Vit D is there, too. But as you go further from the equator, you can get more D without burning, so why not? Maybe Vit D is just a nutrient that is useful in larger quantities than can be safely obtained at the equator.

    • Abraham Lincoln says:

      From what I’ve read, the skin stops synthesizing Vitamin D after a relatively short period–minutes, not hours. Once you have enough, you just don’t need any more. That, at least, is what the article said.

  4. The only hair color that has found a genetic-source is Red hair. 5-6 variants in the MC1R gene explain 80%+ of Red hair. There are other unknown but less popular causes. Brown vs Blue eye color can predicted 80%+ of the time.

    All hair colors besides Red and all skin colors CAN NOT be predicted accurately with known DNA markers yet. There are Light skinned people who lack “Light skin mutations” and Dark skinned people who have “Light skin mutations”.

    So, I don’t see how you can test when, why, how skin color changed with known DNA markers.

  5. Jacob says:

    I was thinking about this the other day and thought of an idea that seemed amusing to me, if perhaps wrong. Sun exposure is tied to reproductive timing in pretty much every temperate species, and in humans’ case, some of the same biochemical pathways seem to be used for libido and for tanning. (There are some amusing stories about chemicals intended to be used as tanning agents when they were given clinical trials.) What if the initial adaptation was, for example, for reduced androgen levels or androgen sensitivity, and then pale skin was a corrective to get the now unfrisky population back in the mood?

  6. Neocolonial says:

    Resistance to cold?

  7. duderino says:

    I wonder if white skin in women provides a more honest signal of aging than darker skin and thus more capable of communicating fertility? Naturally pale women who use the tanning booth look gross by 40.

    • Jacob says:

      More capable of communicating youth, at least. Selecting for youth (as opposed to fertility and health, which are always valuable) only becomes particularly valuable when pair bonds are expected to last a long time.

      In chimpanzees, an entirely promiscuous system, I remember Jane Goodall observing that the males actually preferred older females, presumably because (as she also observed) older mothers kept their infants alive more often. So the selection for youth seems to be at least newer than our separation from chimps (when, 7 million years ago), and could presumably intensify under conditions where pair bonds are more consistently lengthy.

    • shartiste says:

      Yes, some method of sexual selection seems exceedingly likely, especially males selecting females. Although I know the proprietors of this blog disagree. Competing to signal residual fertility (youth) in areas of the world where longer-term partnerships were important for survival of offspring would seem to favor mutations that produce light skin.

  8. Bruce says:

    Isn’t it the case that blue eyes are more common among northern dog breeds?

  9. chrisdavies09 says:

    Adaptation to diet low in phenylalanine?

  10. sprfls says:

    I find it intriguing that all domesticated mammals exhibit depigmentation and other strange color changes. Perhaps, in the absence of needing dark skin near the equator, early agricultural and other advanced types of cultures selected for tameness — for example, by shunning aggressive/uncooperative individuals.

    I don’t really think this is the case and would put way more stock in a purely biological (rather than psychological/social) explanation, but just figured I’d throw this out there.

    “The “Domestication Syndrome” in Mammals: A Unified Explanation Based on Neural Crest Cell Behavior and Genetics” http://www.genetics.org/content/197/3/795.full

  11. RCB says:

    I’m wondering if the argument here really follows. If I understand correctly, there are a lot of loss-of-function G6PD alleles, so we might therefore expect lots of broken SLC24A5 alleles. But what if it’s G6PD that’s the unusual one? Isn’t malaria an unusually strong selective pressure? Perhaps stronger than selection on skin color? That would imply that more of the adaptive alleles would have gotten off the ground in G6PD than at other loci.

    Then again, it seems like these skin color alleles have swept pretty quickly, which means selection isn’t all that weak. Also – are the lightening effects of these alleles recessive? That would strongly argue against selection for light skin.

    Seems like this should be quantifiable: Given what estimates we have for the strength of selection of the alleles, the approx time at which the selective pressure arose, and a ballpark loss-of-function mutation rate, how many broken alleles should we expect to see? How likely is it to see only one?

  12. ctmorey says:

    Perhaps maintaining melanocytes is energy expensive, advantageous to get by with as few as possible.

    • Jacob says:

      This seems plausible, but you should be able to quantify the metabolic effects in this case. And this doesn’t quite explain the Ethiopia issue.

  13. artifact says:

    Social? Lighter skin shows blushing, lighter eyes show pupil dilation. Both betray the emotional state of the person who has them. This would seem to be a disadvantage, unless it translated into more group cohesion (ingroup trust) and cooperation as a result, keeping tribes close-knit and xenophobic. Then the alleles could spread rapidly.

    • gcochran9 says:

      You’re invoking group advantage. Unlikely.

      • artifact says:

        If you can easily tell when your fellow tribesmen are embarrassed or pissed off, and they can tell the same about you… that could influence group social dynamics and selection as a result. Throw in variations in the OXTR gene, which is a two-sided coin involved with both empathy and ethnocentrism… then, greater logistical pressures related to the environment (being forced to spend winters in close quarters, being forced into division of labour and planning to get through the winter, all promoting social dynamics), and we have a cluster of factors driving selection.

        • artifact says:

          …it wouldn’t necessarily have to mean group advantage to start with, either. It would be enough to be an advantage to those at the top of the hierarchy, who most benefitted from sitting atop the social hierarchy, and probably bred far more fruitfully.

        • Space Ghost says:

          Yeah, but why does the first guy to develop that mutation have a reproductive advantage? Everyone can see when he is embarrassed / pissed off / whatever, but he can’t tell if they are. So he should be at a disadvantage in the scenario you’ve proposed.

          • artifact says:

            Well in that case, it shouldn’t have survived at all. King of the outcasts, perhaps? Maybe the factors i’m imagining, if they existed at all, only came into play later as a positive feedback mechanism.

          • artifact says:

            The other thing is, with lighter skin, it’s a lot easier to see blemishes (such as those arising from hormonal imbalance), and wounds/scars. In other words, light skin is a clear litmus for fitness. Again, costlier, because it makes it harder to conceal negatives. But when healthy, a possible driver of sexual selection, I imagine especially in females. Same with losing body hair, I can’t think of why high latitude peoples would have less body hair?

          • Bob says:

            You’re talking about the evolution of communication. It’s easy to see how that might have evolved and survived without the context of large social groups. Babies communicate their emotional states to their mothers, fathers communicate life lessons to their sons, etc.

            In social groups, there may be selective pressure away from communication and towards deception and manipulation.

        • Jacob says:

          Sometimes credible communication is favored, sometimes the ability to lie is favored.

          Eg, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1693069/pdf/12495517.pdf

          • artifact says:

            Very interesting, and reminiscent of the Prisoner’s Dilemma. As I recall from Hofstadter, the winning strategy is to cooperate on the first move, and then play tit-for-tat. Phase-locked cooperation leads to a flourishing society. Which may conceivably engender epigenetic, and then evolutionary shifts.

      • Stephen W says:

        If someone has a trait that makes it hard for them to lie, then I am more likely to ally or trade with them. I would always go to the pale blue eyed shopkeeper if i knew he was more honest than the competition. It is a greater advantage to have more people trust you without being able to betray them. Than it is to have a smaller number people trust you but being able to betray tham.

    • Bruce says:

      Somewhere at the speculative level I’ve seen the claim that light skin is selected for because it makes it more difficult to hide the signs of disease.

      • M says:

        I like the fact that with this idea you could experimentally test it in practical modern terms simply by testing batches of faces with diverse health status against viewers rated health scores (or even using morphs to morph the same faces, although that has risks).

        I don’t actually think it will hold up, because when we have the closest experimental test to it, seeing if women are more sensitive to carotenoid related skin tone shifts in males relating to attractiveness, there is not an effect where the light skinned faces gained more from carotenoid and lost more from it’s lack. Rather participents were simply more sensitive to own ethnic group – http://www.livescience.com/36145-skin-tone-health-attractiveness.html. Not a slam dunk, but I think casts some doubt on the ability that human perception has enough trouble with health perception at the margins that reducing skin pigment helps us discriminate between healthy and unhealthy.

    • Peter Lehmann says:

      Visibility of emotional state -> disadvantage

      Where has that derivation ever been proven empirically? In animals? In hunter-gatherers? (Pointing out that modern societies might differ in this respect doesn’t seem to have explanatory value and is not really obvious, either.)
      In fact, if “I want to mate”, “I want to be invisible to you” etc. are emotional states, then the opposite derivation is true.
      I am thinking of the five volumes Hans-Peter Duerr wrote to contradict Norbert Elias’ “Civilizing Process”, e.g., the expectation of invisibility to other tribe members during sexual intercourse. The capability to blush would seem to be very relevant there. I am not aware of any correlations with skin or eye color.

  14. magusjanus says:

    Curious your thoughts on: http://www.nature.com/ncomms/2015/150818/ncomms8502/full/ncomms8502.html

    regarding polycystic ovary disease. They claim to find some genetic causes; I recall from your paper with Ewald that you made a case it would likely be viral in origin.

  15. Jack says:

    Does it have to do with resistance to frostbite? Has anyone ever followed up on these experiments? Or this data from the Korean War? Black soldiers got frostbite at a higher rate than whites did. The suggestion is that melanocytes freeze up more easily (somehow).
    https://goo.gl/Pcf5s8

  16. teageegeepea says:

    Because this post may be read when Razib’s post is no longer on his front page, I will deposit the link I assume you’re referring to here:
    http://www.unz.com/gnxp/selection-is-often-a-mystery/

  17. Fourth dooman of the apocalypse says:

    So does that mean that all those Indians who use skin lightening creams are on to something?

    And is white skin going away as soon as the President seems to prefer?

  18. TWS says:

    Is there other blond variants than the European and Melanesian?

  19. RationalXX says:

    With regard to light eye colours, I wonder if better low light / night vision could be a selection factor. Among non-domesticated mammals, nocturnal predators (cat species, wolves, foxes) seem to have a tendency towards non-black eyes. I could imagine that this has some advantages in high latitudes for a hunter gatherer, although the recent selection among farmers would make less sense.

    • fvdv says:

      This is worth a look. I’ve heard old baseball players remark that blue eyed players hit better in night games than other players.

    • anonymous says:

      The natural range of blue eyes almost perfectly overlaps the former natural range of Neanderthal. Neanderthal had huge eye orbits and may possibly have been a nocturnal predator. If so, and if Neanderthals were predatory/cannibilistic enemies (as Vendramini has suggested), then it would make sense that selection would have lightened up the eye color of modern humans in the areas where they had to fight off night-attacks by Neanderthals in order to survive.

      FWIW, I don’t buy Vendramini’s theory regarding our physique, nakedness, or gait, since that’s better explained by selection for persistence hunting in a hot savannah (cf Born to Run). However in terms of our psychology, his idea of a millennia-long human versus Neanderthal war makes sense. It also explains uncanny valley reactions, and why we mostly make horror movies about near-humans (werewolves, vampires, zombies, humanoid aliens) rather than, say, hyenas or leopards. That which is ALMOST us is the scariest —

      “…take my advice, when you meet anything that is going to be Human and isn’t yet, or used to be Human once and isn’t now, or ought to be Human and isn’t, you keep your eyes on it and feel for your hatchet.
      — “The Lion, the Witch, and the Wardrobe”, C.S. Lewis

  20. jb says:

    This applies to selection on all traits, not just pigmentation, but it occurs to me that since before the modern period the rate of child mortality was so very high, ones first thought with all evidence of selection ought to be that maybe it had something to do with helping children reach reproductive age.

    A trait that helps adults survive and prosper is nice, but even genetically disadvantaged adults have at least a shot at reproducing, simply by virtue of having made it that far. But a child who doesn’t make it to adulthood has zero chance, so isn’t that where we should always expect to see the most intense selective pressure? And conversely, when we find evidence of unusually intense selective pressure, shouldn’t the possibility that it somehow has something to do with child mortality always be at the top of our list?

  21. dearieme says:

    If it is a response to living in a new climate zone, I wonder whether what matters is an absolute (e.g. winter is cold) or a relative (e.g. the seasonal variation is much bigger).

  22. ctmorey says:

    Hmm, could could SLC24A5 be related to dairy consumption in some way?

  23. Anon says:

    How about resistance to diseases that spread through boils, blisters, rashes and the like?

  24. IC says:

    “THE colonization of caves, and the evolution of the peculiar characteristics of most
    members of the cave fauna, have interested many workers. The cave environment
    is of a specialized nature, its most obvious characteristics being absence of light,
    lack of food, uniformly low temperature’and, in subterranean.waters, low oxygen
    content. Cave animals are likewise specialized and commonly exhibit loss of
    pigmentation,”

    http://jeb.biologists.org/content/18/2/136.full.pdf

    Based on this study, pale skin from north might be for the similar reason. A mutation is well tolerated in low light enviroment.

    • IC says:

      In complicated civilized societies, people who own big land or property can afford to stay in door by collecting rent, or tax on poor subjects. If social status is strongly heritable, these people became new `cave animals’ who can afford pale skin mutation. Such pale mutation might turn around as maker for high status and sexual selection marker. . Pale mutation might be coevolved with mental abilty to live such new cave life in civilizations (similar to SNPs information).

      Just speculation, might be wrong.

      • IC says:

        In classical China, intellectuals were always refered to as pale-faced scholars (bai mian shu sheng).
        Well to doers or lords did not expose to sun light very much. They were always well-clothed and traveling under veiled or umbrella covered wagons. So they were basically caved human beings.
        Classical Chinese literature always described brainy people with pale complexion.

      • IC says:

        If downward social mobility from these pale upper class mutants became stable process under stable civilization (as implied by historical studies of England and China), these pale feature could quickly spread to the entire population. We are all descendants of lords.

        • Paul Conroy says:

          I agree!

          Also, much Eurasian Y-DNA seems to have a Northern origin, so hunting in snowy conditions would favor light pigmentation, as camouflage

      • IC says:

        Less light exposure might also explains relative more pale female than male in human generally since females tend to spend more time in door than male across all cultures. This explanation is more plausible than sexual selection proposed by peter frost since this is backed by evidence of cavernous animals.

        Science is based on evidence (data). Without data, one can only be a person with opinion. Like math or physics rules (following example of gravity equation), truth tends to explain much broader world than narrow application.

        This equation pretty much explains every going on about our solar system.

        • ursiform says:

          No, it really doesn’t. It doesn’t explain why the sun shines, the geology of any planet, or the precession of the perihelion of Mercury. Etc.

          • IC says:

            Well, I have to agree with you. Certainly that equation does not explain thermdynamics, evolution, photosynthesis ect which are also phenomena in solar system.

    • IC says:

      White race in China (joke)

      Even in fujian province (southern China), the light is weak enough to develope natural white primates.

      • IC says:

        Every body just loves babies.

      • IC says:

        These monkeys are all white when they are young. They develop pigmentation in their skin when become adults
        In fact, all fetus or embryos in all animals are white (basically pink without any pigment)

        When adult lacks pigmentation, it is really retained fetal or infant feature very much like lactose tolerant for some people. Perhaps civilizations creats more sheltered life which allows more infant features retained (devolution). Many things in human are products of devolution including depigmentation, loss of body hair, larger head size ect.

      • IC says:

        Btw, devolution is also faster than evolution like downhill process.

  25. Eternal Apparatchik says:

    I see this idea was already brought up, but my guess too is that the main effect is behavioural, based mostly on the gradient of its spread in south and east Asia: whatever the gene is doing, it doesn’t seem to have been advantageous to rice farmers. (I doubt it is a direct dietary adaptation, because we have adapted the grains to our guts a whole lot more than our guts have adapted to the grains.)

  26. dearieme says:

    Could it relate to the near-constant length of daylight at low latitudes, and the great disparities at high?

  27. ctmorey says:

    Clothing!
    and > time spent in shelter (because of cold)

  28. ctmorey says:

    Clothing = wearable melanin

    • dave chamberlin says:

      Clothing and hats are important variables in the evolution of lighter skin tones. It made lighter skin more advantageous in the far north where very little skin area was exposed to vitamin D giving UV light and darker skin less advantageous in equatorial regions where clothing could give protection from the sun.

      But what we really need, and we aren’t even close yet, is enough recovered genomes to make a map covering time and space showing when and where the peoples of Eurasia lightened up. But the way things are moving in paleogenetics I hope to looking at such map at one of my favorite blogs someday.

  29. JayMan says:

    It seems SLC24A5 is the Western counterpart to EDAR, at least in the nature of the mystery presented by both.

  30. IC says:

    Evolution or natural selection does not always gaining some thing new. Appendix for human, limbs for whales or snakes, fingers for horse or antelops are example of losing something which is not longer very useful. Getting rid of something no longer useful is also product of natural selection to be efficient or competitive in Darwin’s world.

    Maybe reducing melanin production for some human being is similar to shrinking appendix for human.

  31. Bryan Bell says:

    Ethiopia was one of the first places in Africa to develop agricultural. With the plough being adopted from the Sabeans ~800BC.

    I’d be interested if SLC24A5 was selected in any non-agricultural populations e.g. the Sami.

    • Sean says:

      http://evoandproud.blogspot.co.uk/2014/02/slc24a5-reply-to-greg-cochran.html

      The plateau reference by Gregory Cochran above makes me wonder if the land is poor. Consider, where land was poor traditional marriage practices like bride price (http://www.forwarduk.org.uk/283-2/) could mean many women had to forgo marriage . So those men could afford the bride price got a choice of many women. Not a harem, just a choice they had to stick with. We are all WEIRD here and so can’t understand a slight skin tone difference being desirable. But such a preference is well supported in anthropological studies. In these societies marriage practices are very tightly tied to resources and reproductive success so the algorithms have a logic beyond us. The average girl we know now darkens her skin and gets tattoos to show she is no prude, and chooses the man who she finds most physically attractive

      Traditional family honour demands a high price for the pure young maiden. I read once that the blood-stained sheet of the marital bed is nailed to the door in Ethiopia.

      http://evoandproud.blogspot.co.uk/2011/06/on-impossibility-of-blue-eyes.html

  32. iffen says:

    If why it happen had a simple explanation, everybody could be privileged.

  33. iffen says:

    many partial loss-of-function alleles would be favored, rather like what we see with G6PD deficiency (a malaria defense)

    I get the impression that the selection for this SLC24A5 was recent and occurred relatively quickly. Is this the same for the various malarial defenses ? Or is that even something that can be determined?

    • Bryan Bell says:

      Yes, it can be determined. It’s more complicated than being the same or not the same since malaria itself also evolves. John Hawks’s lecture on malaria is good viewing on this, https://www.youtube.com/watch?v=BnQF2DRjIkg.

    • Tom Bri says:

      I wonder if it isn’t a malarial defense. Mosquitoes prefer attacking darker colors, thus the advice to wear white clothing when mosquitoes are a problem. Imagine young kids running around naked in the summer, and the darker ones getting bit somewhat more than the lighter ones. A slight effect, over many generations might do it, with more sunny realms balancing this against the need for more protection from the sun. Anyone know if darker-skinned people get bit more?

  34. Sean says:

    Re recessiveness: http://evoandproud.blogspot.co.uk/2011/06/on-impossibility-of-blue-eyes.html
    “Although eye color is polygenic, alleles at two STPs (rs12913832 and rs1129038) seem to account for most cases of blue eyes (Eiberg et al., 2008). In a Polish sample, 89% of the blue-eyed individuals had both copies of the ‘C’ allele at rs12913832 and no copies of the alternate ‘T’ allele (Branicki et al., 2009).[…] Among CT heterozygotes, 16% had blue or grey eyes, 10% green eyes, 47% hazel eyes, and only 27% brown eyes.[…] Needless to say, this outcome is possible only when the operational sex ratio is very lopsided[…]. ”

    ETHIOPIA: Bride-Price Key in Increasing Rate of Rape. In traditional Ethiopian society the newlyweds’ bed sheets were nailed to the front door so everyone could see that a pure maiden had been supplied for the bride price. Greg Cochran is is rather gnomic, but it always pays to read his replies to comments carefully, . The reference by Greg to the Ethiopian plateau made me recall the resource scarcity explanation for Tibetan plateau polyandry. What is the quality of the land in Ethiopia?

    https://en.wikipedia.org/wiki/Agriculture_in_Ethiopia#Land_use
    “Most agricultural producers are subsistence farmers with small holdings, often broken into several plots. Most of these farmers lived in the Ethiopian Highlands, mainly at elevations of 1,500 to 3,000 meters. There are two predominant soil types in the highlands. The first, found in areas with relatively good drainage, consists of red-to-reddish-brown clayey loams that hold moisture and are well endowed with needed minerals, with the exception of phosphorus. These types of soils are found in much of the Southern Nations, Nationalities, and People’s Region (SNNPR). The second type consists of brownish-to-gray and black soils with a high clay content. These soils are found in both the northern and the southern highlands in areas with poor drainage. They are sticky when wet, hard when dry, and difficult to work”

    The family honour demands a bride price be paid , but perhaps the soils and climate made raising the bride price demanded so difficult that a proportion of girls lost the chance to secure a husband . If so, there would be an effectively skewed sex ratio, and the man with the wherewithal to afford to marry would (in a nutshell) get to choose if a girl reproduced or not. In a wide range of traditional societies the relatively light skinned young maiden is favoured. We of the WEIRD WEIRD West can’t understand this preference, which is tightly linked to successful reproduction.

    http://evoandproud.blogspot.co.uk/2014/02/slc24a5-reply-to-greg-cochran.html
    “Ethiopian manuscript paintings (source: A. Davey). Ethiopians have a self-image that is lighter-skinned than their actual selves. If the prevalence of SLC24A5 is higher in Ethiopia than the degree of admixture from lighter-skinned peoples across the Red Sea, this discrepancy may be explained by social selection for lighter skin.”

    No, sexual not social. Peter Frost is wrong about SLC24A5.

  35. pam32 says:

    Most mammals have light skin that is covered by fur
    https://en.wikipedia.org/wiki/Hair#Human_hairlessness

    Burt

    • Sean says:

      The modern human hair colour gene MC1R, has a number of Neanderthal alleles. It would make sense for red hair to go with very light skin if red hair alleles came from Neanderthals (whose skin was hidden under a coat of fur because Neanderthals had no tailored clothing. The neanderthals coat color was likely a type of camouflage pattern, we know they couldn’t run prey down so they had to ambush.

      Gregory Cochran has been proved right about Neanderthal introgression. But in Europeans the Neanderthal alleles are nonsynonymous, each confers a distinct phenotypic variant.

  36. Nobody says:

    Off-topic: I’ve donated to your BTC account.

  37. Julian says:

    OT. Greg or Henry may be interested in this:
    “Genome structure variants identified using archaic human genomes

    Zip archive of structural variant calls identified using the sequenced genomes of archaic humans is available here.

    Manuscripts using genome structure calls prior to publication must cite Rogers 2015 “Chromosomal rearrangements as barriers to genetic homogenization between modern and archaic humans” available from ArXiv. The data set may not be used prior to publication without preprint citation under any circumstances.

    This dataset is released to facilitate any related work in human genetics where genome structure information may be useful. This dataset will be updated if changes are made to the manuscript and dataset. Please note the date of access and release number in publications.”

    http://evolscientist.com/NeanderthalStruct.html

  38. j says:

    The Vit. D argument was wrong from the start. Humans universally prefer white skin and colored eyes and hair. In mulatto families, the whiter child is better treated. A couple seeking to adopt a white baby is charged $35,000 and $4,000 for a dark one. This is not only in America. Slave markets since Rome to Saudia valued white girls twenty times more. All the angels, from pagan times, are blond, the devils, black. However I reject this mindless discrimination, it seems sufficient to explain the white sweep as soon temperate and cold lands were settled.

  39. melendwyr says:

    Perhaps there is a resource competition involved. Is there a precursor for melanin that is also required for a vitally-important substance?

    • setstamov says:

      Serotonin (via conversion to melatonin, that stimulates melanin release) . Excess melatonin = depression;lack of melatonin = broken circadian cycle. OT, congratulations to all US people for the bravery of their brethren in that Amsterdam train, regardless if it was evolutionary justified or not. Long live the altruistic allele. Hundreds would have been dead in that train without it.

  40. Harry E. says:

    Whites can tan when and where required, so it’s at least not totally “dumb” loss of function. Similar to muscle, which gives weight to arguments about being economical with resources and precursors, maybe L-DOPA.

  41. ctmorey says:

    Ok rather than metabolic savings, perhaps having less melanocytes frees up resources for other neural tissue :

    “Melanocytes share common embryologic origin, signaling molecules, receptors, and signaling pathways with cells of the nervous system”

    http://www.nature.com/jid/journal/v132/n3-2/full/jid2011386a.html

  42. Reblogged this on Philosophies of a Disenchanted Scholar and commented:

    I would also chip in pheomelanin, which can affect overall colouring from the hair, to the skin, to the eyes. When most people discuss melanin, they mean eumelanin.

  43. John Carney says:

    My guess is that the pigment lightening SLC24A5 variant made lactose tolerance more advantageous. It is thought that SLC24A5 is related to a mechanism for controlling the maintenance of cholesterol homeostasis. In particular, it appears to protect the melanogenic process from fluctuations in dietary cholesterol.

    The introduction of milk and cheese into the diet of the steppe folks may have increased cholesterol levels, interfering with melanogensis.. The variant could have acted to ameliorate this effect. Paleness might have been, well, a side effect of helping the body deal with the effects of the new diet.

    Related papers:
    http://www.ncbi.nlm.nih.gov/pubmed/23224873

    http://www.ncbi.nlm.nih.gov/pubmed/19469904

  44. Pingback: Biology is not physics: the perils of reasoning a priori | Julian O'Dea

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