Hairlessness, kin selection and sexual selection

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Would it be possible that hairlessness and skin color evolved to allow fathers to identify their direct biological offspring, so as not to have to support another male’s genes. In many other species, when a new male/group of males takes over they kill the existing pups, so there seems to be pretty good evidence for parents to prefer their direct offspring to the offspring of others. Now it’s difficult to say that this is natural selection as there’s no particular reason to think that the existing brood has “less fit” genes than the new brood would provide. It would also be interesting to see whether or not there is greater morphological variation in appearance in humans than in other great apes. This would seemingly bolster the argument that much of human culture evolved around allowing fathers to identify their specific offspring and prefer them over other male’s offspring. Men would tend to prefer women with greater morphological variation and hairlessness because these women’s children would tend to exhibit features that would allow the male to more readily identify parentage. I have done a fair amount of googling and have not been able to find anyone else making this hypothesis but I would be happy if someone could provide any insight into prior references to this sort of theory, namely that hairlessness was sexually selected so that fathers could identify their specific biological offspring.


The one hypothesis posited by Judith Rich Harris seems to assert post-modernist sounding psychological factors; her “us hairless/them hairy” sounds like something straight out of post-colonialist theory or the Frankfurt School. If we are going to seriously consider the role of genetics and evolutionary history in current human affairs we need to prefer strict genetic causes over psychological-ideal “causes”, which are really manifestations of the ghost-in-the-machine mentality.

My interest in genetics comes from my interest in ethical philosophy and economics, so I’ll admit up-front that I have no expertise here and I’ll ask for your indulgence. As I understand it sexual selection for dimorphism takes a long time. However, traits that are sexually monomorphic can manifest relatively quickly in a population. Significant selective pressure for less hairy females would result in offspring of both sexes exhibiting lower levels of fur over probably a relatively short period of time. I just wanted to add that last part because I want to state up-front that I have no specific expertise in this area.

If there is one thing that I believe that I cannot yet demonstrate it’s that much of varieties of human life and culture evolved to allow human beings to identify their kin and prefer those kin to non-kin.

A final note: most people agree that sexual selection takes place on the male which is why males in almost every attribute exhibit. This is true. But imagine a situation where a particular region has developed high rates of pair-bonding and males and females raise their offspring as a pair. Males might not be willing to care for a child that they could not specifically recognize as their own, therefore, the female would continue eliminating children until she produced an individual child for which a specific male would/could identify as his own and take responsibility. So, what I am doing is taking a cue from Harris and combining parental selection as a cognitively complex variation on sexual selection; this is kin selection plus sexual selection.

P.S. this is my first post so be gentle

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30 Comments

  1. It doesn’t sound compelling to me. The ability to detect cheating (a baby fathered by someone other than you) would have evolutionary value for a man, but the genes you are wanting to favor in your scenario are the ones on the female side – and on that side we would expect morphological variations that make deception *easier* to be favored. Also, it’s hardly clear that hairlessness and paler skin (in the mother) would be a big aid in figuring out whether the baby looked like the father or not. 
    Other theories (women need vitamin D more, hairiness and dark skin are markers of aging and selected against in women for that reason, women have more subcutaneous fat and so don’t need hair anyway) seem more sensible to me.

  2. Actually, I don’t think that’s quite what I’m saying. The genes that are being selected are not directly the mother’s genes but rather the genes of the offspring. A mother that could demonstrate to her partner that he was, indeed, the father would have a reproductive advantage over women who could not demonstrate this. But the trait would actually be selected for both genders because the father would want to know the parentage whether the child was male or female. 
     
    I simply can’t see how additional need for vitamin D explains the sexual dimorphism between men and women. Remember, dimorphism, as I understand it (I could be wrong) usually requires at least a partial basis in sexual selection and I can’t see the vitamin D explanation providing that. Finally, remember that sexual selection, by and large, takes place with females doing the selecting. Many other theories have posited all sorts of plausible explanations (e.g. individuals with fur caught fire when humans began learning to burn things) but I can’t see how they are either strong enough or how they explain the slight dimorphism.

  3. BTW, I can understand why skin color isn’t such a good example, but hairlessness? Why are people in very sunny climes hairless?

  4. Finally, I think that if you look at two men next to each other with full beards on their faces you will see markedly less difference than if you compare them when beardless. Sorry about the successive posts, but these things are coming to me while I’m engaged elsewhere.

  5. A request to be gentle naturally provokes the urge to slaughter. At least the NSFW posts don’t take themselves seriously. The risk of coming down on newbies is that they will think thrice before posting their personal theories again. God, I hope that’s true. 
     
    1) If you want to post a new sweeping hypothesis based (however loosely) on pre-existing data, provide references. In “many other species”, conquering males “kill the existing pups.” I’m sure they do. Please provide a link to something credible. And so on, throughout. 
     
    2) “…much of human culture evolved around allowing fathers to identify their specific offspring” This is your theory, I take it? Could you perhaps provide examples to defend this idea about “much of human culture?” You mean, like storytelling, religion, art, and food preparation? Do tell. 
     
    3) When presenting a novel theory, kindly present falsifiable hypotheses (in principle if not practice) which distinguish it from other theories. If other theories do not exist, or are lame, as asserted, kindly begin by convincing us that there is indeed a problem to be solved. 
     
    To the theory. First, skin color. Let us speculate that most invading males during the course of human evolution were from nearby rival groups and had the same skin color. How does skin color help in recognizing your kids? 
     
    Second, hairlessness. Why is a hairless individual less likely to be recognizable as your progeny than a hairy one? Hair (distribution, texture, thickness) would seem to be among the most recognizable of human traits. 
     
    Third, what part of this theory do chimps not satisfy? They have largely hairless faces. Are you theorizing about hairless bodies (at which point the theory’s merit revolves around our ability to recognize each other’s lumbar vertebra, I suppose) or hairless heads? Please fill in the basics for us. 
     
    Happy holidays.

  6. hairless individual less likelyExcuse — more likely.

  7. I’ve had a similar idea with respect to birthmarks and other little odd traits that don’t serve any obvious purpose. In my family, most people have a crooked second toe on each foot, and so does my son, so that strengthens my belief that he’s mine. But I wouldn’t think it would work so well with skin color and hairiness, because those traits change so much from infancy to adulthood.

  8. In many other species, when a new male/group of males takes over they kill the existing pups, so there seems to be pretty good evidence for parents to prefer their direct offspring to the offspring of others. Now it’s difficult to say that this is natural selection as there’s no particular reason to think that the existing brood has “less fit” genes than the new brood would provide. 
     
    Except for the fact that the “existing brood” is dead, and the “new brood” is not.  
     
    Both preference for one’s own offspring and natural selection are at work here. Over time, the practice of murdering the offspring of a defeated foe serves to weed a species of its wimpiness.

  9. 1) you should look into greenbeards. 
     
    2) variation in skin color is something we know about 
     
    a) it’s clinal. that is, the men most likely to impregnate “your women” will have the same range of skin color. it isn’t like black men migrated en mass to europe during the ice age and what not. interdeme conflicts would be between groups whose gene frequencies really wouldn’t differ much. this is why tatoos and body painting are popular at that level of granularity. it seems that if there was diversifying selection it occured over transcontinental ranges, which doesn’t make sense (as i said, those who would turn you into a cuckold are within range of your women, not separated by a thousand miles or more).  
     
    b) it seems like frequency dependence would preserve lots of variation in many populations. consider this, assume i, a brown skinned man, am single. now, unleash me on a harem of swedish women. i am rather confident of my paternity certainty if the only other competitors are ethnic swedish men, because i look so different. why is it that loci for light skin are fixed in so many populations when negative frequency dependent selection should preserve polymorphism? e.g., SLC24A5 has fixed in europeans within the last 10,000 years in all likelihood.  
     
    c) in populations where it does vary it segregates on a few loci of large effect. this means in a brownish population there is going to a non-trivial amount of variance around the expectation of the skin tone of the offspring of any two individuals (remember that the trait is pretty much additive/independent). so it isn’t a really good character to use where extant variation exists! (at least the blue eye hypothesis has the benefit that most of the variation is localized on one locus and the inheritance pattern is 3/4 monogenic). 
     
    3) the between population difference in skin color might simply be a byproduct of the fact that the melanin production pathways interact with various other pathways (e.g., more testosterone upregs melanocortin i think). i mean, note that women are much more likely to have green eyes than men (who are more likely to have blue eyes). i don’t think we need an adaptive explanation…there is probably selection for skin color via OCA2, which controls eye color, and the pathways around OCA2 interact with sex conditional hormones (there is also selection on a locus associated with blonde hair, and that locus as a skin color effect). 
     
    4) if skin color differences are driven by sexual selection it is notable that the data on international skin color dimorphism is very muddy. some of it suggest no difference across regions (if light skin is driven by sexual selection there should be more dimorphism in those regions). while some research shows the most dimorphism in the middle latitudes of the world (where there’s more segregating variation on the loci to begin with). 
     
    5) finally, we have lots of recent genomic data that skin color variation might be new across much of eurasia. so from that one might infer (with the model) that men become concerned about paternity very recently? (on the order of 15-5 thousand years ago?) a good check on this would be increased sexual dimorphism in size as well. 
     
    6) there might be different selective forces at work at work together. i think the data out there makes some sort of non-sexual selective pressure pretty convincing. it would be strange that a runaway effect in europe happened just in the direction of skin which results in increased vit. D production (usually runaway selection runs up against maladaptation). that being said, the genetic architecture which exogenous ecological selection has wrought can be raw material for cultural and gene-culture dynamics. but these would be secondary, and constrained by the parameters of ecological selection (frequency dependence would not drive the increase of dark skin in scandinavia because that is a recipe for rickets in the pre-modern period). 
     
    7) sexual selective hypotheses should not be dismissed out of hand, but, my own opinion is that we should check for ecological selection first. we’re not a species with an enormous amount of reproductive skew, at least outside of peculiar (and recent) historical conditions. also, we should evaluate different forms, such as sensory bias, “good genes,” and runaway, instead of lumping them together. 
     
    8) do note that the likelihood that ancestral h. sap had dark skin is because we lost our fur. in other words, the evolution toward a consensus sequence on loci such as MC1R among dark peoples was driven probably by ecological selective pressures such as the destruction of folate and so increased birth defects in pregnant women. so it seems likely that the emergence of dark skin was a matter of non-sexual selection, so i think that should modulate our priors as to the origins of light skin.

  10. as for fur. the explanations for why we lost it are not persuasive to me. but i would like someone who knows more anthropology to weigh in. nevertheless, most of the skin color genes are the same ones that affect coat color, fwiw.  
     
    also, we are the most varied in phenotype of the apes. in fact, it is argued that we are the most varied large mammal, the only one to beat us are dogs. but, as i said, that variation is extant over wide geographic distances. i don’t think diversifying selection is really an issue, rather, we’re a mammal that lives on every continent but antarctica. that’s a lot of ecological variation, and, in pre-modern times genetic isolation might have been great enough for lots of little distinct evolutionary experiments to occur.

  11. Razib could’ve made several posts out of his comments.

  12. ben g, i have. here.

  13. for some reason the scienceblogs search query didn’t work above. 
     
    http://www.google.com/search?q=skin+color&btnG=Search!&domains=scienceblogs.com%2Fgnxp&sitesearch=scienceblogs.com%2Fgnxp

  14. A mother that could demonstrate to her partner that he was, indeed, the father would have a reproductive advantage over women who could not demonstrate this. 
     
    Yeah, but a mother who can cheat effectively (i.e. get the best genes in a one-night stand with the alpha male and then use one of the betas as a meal ticket) arguably has a greater advantage.

  15. Yeah, but a mother who can cheat effectively (i.e. get the best genes in a one-night stand with the alpha male and then use one of the betas as a meal ticket) arguably has a greater advantage. 
     
    these sorts of though exp. can open up infinite doors. imagine a man whose physical traits are all ‘recessive.’ most of his genome content could slip into what should be another man’s reproductive output but the physical traits could be those of the mother because of the ‘dominance’ of her traits. a man would just assume that his offspring favored their mother.

  16. also, regarding paternity. i think one could argue that a variety of strategies could result in high fitness, we don’t need to posit obligate behavior. i would observe that cross-culturally the two regions with the highest fertility and increase probably exhibit the two extreme variants: the middle east (kill purely on suspicion of extra-pair intercourse) and africa (where high rates of polygyny are combined in some regions with female economic independent which allows for de facto polyandry).

  17. Welcome to Asher, and I hope you keep posting. 
     
    I haven’t thought through your hypothesis very far, but one quick thought: the kind of markings (moles, birthmarks, etc) that are exposed by hairlessness are probably not usually inherited, so they would not help in paternity recognition. 
     
    Someone questioned whether it is common for incoming males to kill existing young. This has been reported in lions, langur monkeys, and some other primates, but the evidence isn’t always clear. See Schaller on lions, but I don’t have other references to hand.

  18. Some points: 
     
    “Now it’s difficult to say that this is natural selection as there’s no particular reason to think that the existing brood has “less fit” genes than the new brood would provide.” 
     
    The above is a group selection-based argument, and therefore wrong. The fitness advantage is realized through increased mating opportunities of an infanticidal male. A gene that results in a conquering male killing another male’s offspring will proliferate because the conquering male will produce more offspring than a male that doesn’t kill the offspring. This is because he will return his females to estrus sooner.  
     
    Infanticide is pretty well established in polyandrous species with parental investment. This is probably best studied in primates. Gorillas, Hamadryas but not other baboons, langurs, etc. Infanticide requires high confidence of non-paternity, and high probability that the female will be receptive sooner with the loss of her offspring (almost always the case with dependent young).

  19. The above is a group selection-based argument, and therefore wrong.  
     
    i would assert that a group selection is not ipso facto invalid. just demands a lot of skepticism because of its relative lack of parsimony and the nimbleness of lower level selection. after all, james crow accepts group selection as likely for humans and he isn’t exactly a shallow thinker. 
     
    (i though i remain skeptical of group selective arguments)

  20. razib, 
     
    Fair enough. …it is, however, wrong in this case.

  21. I haven’t carefully read all the comments, so forgive me if this is redundant. Anyways, regarding infanticide and “hairlessness”, Sarah Hrdy among others speculated that flamboyant infant fur color might be a strategy to allay infanticide (which, by the way, is unequivocally established as a sexually-selected post-zygotic male strategy to increase fitness). In some primate species, infants are born with brightly colored fur that differs markedly with the permanent adult fur color. Adrian Treves did a pretty cool study on infant fur color and infanticide risk: ” Primate natal coats: A preliminary analysis of distribution and function” in American Journal of Physical Anthropology, vol 104: 47-70. It discusses infant coloration with respect to various hypotheses, including infanticide. Asher, this study might tie into what you’re talking about with respect to hairlessness in human infants…? 
     
    Subsequently, infanticide by males is most likely to occur when the period of lactation lasts longer than gestation (since moms are nursing offspring and not ovulating, so any male unrelated to the nursed infant is sitting around waiting for her to wean the infant…which might select for the following invasibable strategy in males unrelated to the infant: why not kill the infant and induce “mom” into ovulation again?).

  22. Cleary, human skin has acquired strong erogeneous properties. This statement may seem obvious, even trite, but it needs to be made. In other primates, pelage is much more critical than skin in stimulating sexual and/or parental responses. Yes, there is evidence from other primates that light pink skin elicits nurturance, but the mental algorithm seems to be limited to adult/infant interactions. It may be that this algorithm has been extended to male/female interactions. 
     
    I have trouble with the statement “most people agree that sexual selection takes place on the male.” This is a biological trend not a biological law. Sexual selection generally acts on males because paternal investment is usually much lower than maternal investment, so at any one time too many males are competing for access to too few females. 
     
    In humans, this trend can be reversed, particularly in environments where women are heavily dependent on men for food provisioning.

  23. peter, you should link to your posts. lots of people are probably not familiar with your model. whether you agree with the theory or not, you’ve collected a lot of data people should know about.

  24. The one hypothesis posited by Judith Rich Harris seems to assert post-modernist sounding psychological factors. her “us hairless/them hairy” sounds like something straight out of post-colonialist theory or the Frankfurt School. 
     
    This point seems not only false, but ridiculous. JR Harris is not making a “postmodernist” or “Frankfurt School” argument, and she surely doesn’t believe in or assume a ghost in the shell… I’m surprised no one else on the comments has objected to these claims, despite the otherwise edifying torrent of criticism.

  25. I wonder why humans as a species have such a wide variation in hairiness, for example Sub-Saharan Africans and East Asians having much less than Europeans and other Caucasoids? 
     
    Anyone have an explanation?

  26. This statement may seem obvious, even trite, but it needs to be made. In other primates, pelage is much more critical than skin in stimulating sexual and/or parental responses

  27. Razib, 
     
    Fine, your readers might want to read my essay on sexual dimorphism in skin color at: 
     
    http://pages.globetrotter.net/peter_frost61z/fwdm.htm 
     
    I also have a post on Judith Harris’ theory at: 
     
    http://evoandproud.blogspot.com/2007/04/parental-selection-human-hairlessness.html

  28. Perhaps I’m just missing something here, but wouldn’t the visible phenotypic trade-off be pretty doubtful… 
     
    Now we all agree that there’s a clear benefit in males *detecting* offspring who aren’t their own and killing them, so detection-modules are certainly favored. 
     
    And if a male’s children had distinguishable characteristics, it would become much, much easier for him to detect those not his own, and kill them once he seized control over the females. But it would be equivalently that much easier for competing males to detect and kill *his* children if and when he was the one driven out. So *reducing* the visible markers of one’s children would arguably be as good a male parental strategy as *increasing* them. 
     
    The two absolutely-beneficial characteristics would be (1) increased ability to seize control of a herd of females and drive off rival males and (2) mental-modules for detecting parentage of children. 
     
    By contrast, producing visible indicators of parentage would be—at best—a wash.

  29. I doubt it. Hairiness is fairly well correlated with testosterone levels, and men want low testosterone females. (This ndicates fertility)

  30. I guess I’m the only oddball around here that thinks that hairlessness in humans evolved for thermoregulatory reasons. When you look at the 4000 or so species of mammals, the amount of fur is almost exclusively determined by the thermoregulatory needs of the animal. Why should humans be presumed to be different in this regard? Indeed, for the sake of parsimony, it seems to me that a thermoregulatory explanation should be presumed until strong evidence emerges to the contrary.  
     
    While most hairless mammals are much larger than humans, we are an extreme outlier in terms of encephalization quotient (EQ). And the brain is the organ most susceptible to hyperthermia. In order to achieve such an extreme EQ, it seems obvious that some sort of adaptation for cooling the brain would be required. Dean Falk has addressed this with her Radiator Hypothesis. She does not include (as far as I’m aware) that human hairlessness has evolved to cool the brain, but it seems a logical extension of her Radiator Hypothesis.

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