Sexual Selection and the Color Line

Essays on the Color Line and the One-Drop Rule
by Frank W Sweet
August 1, 2007

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s the extraordinary paleness of the natives of the Baltic region caused by sexual selection? Are the different skin tones of men and women caused by sexual selection or are they a side-effect? Many women believe that a pale skin attracts men. Is this genetic? The answers are presented in seven topics: Nature selects among competing alleles, not among species, nor individuals. Selection can help an allele but hurt its host species. Sexual selection happens when two alleles conspire. The five hallmarks of sexual selection. Is the Baltic paleness adaptation sexual selection? Was skin-tone dimorphism caused by sexual selection? And do current fashions prefer paler skin?

Nature Selects Among Competing Alleles, not Among Species nor Individuals.

Chromosomes are data storage media. The data files that they contain are called genes. All organisms are built and maintained following the information contained in their genes, copies of which are stored in the nucleus of each cell. Humans have about 30,000 pairs of genes, stored as 750 megabytes of data (3 billion base-pairs, carrying two bits of data each, at eight bits per byte). The information carried by each gene produces a specific protein which, in turn, performs a specific function in the organism. Each gene can come in variants that act slightly differently. Each variant of a gene is called an allele of that gene. And some alleles are more successful at cloning themselves than others.

Since the publication of Richard Dawkins’s groundbreaking The Extended Phenotype: The Gene as the Unit of Selection (Oxford: Freeman, 1982) or his more popular The Selfish Gene, 2 ed. (Oxford: Oxford University, 1989), the consensus among evolutionary biologists has been that natural selection does not work by selecting the most “fit” among species nor even among individuals. Instead, it works by rewarding certain gene variants (alleles). To be precise, those alleles that clone themselves most successfully eventually replace less successful versions of those same genes.

Selection can Help an Allele but Hurt its Host Species.

With few exceptions, the only way that an allele in a multicellular organism can propagate is within a gamete (egg or sperm) of the individual that carries it. And so, with the same few exceptions, what is good for an individual’s propagation is good for its alleles’ and vice-versa. This is why it is still a useful approximation to say that natural selection chooses the most successful individuals of each generation (in the obvious sense that those who successfully rear more offspring have more descendants). But what is really happening is that those alleles that make their host organisms more successful leave more clones of themselves. When you get down to it, nature selects among competing alleles, not among competing individuals, and certainly not among competing species.

The disconnect between benefit for an allele and benefit for the species is most evident in those cases where an allele propagates at the expense of the species that it finds itself in. That is, where the spread of an allele can drive its own host species to extinction. The most common such situation is runaway sexual selection.

Sexual Selection Happens When Two Alleles Conspire.

The classical example of sexual selection is the peacock’s tail. Imagine prehistoric peacocks where males and females both had ordinary chicken-like tails. By sheer chance, a mutation produced a gene variant (an allele) that produced a slightly longer tail if it found itself in a male individual but not in a female. Such sex-linked alleles are not uncommon. The one gene that switches between sexual traits in any embryo works by triggering a cascade of thousands of other genes that produce the physical differences between sexes. A longer-tailed-male allele is just a random variation. Random odd variants caused by mutations are common but most go nowhere. In this case, if that were all there was to the story, the sons of the creature with the mutation would have had slightly longer tails but the daughters would not. Since there was no particular advantage for a male to have a longer tail, copies of the longer-tailed-male allele would have disappeared after a few generations and that would have been the end of it.

But what actually happened was this. While copies of the longer-tailed-male allele were still being passed down from a few individuals to their offspring, before disappearing entirely, a second mutation arose, again by sheer chance. The second allele, a variant of an entirely different gene, when in a female produced a slight preference to mate with longer-tailed partners. Females lacking the prefer-longer-tail allele mated randomly, but those with the prefer-longer-tail allele mated more often with males carrying the longer-tailed-male allele.

That is all it took: two alleles in collusion, each helping the other to propagate. The math is simple. In a geological eye-blink, every peacock carried both genes, every male had a longer tail and every female preferred those males with the longest tails of all. In an evolutionary explosion, male tails became ever longer and females became ever more choosy, demanding tails so long that they jeopardized the very survival of the males who carried them. The alleles neither know nor care what is happening to the peacocks. Alleles are just data files. Similar runaway sexual selection drove the Irish Elk to extinction when the species’ food supply could no longer support the annual production of ever-larger male antlers. Runaway sexual selection may yet drive peacocks to extinction.

Not all sexual selection produces a runaway spiral that destroys the alleles’ host species. Most cases taper off when the trait becomes so extreme and harmful that it reduces fecundity despite its carriers’ preferential mating. In such cases, reduced fecundity eventually matches increased chance of mating and the spiral levels out. For example, there is a well-known primate species where females began preferentially to mate with males who were aggressive among themselves. Two genes in collusion: aggression in males, coupled with mating preference for aggressive partners in females. Soon the males grew ever more aggressive among themselves and the females became ever choosier at mating only with the most aggressive of all. Fortunately, this creature’s sexual selection spiral eventually leveled off before it became extinct. Males less aggressive than the final balance point died childless because they could not attract mates. Males more aggressive than the final balance point died childless because they were killed before they could help to rear a family. Unfortunately, after about 120 millennia this organism suddenly invented firearms, so it may yet become extinct, but this takes us away from the main story.

The Five Hallmarks of Sexual Selection

Summarizing, sexual selection produces an evolutionary change in an organ or behavior that makes its carrier more attractive to the opposite sex. You can recognize sexual selection by its five hallmarks:

    1. The change enhances the individual’s chance of mating.
    2. It does not enhance the individual’s survival; indeed it can harm it.
    3. It does not benefit the species; indeed it may drive it to extinction.
    4. It produces sexual dimorphism (clearly visible differences between males and females).
    5. It happens lightning fast, within a few hundred generations.

Keeping those five hallmarks in mind, examine the two hypotheses connecting human skin tone with sexual selection. The first hypothesis is about northern European depigmentation. The second is about skin-tone dimorphism.

Is the Baltic Paleness Adaptation Sexual Selection?

Natives of the Baltic region are paler than any other population on the planet, even populations at higher latitudes. The first hypothesis says that this is due to sexual selection. It says that: an allele that destroys the skin’s ability to synthesize and distribute melanin just happened to appear at the same time as one for pale-skin preference in mate-selection. The interaction between the two colluding genes produced ever-paler individuals. A balance was finally struck between being too dark to attract a mate and too pale to survive skin cancer long enough to raise children. This theory is best presented today by Peter Frost, whose argument is available at http://pages.globetrotter.net/peter_frost61z/. It is also mentioned in Luigi Luca Cavalli-Sforza, Paolo Menozzi, and Alberto Piazza, The History and Geography of Human Genes, trans. Sarah Thorne (Princeton: Princeton University, 1994), page 145.

The strength of Frost’s hypothesis is that it fits hallmarks 2, 3, and 5. It fits 2 and 3 because paleness worsens the risk of squamous cell carcinoma and the more lethal melanomas. It fits hallmark 5 because there is strong evidence that Europeans were still medium-brown (like Berbers, say) as recently as the mesolithic period.

The weakness of the hypothesis is that does not fit hallmark 4 very well. There is a difference in skin-tone between men and women but it is not very great (not as dramatic, for example, as human females’ protruding mammary glands, which show all five hallmarks). Furthermore, if the Baltic paleness were due to sexual selection, then its dimorphism (the difference in skin tone between males and females) would be greater in northern Europe than in central Africa. In fact, this is not the case. Central Africans are much darker than northern Europeans, of course, but the relative difference between males and females is about the same in both regions.

According to Madrigal and Kelly in “Human skin-color sexual dimorphism: a test of the sexual selection hypothesis” Am J Phys Anthropol. 2007 Mar;132(3):470-82:

Applied to skin color, the sexual selection hypothesis proposes that male preference for light-skinned females explains the presence of light skin in areas of low solar radiation. According to this proposal, in areas of high solar radiation, natural selection for dark skin overrides the universal preference of males for light females. But in areas in which natural selection ceases to act, sexual selection becomes more important, and causes human populations to become light-skinned, and females to be lighter than males. The sexual selection hypothesis proposes that human sexual dimorphism of skin color should be positively correlated with distance from the equator. We tested the prediction that sexual dimorphism should increase with increasing latitude, using adult-only data sets derived from measurements with standard reflectance spectrophotometric devices. Our analysis failed to support the prediction of a positive correlation between increasing distance from the equator and increased sexual dimorphism. We found no evidence in support of the sexual selection hypothesis.

Finally, there is an alternative hypothesis for the Baltic paleness adaptation that matches the known facts better than the sexual selection hypothesis. Compared to other high-latitude dwellers not exposed to ambient UV, European diet was uniquely cereal-based and so deficient in vitamin D. See The Paleo-Etiology of Human Skin Tone. In conclusion, the sexual selection hypothesis for the Baltic paleness adaptation seems unlikely.

Was Skin-Tone Dimorphism Caused by Sexual Selection?

There is no longer any question that men are slightly darker than women. But the hypothesis that Baltic paleness was an adaptation to vitamin D deficiency does not help solve the puzzle of this dimorphism. This is because men and women are exposed to the same sunlight and the same foods. Furthermore, human skin-tone dimorphism (the sex-difference in skin tone, not the skin tone itself) hits all five of the hallmarks. This is very strong evidence that it was indeed caused by sexual selection of some sort.

The only question is, was the skin tone dimorphism a direct consequence of males preferring pale females, or was it a side-effect of some other male preference? Specifically, was it a side-effect of men preferring cute women?

Human females do not grow facial hair, their faces remain childlike and cute much older than males, and of course they are shorter than males. There is no doubt that these dimorphisms resulted from sexual selection. The two colluding genes in this case are, in males, a preference for childlike features in their mates and, in females, the retention of childlike features well into maturity. The retention of childlike features (called “neoteny”) is a very common adaptation among organisms because one allele in the maturation cycle can affect many other genes via triggering and cascade effects. The physical changes associated with reaching maturity must be coordinated and so a single odd allele in the coordinating gene can easily delay the entire process. The neoteny evident in the features of female humans was undoubtedly produced by sexual selection. The underlying allele that produced the neoteny (that delayed the maturation change in the features of human females) affected hundreds or thousands of other genes.

Human children are paler than adults. By delaying all the maturation features, the neoteny allele in human females might have also delayed the skin-darkening that normally occurs at puberty. It is quite possible that human males do not particularly prefer paler human females. It is quite possible that human males prefer cuter human females, and their relative paleness just got dragged along, with all of their other childlike traits that were delayed in order to make human females cuter for longer.

There is no way to tell. Men might have a genetic preference for lighter-skinned mates. If so, it is a barely detectable preference judging by the degree of dimorphism. Or, alternatively, men might have a genetic preference for cute mates, and the cuteness gene just drags the shortness, paleness, and hairless-chin genes along with it. We may never know.

Do Current Fashions Prefer Paler Skin?

Many anthropologists of the 1920s and 1930s who studied the African-American community reported detecting a male mate-selection preference for fair women (or a female preference for darker husbands, which would produce the same result). Melville J. Herskovitz, Gunnar Myrdal, Edward Reuter, Ralph Linton, Gustavas Steward, and Robert E. Park observed that husbands were darker than wives more often than the reverse. Their conclusions were overturned when studies of siblings showed that females are often less pigmented than males, just as children are less pigmented than adults. As mentioned above, it is simply one of the neotenous features associated with human sexual dimorphism. According to Joel Williamson, the current consensus is that mating within the U.S. Black community has been essentially random for skin tone.

On the other hand, whether people actually prefer paler mates is a separate question from whether they believe that the opposite sex prefers paler or darker mates. A large segment of the international cosmetics industry supplies the market for skin darkeners and skin lighteners.

According to The American Tanning Insitute, there are about 30,000 indoor tanning facilities in the United States, employing 160,000 individuals. This is roughly a $500 million annual industry. In addition, cosmetics companies market an increasingly diverse line of “cancer-safe” skin-darkening lotions. Google “skin darkeners” and you get 674 hits.

Nevertheless, the world-wide skin-lightening market is orders of magnitude larger than that for skin darkeners. According to Amina Mire, “Pigmentation and Empire: The Emerging Skin-Whitening Industry”, Counterpunch July 28, 2005, in Japan and China alone (the only nations for which data are published) skin-lightening cosmetics amount to a $7 billion per year industry. Europe, Africa, and the Americas together probably sum to ten times that figure. Google “skin lighteners” and you get 105,000 hits. Hydroquinone medications (the kind that Michael Jackson used) are widely sold throughout Africa as cosmetics to women who want to become paler. Women in both Nigeria and Mexico have been hospitalized for poisoning because they applied skin lighteners that contained mercury. Dangerous or not, misused or not, skin lighteners are a multi-billion dollar worldwide industry fueled by the desire to look paler in order to attract a mate.

There is no doubt that many people around the world believe that the opposite sex prefers those with paler skin-tone. But fashions come and go. Hemlines rise and fall. In Reubens’s time, well-rounded women were seen as more attractive. Today the emaciated look is all the rage. Anything that changes within the space of a single generation cannot possibly be genetically determined.

All in all, sexual selection is unlikely to have caused the Baltic paleness adaptation because the hypothesis does not fit the facts. Skin-tone dimorphism was caused by sexual selection, but whether female paleness was selected for directly, or simply a by-product of neotenous cuteness selection is unknowable. The latter seems more likely, due to its barely detectable level. Finally, people around the world apparently believe that paler is more attractive and back the belief up with their cash. Some may even justify their choice of cosmetics as being genetically unavoidable. But this seems the least likely hypothesis of all.

For the details of sexual selection, with particular emphasis on humans, read Matt Ridley, The Red Queen: Sex and the Evolution of Human Nature (New York: HarperCollins, 1993).

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Frank W. Sweet is the author of Legal History of the Color Line (ISBN 9780939479238), an analysis of the nearly 300 appealed cases that determined Americans’ “racial” identity over the centuries. It is the most thorough study of the legal history of this topic yet published. He was accepted to Ph.D. candidacy in history with a minor in molecular anthropology at the University of Florida in 2003 and has completed all but his dissertation defense. He earned an M.A. in History from American Military University in 2001. He is also the author of several state park historical booklets and published historical essays. He was a member of the editorial board of the magazine Interracial Voice, and is a regular lecturer and panelist at historical and genealogical conferences. To send email, click here.


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