Skin color differences between the sexes are always discussed in terms of women being lighter than men, but never men being darker than women. This is seen in numerous animal studies (some reviewed by Rushton and Templer, 2012; read rebuttal here; also see Ducrest, Keller, and Roulin, 2008). Though, the colors that evolved on the animal’s fur due to whatever mate choices are irrelevant to the survival capabilities that the fur, feathers etc give to the organism in question. So, when we look at humans, we lost our protective body hair millions of years ago (Lieberman, 2015), and with that, we could then sweat. So since furlessness evolved in the lineage Homo, there was little flexibility in what could occur due to environmental pressures on skin color in Africa. It should be further noted that, as Nina Jablonski writes in her book Living Color: The Biological and Social Meaning of Skin Color (2012, pg 74)
No researchers, by the way, have explored the opposite possibility, that women deliberately selected darker men!
One hypothesis proposes that lighter skin in women first arose as a byproduct due to the actions of differing levels of hormones in the sexes—with men obviously having higher levels of testosterone, making them darker them women. So according to this hypothesis, light-skinned women evolved since men could tell high-quality from low-quality mates as well as measure hormonal status and childbearing potential, which was much easier to do with lighter- than darker-skinned women.
Another hypothesis put forth is that further from the equator, sexual competition between women would have increased for mates since mates were depleted, and so light skin evolved since men found it more beautiful. Thus, women living at higher latitudes were lighter than women living at lower latitudes because men had to go further to hunt which meant they were more likely to die which caused even greater competition between females, lightening their skin even more. And another, related, argument, proposed that light skin in women evolved due to a complex of childlike traits which includes a higher voice, smoother skin and childlike facial features, which then reduced male competition and aggressiveness. But women did not stay around waiting to be provisioned and they got out and gathered, and hunted sometimes, too.
Harris (2005) proposes that light skin evolved due to parental selection—mothers choosing the lightest daughters to survive, killing off the darker ones. All babies are born pale—or at least lacking the amount of pigment they have later in life. So how would parental—mostly maternal—selection have caused selection for lighter skin in girls as Harris (2005) proposes? It’d be a pretty large guessing game.
The role of sexual selection in regard to human skin color, though, has been tested and falsified. Madrigal and Kelly (2007a) tested the hypothesis that skin reflectance should be positively correlated with distance from the equator. It was proposed by other authors that as our ancestors migrated out of Africa, environmental selection relaxed and sexual selection took over. Their data did not lend credence to the hypothesis and falsified it.
Madrigal and Kelly (2007a: 475) write (emphasis mine):
We tested the hypothesis that human sexual dimorphism in skin color should be positively correlated with distance from the equator, a proposal generated by the sexual selection hypothesis. We found no support for that proposition. Before this paper was written, the sexual selection hypothesis was based on stated male preference data in a number of human groups. Here, we focused on the actual pattern of sexual dimorphism. We report that the distribution of human sexual dimorphism in relation to latitude is not that which is predicted by the sexual selection hypothesis. According to that hypothesis, in areas of low solar radiation, there should be greater sexual dimorphism, because sexual selection for lighter females is not counterbalanced by natural selection for dark skin. Our data analysis does not support this prediction.
Though Frost (2007) replied, stating that Madrigal and Kelly (2007a) presumed that sexual selection was equal in all areas. Madrigal and Kelly (2007b) responded, stating that they tested one specific hypothesis regarding sexual selection and found it to be false. Frost (2007) proposed two hypotheses in order to test his version, but, again, no one has proposed that women select darker men, which could be a cause of lighter-skinned women (though sexual selection does not—and cannot—explain the observed gradation in skin color between men and women).
Skin color differences between men and women first arose to ensure women enough calcium for lactation and pregnancies. Since skin pigmentation protects against UVR but also must generate vitamin D, it must be light or dark enough to ensure ample vitamin D production in that certain climate, along with protecting against the UVR in that climate. So women needed sufficient vitamin D, which meant they needed sufficient calcium to ensure a strong skeleton for the fetus, for breastfeeding and for the mother’s own overall health.
However, breastfeeding new babes is demanding on the mother’s body (calcium reserves are depleted four times quicker), and the calcium the babe needs to grow its skeleton comes directly from the mother’s bones. Even a mother deficient in vitamin D will still give calcium to the babe at the expense of her own health. But she then needs to increase her reserves of calcium in order to ensure future pregnancies aren’t fatal for her or her offspring.
Though, at the moment to the best of my knowledge, there are no studies on calcium absorption, vitamin D levels and the recovery of the female skeleton after breastfeeding. (Though n3 fatty acids are paramount as well, and so a mother must have sufficient fat stores; see Lassek and Gaulin, 2008.) Thus, light-skinned women are most likely at an advantage when it comes to vitamin D production: The lighter they are, the more vitamin D and calcium they can produce for more pregnancies. Since light skin synthesizes vitamin D more efficiently, the body could then synthesize and use calcium more efficiently. The body cannot use and absorb calcium unless vitamin D is present. Since the fetus takes calcium from the mother’s skeleton, ample amounts of vitamin D must be present. For ample amounts of vitamin D to be present, the skin must be light enough to ensure vitamin D synthesis which would be needed for calcium absorption (Cashman, 2007; Gallagher, Yalamanchili, and Smith, 2012; Aloia et al, 2013).
Nina Jablonski writes in her book (2012, 77):
Women who are chronically deficient in vitamin D because of successive pregnancies and periods of breastfeeding experience a form of bone degeneration called osteomalacia. This has serious consequences for infants born of later pregnancies and for mothers themselves, who are at greater risk of breaking bones. It makes sense that protection of female health during the reproductive years would be a top evolutionary priority, so we are now investigating whether, in fact, slightly lighter skin in women might be a fairly simple way of ensuring that women get enough vitamin D after pregnancy and breastfeeding to enable their bodies to recover quickly. The need for maintaining strong female skeletons through multiple pregnancies may have been the ultimate evolutionary reason for the origin of differences in skin color between men and women.
While Jablonski and Chaplin (2000: 78) write:
We suggest that lighter pigmentation in human females began as a trait directly tied to increased fitness and was subsequently reinforced and enhanced in many human populations by sexual selection.
It is obvious that skin color in women represents a complex balancing act between giving the body the ability to synthesize ample vitamin D and protect from UVR. Skin coloration in humans is very clearly highly adaptive to UVR, and so, with differing average levels of UVR in certain geographic locales, skin color would have evolved to accommodate the human body to whichever climate it found itself in—because human physiology is perhaps the ultimate adaptation.
Sexual selection for skin color played a secondary, not primary role (Jablonski, 2004: 609) in the evolution of skin color differences between men and women. There is a delicate balancing act between skin color, vitamin D synthesis, and UVR protection. Women need to produce enough vitamin D in order to ensure enough calcium and its absorption to the baby and then ensure there are ample amounts to replace what the baby took while in the womb in order for future pregnancies to be successful. Sexual selection cannot explain the observed gradation in skin color between the races and ethnies of the human race. In my opinion, the only explanation for the observed explanation is the fact that skin color evolved due to climatic demands, while independent justification exists for the hypothesis as a whole (Jablonski and Chaplin, 2010).
I don’t see any way that sexual selection can explain the observed gradation in skin color around the world. Skin color is very clearly an adaptation to climate, though of course, cultural customs could widen the skin color differences between the sexes, and make women lighter over time. Nevertheless, what explains the observed skin gradation is adaptation to climate to ensure vitamin D synthesis among a slew of other factors (Jones et al, 2018). Sexual selection, while it may explain small differences between the sexes, cannot explain the differences noted between the native human races.