The Sexes Share Genes

There are three factors that could have kept male human minds similar to female human minds despite strong sexual selection. The first factor is called "genetic correlation between the sexes." Males and females in every species share almost the same genes. There is a very high genetic correlation between the sexes. In humans for example, 22 pairs of our chromosomes are shared by both sexes, while only one pair, the X and Y sex chromosomes, are sexually distinct.

The genetic correlation between the sexes inhibits the evolution of sex differences, at least in the short term. Sex differences do not spring up automatically just because sexual selection is at work. Sex differences have to evolve gradually, like everything else. Consider the example of runaway sexual selection for long tails in birds. We assumed that the long tails would be passed on only from father to son. That might happen after many generations, but it is very unlikely to happen that way at first. It is much more likely that a mutation that increases tail length will i : passed along to both sexes. Both male and female offspring will inherit longer-than-average tails from their sexually attractive fathers. Initially, tail length will increase with equal speed in both sexes. And both male and female offspring may tend to inherit their mother's sexual preference for longer tails. So, female tail length will ride along on the genetic coattails of male tail length, and male sexual preferences will ride in tandem with female sexual preferences.

Darwin understood the genetic correlation between the sexes in a sketchy way, calling it "the law of equal transmission." In The Descent of Man he argued that male human intelligence and imagination evolved mainly through sexual competition, and wrote that "It is, indeed, fortunate that the law of the equal transmission of characters to both sexes has commonly prevailed throughout the whole class of mammals; otherwise it is probable that man would have become as superior in mental endowment to women, as the peacock is in ornamental plumage to the peahen." Basically, Darwin viewed the female brain as riding along on the genetic coattails of sexually selected male brains.

Genetic correlations between the sexes can be measured, and are often fairly strong. Anthropologist Alan Rogers found a very high genetic correlation between male and female height in humans, in a paper he published in 1992. This does not mean that men and women are the same average height. Nor does itjust mean that tall fathers have tall daughters, and that tall mothers have tall sons. Technically, it means that a tall parent's opposite-sex offspring are almost as extreme in their height, compared to others of their sex, as their samesex offspring are, compared to others of their own sex. Rogers saw the implications for sexual selection. If females favored taller-than-average males as sexual partners, then of course male height would increase over evolutionary time because of the sexual selection. But Rogers calculated that female height would also increase, due to the genetic correlation with male height. In fact, female height would increase

98 percent as fast as male height. As you can see, a very unequal sexual selection pressure can produce a very equal outcome.

However, these genetic correlation effects are transient. Eventually, male choosiness should decrease, and the costs of female ornamentation should increase, and these effects will break down the genetic correlation. Male choosiness would probably be eliminated first. Coming back to our long-tailed bird example, any male who rejects a short-tailed female will produce fewer offspring than a male who is less choosy. In most species, the pressures against male choosiness are very strong, causing sex differences in choosiness to evolve very fast. Sex differences in ornamentation might take a bit longer. Females with long tails will be inconvenienced by their cost, and if males do not prefer them to short-tailed females, they should evolve inhibitions against expressing the runaway male ornament. (Typically, this means that they evolve a gene expression mechanism that is sensitive to sex hormones during development, so the genes for long male tails are not turned on in female bodies.)

If genetic correlations between the sexes were not transient, we would never see dramatic sex differences in nature. Peahens would have the same tails as peacocks. Female nightingales would sing like males. The human clitoris would be as large as the human penis. Darwin's coattail theory of female brain evolution doesn't work except in the short term, because sex differences will eventually evolve if the sexes derive different benefits from ornamentation and sexual choice. Genetic correlations between the sexes can explain transient increases in female ornamentation and in male choosiness, but these increases are not evolutionarily stable. Fortunately, there is a second factor that is much more potent over the long term in keeping the sexes similar.

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