The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature by Geoffrey Miller Page B

selection he compared to a rather different sort of artificial selection more familiar to the leisured classes, and more relevant to gorgeous ornamentation: breeding pet birds to make them look unusual and attractive. In The Origin he argued that "if man can in a short time give beauty and an elegant carriage to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect."
    The analogy between artificial selection by human breeders and sexual selection by female animals may seem strained. But for Darwin there was no essential difference between human minds and animal minds: both could work as selective forces in evolution. As a dog-lover and an experienced horseman, Darwin felt comfortable attributing intelligence to animals. He reasoned
that if humans can breed dogs, cats, and birds according to our aesthetic tastes, why shouldn't these animals be able to breed themselves according to their own sexual tastes?
Biology students now are usually taught that sexual selection is a subset of natural selection, and that natural selection is only loosely analogous to artificial selection by human breeders. This was not Darwin's view: he saw sexual selection as an autonomous process that was midway between natural and artificial selection. Darwin was fairly careful about his terms. For him, artificial selection meant the selective breeding of domesticated species by humans for their economic, aesthetic, or alimentary value. Natural selection referred to competition within or between species that affects relative survival ability Sexual selection referred to sexual competition within a species that affects relative rates of reproduction. Darwin knew that Herbert Spencer's term "survival of the fittest" could be misleading Heritable differences in reproduction ability were as important in evolution as heritable differences in survival ability.
However, whereas natural and artificial selection can apply equally well to mushrooms, lemon trees, and oysters, Darwin believed that sexual selection acts most strongly in the higher animals. This is because courtship behavior and selective mate choice behavior are best carried out by mobile animals with eyes, ears, and nervous systems. The mate choice mechanisms that drive sexual selection are much more similar to artificial selection by humans than to blind forms of natural selection by physical or ecological environments. Darwin understood that sexual selection's dependence on active choice might create distinct evolutionary patterns such as fashion cycles and rapid divergence between closely related species.
Males Court, Females Choose
Darwin was more interested in explaining ornamentation than in explaining sex differences. Still, he could not help but notice that male animals are almost always more heavily ornamented than females. He also noticed that most of the differences between
males and females are either specializations for making eggs or sperm, or specializations in the weaponry and ornamentation used during sexual competition. Sexual selection was not only useful in explaining ornamental traits that natural selection could not explain. It could also account for almost all differences between the sexes.
This made a rather neat story. Males usually compete to inseminate females. They do this by intimidating other males with weaponry and by attracting females with ornaments. Females exercise sexual choice, picking the stronger and more attractive males over the weaker and plainer. Over generations, male weaponry evolves to be more intimidating and male ornamentation evolves to be more impressive. There are two results. First, within each sexual species, males diverge from the female norm. Mature males become more strongly differentiated, compared with females, compared with young animals,