And while we are on the subject of sexual selection…

Social mobility, families, breeding, genetics, the persistence of social classes, lead me to the subject of sex. As what doesn’t?

You will recall that Charles Darwin published the Origin of Species in 1859. You will probably not recall that he published another massive tome twelve years later entitled The Descent of Man, or Selection in Relation to Sex. In it, Darwin argued for a different force of evolution, the selection of males by females and of females by males. I have been amazed by the number of people who rail for or against Darwin who remain completely ignorant of his second book, in which he argues for a force of evolution that is a) non-random, or directed, b) mutual as regards the sexes and c) because it is not random,  therefore efficient, rapid and sharing none of the characteristics of the first method proposed in Origin of Species.

I confess I don’t get it – why the second theory has been  politely ignored by most people, Darwinists and anti-Darwinists alike.

Entire careers seem to have been built defending or opposing Darwin – as if he had written only one book, proposing one theory, when in fact he wrote several, and proposed at least two -maybe more – independent theories of how evolution occurs. One theory -natural selection – does not contradict the other; each operates. But  one theory offers an efficient method of selection by members of the species itself of who should breed and who should not.

I have heard two British biologists explain away the second book by saying that since sex is natural, then sexual selection must be just another aspect of natural selection, and therefore the gap between the two theories can be papered over. Whistling past the graveyard, in my opinion.

Geoffrey Miller covered this phenomenon in his marvellous book, The Mating Mind.

Darwin’s second theory has been taken up by other biologists of late. I came across this article in, called DUCK SEX, AESTHETIC EVOLUTION, AND THE ORIGIN OF BEAUTY, by Richard Prum. With a title like that, I was powerless to resist.

Sexual selection was distinct from natural selection in that it had to do with differential reproductive success. Not survival up until the moment of mating, but differential access to mates as a result of two possible mechanisms: One was male-male competition or competition within the sex, the other was female choice or mate choice of the one sex for members of the opposite sex. Darwin elaborated and predicted how male-male competition should give rise to armaments like antlers and large body size like elephant seals, and that nature should give rise to ornaments like birdsong, beautiful bird plumage and many other ornamental features. Darwin used explicitly aesthetic language to describe his theory. He described the mating preferences of birds as standards of beauty. He described female birds as having an aesthetic faculty. He described birds as the most aesthetic of all organisms, excepting of course man, and he was greatly criticized at the time.

In fact, his theory implied that female aesthetic judgments were a major force in evolution, and that was countered immediately by misogynistic responses who described female choice as “vicious feminine caprice.” In those days vicious meant full of vice. In other words, it was even immoral, this theory. In particular, Darwin was criticized for proposing that there was some other theory that might explain evolution other than natural selection, that the power of natural selection was its capacity to explain everything and to be a universal explanation to the origin of biodiversity.

Prum explains the abandonment of sexual (mate) selection as a legitimate theory under the influence of Alfred Russel Wallace, the biologist who is co-author of the theory of natural selection. It is fascinating reading for those who interested in the suppression of ideas, or fashions in theory. Much can be explained by a Victorian squeamishness about the idea that female selection of males was normal.

My interest here is what has happened in the human species as a result of female selection of males: to what ends have we been selected? Prum writes:

I’m interested in the possibility that aesthetic mate choice in humans—female choice—could have played a critical role in the remodeling of male-male competition, essentially by establishing that those features of males that are associated directly with violent competition are unsexy, or more positively, that those features that are associated with advancing female autonomy evolved to be a new from of sexy. That is the kind of dynamic interaction you get between sexual conflict and aesthetic mate choice that we see in birds like bowerbirds and lekking birds and throughout the bird world.

What would these traits be? Well, one of the interesting things is that even though human beings evolved to be much larger than their chimp-like ancestors in body size, they actually have gotten less different in size. Males and females are more similar in size than are chimpanzees. This is exactly against the laws of allometry, which indicate that as you get bigger any differences between the sexes should get broader. That means there’s been active selection to reduce the difference in body size between males and females, and that’s very likely to have evolved through female mate choice.

Prum speaks of sexual selection as being based on an aesthetic agency, and it is true as far as it goes. But female selection of human males goes further; it selects on the basis of moral qualities. One of the qualities selected for, he writes, is  the capacity of males to get along with each other. Selecting on this basis over time has meant that males do not kill the children sired by the previous mate of a female, when the latter remarry/remate. We have been gentled sexual selection.

Other theories have been advanced why life has become  less violent  – notably Steven Pinker’s reworking of Norbert Elias – Prum’s ideas on sexual selection are worth a close reading. The thought that males and females actually select one another, and have been doing so for aeons, and that this drives human evolution, should be a cause of hope.