Darwinism and sex

Since Darwin, sex has been at the very heart of biology. Within Darwinian theory, the winnowing effect of natural selection drives evolution. But natural selection only operates at all because of the variations between organisms thrown up during the processes of reproduction. One source of these variations is the mutation or miscopying of genetic material. Another is the random selection and recombination of genes through sexual reproduction. Sex enables variations that arise in individuals to spread across populations, ultimately shaping whole species.

Sex also introduces a new element of competition into the process of evolution, or rather two. In intrasexual selection, as a rule, male animals – stags, for example, or bull elephant seals – battle it out between themselves for the right to breed. Natural selection thus becomes no longer simply a matter of survival but also of dominance. More remarkably, sex introduces an element of deliberate choice into evolution through intersexual or mate selection. It is still debated whether mate choice acts as a surrogate for natural selection, as Wallace suggested, or a supplement to it, as Darwin argued.

According to Wallace, even this kind of sexual selection is essentially natural selection by proxy, as female and, less frequently, male animals choose their mates on the basis of indicators of their general fitness in the struggle for life. The astonishing results of research on black grouse in Sweden, where the only variable that correlated with the choices made by the females was whether or not the male was still alive six months after mating, lend dramatic support to this hypothesis (Stamp Dawkins 1998: 32–4).