WILD THINGS

Why do animals have sex, anyway?

- ELIZABETH KOLBERT

Across the natural world, reproduction resists tidy categories and easy moral lessons.

Darwin found sex a mystery. He was perplexed not by the practice—he and his wife, who also happened to be his cousin, had ten children—but by the point of it. Darwin spent the last part of his life experimenting with plants, and he was particularly intrigued by cowslips, whose flowers come in two very different forms. Each form can fertilize itself, yet each requires the services of the other in order to be, in Darwin’s words, “perfectly fertile.” Why should nature insist on such “intercrossing of distinct individuals?” he asked. And why bother with sex at all, when, via processes like budding or fission, it was evidently possible to multiply without it? “We do not even in the least know the final cause of sexuality,” Darwin lamented in a paper he published on cowslips in 1862. “The whole subject is as yet hidden in darkness.”

In Darwin's day, of course, the processes by which gametes are formed and then fused had yet to be discovered. As more was learned about genes, chromosomes, and meiosis, the mystery of sex only deepened. In the nineteen-seventies, the British evolutionary biologist John Maynard Smith wrote an entire book about the puzzle. In it, he proposed a thought experiment. Suppose a chance mutation in a species granted a random female the ability to reproduce all on her own, through parthenogenesis. (Many lizards do, in fact, reproduce this way.) This single mom would give birth only to daughters, who would produce only daughters, and so on. By dispensing with sons and lovers, the tribe of sexless females would double its rate of reproduction and so, quite quickly, take over. Maynard Smith labelled this the “twofold advantage of parthenogenesis”; it’s often, more provocatively, referred to as the “twofold cost of sex.” Given the math, sex must confer some important advantage. But what is it?