Imagine an animal that reproduced by budding off genetically identical clones. This asexual creature doesn’t have to bother with finding or attracting mates: it is a self-contained factory for making more of itself. This sounds like a recipe for success, but asexual animals are far from successful. They exist, but they tend to be rare and precarious twigs on the tree of life—recently evolved, and likely to snap off at any time. By and large, the vast majority of animal life practices sex.

The asexual lifestyle falters because it presents a sitting target. If every new generation is genetically identical to the last, then predators, parasites, and rivals can easily evolve ways of outmanoeuvring them all.

Sex mixes things up—literally so. When