What’s sex all about? This question has been exercising biologists since well before Williams’s time, but in the 1970’s, with the rise of ‘gene-centrism’ and the related controversy over group selection, a succession of prominent authors grappled with the problem, trying to fit it with current evolutionary theory to no-one’s particular satisfaction. Males were deemed an impediment to a female’s efforts to maximise her reproductive output, time wasted on these feckless types resulting in her only passing on 50% of her genes per offspring. From the perspective of a ‘selfish gene’, meanwhile, getting into every offspring seems a preferable fate to only getting into half of them. On the basis of these apparent large costs, a cryptic offsetting benefit of corresponding magnitude was assumed. Like Godot, it is yet to appear.

Yet sex is widespread. All eukaryotes either do it now, or possess tell-tale signs that their recent ancestors did. Given that it appears costly to individuals, and genes, how did it evolve and why does it persist?