Sex, death, and the red queen

Sex is hard to explain. Since males can't reproduce by themselves and often contribute nothing except genes to offspring, a population of asexual females can grow at double the rate of a population that reproduces sexually (1). Why then, given this “cost of males,” do most plants and animals indulge in biparental sex? One possible solution is that sex accelerates adaptation; the Red Queen hypothesis, for example, proposes that sex gives plants and animals an edge in the never-ending battle against their coevolving parasites (2–4). Although researchers have collected empirical field data consistent with the Red Queen hypothesis from a range of natural host-parasite systems, direct experimental evidence that coevolving parasites select for sex in their hosts has proven elusive. On page 216 of this issue, Morran et al. (5) pin down some of that direct evidence. In laboratory experiments, they grew several populations of nematode worms, some with and some without a bacterial parasite, to provide the most definitive support yet for the Red Queen's answer to why sex evolved.