[dropcap]”S[/dropcap]ex is hard to explain,” writes Michael Brothurst in a recent article in the journal Science. Like others in his field, Brothurst, who studies the evolution of sexual reproduction at the University of Liverpool, doesn’t “get” men.
“Since males can’t reproduce by themselves and often contribute nothing except genes to their offspring, a population of asexual females can grow at double the rate of a population that reproduces sexually,” he writes.
Why, then, do males exist at all? Why do most plants and animals have two sexes — that is, two sexes who have sex with each other — instead of just one?
The most likely explanation is known as the Red Queen hypothesis, named after the monarch in Lewis Carroll’s “Through the Looking Glass.” In that novella, Alice and the Red Queen hold a race in which they run in place but never get anywhere. Somewhat analogously, the Red Queen hypothesis holds that organisms and the parasites that live on them are running a race in which they constantly evolve in response to each other’s genetic mutations , maintaining an overall balance.
As parasites evolve to take advantage of the weaknesses of a typical host organism, Brothurst explains, host organisms with rare versions of genes, known as alleles, are less susceptible to the parasites, and so stand a better chance of surviving to their reproductive age; likewise, their offspring are endowed with these advantageous alleles. As a result, over generations these organisms’ rare alleles become more common inthepopulation, soparasites start evolving to