On Valentine's Day, what better subject to tackle than sex? Well, maybe love, but that's not what gets evolutionary biologists all hot and bothered. Sex is where it's at -- the battle between the sexes. Males and females interact like two superpowers engaged in an arms race -- each escalation in arms is matched by escalation on the other side. A relationship of apparent mutual harmony might in fact be more one of mutually assured destruction.

John Maynard Smith at the University of Sussex said it was an "evolutionary scandal" that biologists still couldn't explain the purpose of sex -- why males and females mix DNA. After all, many species of insects, amphibians and reptiles reproduce without having sex -- they clone themselves. Mark Ridley of the University of Oxford called sex "the ultimate existential absurdity."

The problem is this: Sexual reproduction forces us -- and all other sexual organisms -- to reproduce at half the rate that we could by parthenogenesis, that is, by cloning. Half the rate, because sexual organisms have to waste half their resources in making males -- clonal organisms like aphids don't need sperm, so they just produce females. Since natural selection automatically favors organisms that reproduce most efficiently, why haven't clonal reproduction methods replaced sexual ones? There must be some advantage to sex, that offsets the numerical advantage gained by cloning. Just what that advantage is remains the subject of intense research.