Why Sex Survives

The Red Queen race may explain one of biology's old puzzles: why sexual reproduction is so common when it is so costly. Making a baby with a partner means passing on only half your genes, yet most complex species do it anyway.

The payoff is variety. Shuffling two parents' genes produces offspring that are all slightly different, a moving target that parasites tuned to last year's host can no longer lock onto. Sameness is what parasites exploit.

Proof in a Snail

New Zealand's freshwater mud snails put the idea to the test. Some populations reproduce sexually; others are all-female clones. In lakes where a worm parasite was common, the clones got hammered, because every clone shared the same lock and the parasite already had the key. The sexual snails held on, their reshuffled genes keeping the parasite guessing. The arms race was visible in a single lake.