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Planet Earth

The Real Dirty Secret about Sex

Life doesn’t need it, so why do we do it?

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A hammerhead shark in a Nebraska zoo didn’t have sex for three years and still had a pup (the DNA maternity results were confirmed in May). She’s not the first to reproduce without a mate. A virgin Komodo dragon in England laid a clutch that hatched in time for Christmas in 2006. Some female lizards can reproduce without sperm penetrating the egg, and the bdelloid rotifer, a microscopic moss dweller, has been asexual for 85 million years. So why do so many animals bother with sex when asexuality is so much simpler?

Evolutionary biologists tackle this problem like an economist: What is the cost-benefit ratio? Asexuality is quick, requires no special appendages, and produces twice as many offspring per parent as sexual reproduction. Chaste females should be so successful, they’d just crowd out the rest. Males clearly aren’t necessary. Yeast, for example, can skip the romance and just split in two. “It seems perfect,” says McGill University evolutionary biologist Graham Bell. “From an individual point of view, [sex] is a waste of time.” Yet sex is everywhere, the favored mode of reproduction of nearly every vertebrate.

There is plenty of guessing on how sex overcomes its cost, and all the theories involve diversifying the gene pool. Sex tosses out bad genes and brings together good ones. Without sex, the union of Harry and Sally’s beneficial mutations could take millions of years; this way it takes only a one-night stand. Or maybe sex makes siblings so different from each other that at least one of them ought to survive, whatever the future holds. Bell’s own theory, the Red Queen hypothesis, is named after the character in Through the Looking-Glass who keeps running and running just to stay in the same place. Like the Queen’s never-ending workout, the continual genetic renewal of sex helps us to adapt to environments that are always changing.

Whatever the reason for sex turns out to be, men, you’d better hope it’s a good one. Scientists can already make mammal eggs stand in for sperm in a lab. It may be only a matter of time until nature figures it out as well.

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