Why does sex exist? After all, plenty of organisms, from dividing microbes to plants that grow from cuttings, do perfectly well without it. Although researchers can’t say decisively what sex is for, they have now ruled out one common explanation: It’s not for weeding out mutations.
Love in the time of the Cretaceous.
Sexual reproduction is extremely inefficient compared to asexual cloning, because half of a population doesn’t produce any offspring. Puzzling over why organisms bother, some scientists have claimed that sex helps rid a population of harmful mutations. Because sexual reproduction mixes and reshuffles genomes, some offspring might escape with few or none of the genetic errors that burden their parents. This idea, launched in the 1960s by geneticist Hermann Muller, has remained among evolutionary biologists’ pet theories ever since.
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But theoretical models have shown that sex could be beneficial this way only if mutations appeared frequently in a population. Until recently, good estimates of mutation rates have been hard to come by. But the wealth of new gene sequences provided evolutionary geneticists Peter Keightley of the University of Edinburgh, U.K., and Adam Eyre-Walker of the University of Sussex in Brighton, U.K., with enough data for calculating reliable rates. They downloaded sequences of more than 700 genes from organisms ranging from fruit flies to humans and compared genes from closely related species. This revealed genes that had mutated over evolutionary time. For most species, the overall rate of harmful mutations was much less than 0.5 per individual per generation, the researchers report in the 13 October issue of Science–too low to justify sexual reproduction.
The new study is “excellent” and is an important step in understanding the evolution of sex, says James Crow, a geneticist at the University of Wisconsin, Madison. But none of the alternative theories–for instance, that sex generates diversity to help cope with a changing environment–seem entirely adequate either, Crow adds. The mystery of sex still stands.
BY WIRED SCIENCE STAFF
Illustration: Leo Espinosa
Early in Masters of Sex, Showtime’s new Mad Men-era drama about pioneering sex researchers William Masters (a respected ob-gyn) and Virginia Johnson (his secretary turned partner), Masters proposes a seemingly modest scientific inquiry: “What happens to the body during sex?” Over more than three decades of highly charged collaboration, the pair gathered data from thousands of “complete cycles of sexual response”—and also experimented on each other. But even with all the research that Masters and Johnson did and inspired, researchers haven’t stopped asking a lot of basic questions about sex. Here’s what science is still wondering.
How does external stimulation produce an orgasm?
Masters and Johnson outlined an enduring framework for the stages of sex. But scientists are still trying to understand arousal. Some people can think themselves to orgasm; others need zero mental preparation. Debby Herbenick, a sexual health educator at the Kinsey Institute, is studying the phenomenon of exercise-induced orgasm: no partner — and for men sometimes no erection—required.
Why don’t some women orgasm from intercourse?
Women faking orgasms was another of Masters’ obsessions. One recent study found that women who report having more orgasms boast a shorter distance between their clitoris and their vagina. But psychological causes are harder to establish: “If you find a woman who’s happily partnered, you can’t randomly assign her to someone else,” Herbenick says.
How can sex scientists parse the distinction between nature and nurture?
Modern studies of caged monkeys mating and lab-rat vaginas have helped inform the animalistic qualities of our own sex lives. But culture isn’t so easily corralled. (And the most convenient lab rat for sexuality studies—the college student—fails to explain the practices of the wider human population.)
“When sex becomes a human characteristic, it becomes so much more complicated,” says J. Dennis Fortenberry, senior research scientist at Kinsey.
How can scientists re-create sex in a laboratory?
Masters and Johnson conducted research with the help of a motor-powered plexiglass phallus nicknamed Ulysses. Though their gadgets are growing ever more sophisticated, sex scientists are still not sure how their tools affect the act, and therefore the data. One approach: Have women recline in a La-Z-Boy and watch sexually explicit images with a blood-flow-sensing plethysmograph attached to their vaginas. But that’s not something you do every day. Probably.