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.
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.
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.