Bacteria don't do it, fungi don't do it, certain sea cucumbers don't do it--have sex, that is. These organisms reproduce asexually, and their method makes our own reproductive strategy seem a bit clunky. Why go to all the trouble of sex when asexual reproduction is so much more efficient? One popular explanation holds that sexual reproduction reduces the number of harmful mutations that can accumulate in a genome. By that reasoning, the higher the rate of dangerous mutations, the more advantageous sexual reproduction should be. Peter D. Keightley of the University of Edinburgh and Adam Eyre-Walker of the University of Sussex set out to test that hypothesis by calculating mutation rates in a number of species. And the results of their study, published today in the journal Science,call the mutation-reduction idea into question.

Examinations of sexually reproducing species ranging from fruit flies to humans revealed that each of the organisms appears to have a lower mutation rate than that necessary to benefit from sexual reproduction, "suggesting that sex is not maintained by its capacity to purge the genome of deleterious mutations," Keightly and Eyre-Walker report. Another widely held explanation--namely that sexual reproduction enables organisms to adapt more quickly to changing environments--may fare better. "While this [study] leaves many other hypotheses to test," they write, "they all share a common feature: it is adaptive evolution that principally drives the evolution of sex, perhaps in combination with other mechanism."