Why sex has evolved is an age-old problem. The prevalence of sexually reproducing species indicates that this mode of reproduction is superior to the asexual, non-recombining kind, but none of the proposed models for why sex evolved — that sexual recombination brings together beneficial mutations, purges the genome from harmful mutations or frees mutations from the burden of their genetic backgrounds — have been entirely satisfactory. The relative merits of each model have largely been fought on theoretical grounds; however, now Rice and Chippindale provide some welcome experimental backing for the last of these theories. By following the fate of a beneficial mutation in Drosophila strains, they have found that it is much more successful in recombining than in non-recombining lines, indicating that sexual recombination is required to bring new beneficial mutations into genetic backgrounds in which they can thrive.

The thinking and the experimental design behind the experiments in this paper are as follows. A mutation that arises on a non-recombining background has little say on the genetic company in which it finds itself. This is bad news for a mildly advantageous mutation, which can only hope to be fixed (that is, achieve a frequency of 100%) if it arises in a genome at the higher end of the reproductive fitness scale. If it doesn't, the advantage conferred by the mutation will be swamped by the overall low fitness of the genome in which it occurs. The same mutation occurring in a sexually reproducing genome, however, has more opportunities to sample different genetic environments — those produced through recombination — and to land in ones in which its success is unhindered by an unfavourable genetic background. This idea predicts that the frequency of a beneficial mutation will increase when propagated in a recombining versus a non-recombining lineage.

Using Drosophila, the authors followed the success of a beneficial white + allele (conferring red eyes) as it was passed down through ten generations of white− (white-eyed) flies that were either recombining or non-recombining. Non-recombining flies were obtained by manipulating the female genome so that it propagated itself clonally (Drosophila males normally do not recombine). The experiment — repeated 34 times — invariably showed that the white+ allele accumulated more rapidly in the sexual population, supporting the idea that sex is advantageous because it accelerates adaptive evolution.

Needless to say, the story doesn't end here. As Richard Lenski points out in an accompanying Perspective, two puzzles surround why sex evolved: why sexual reproduction came about in the first place and why it is maintained. This paper has found an answer (although probably not the only one) to the second of these questions, and also shows how incisive experimental design can complement theoretical studies of long-standing evolutionary issues.