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Most organisms pass on only half their genes when they reproduce, mingling their genome with that of another. Sexual reproduction not only halves an individual's contribution to the next generation, it leads to all sorts of trouble and expense. Scientists have thus long wondered why sex evolved in the first place. Indeed, as William R. Rice, a biologist at the University of California, Santa Barbara, puts it: "My son only has half of my genes; the other half are from his mother.... However, if I were an asexual female, my offspring would carry all of my genome. I would put twice as many genes into the next generation." Now a study described in the current issue of the journal Science offers new insight.
Popular explanations for the evolution of sexual reproduction hold that sexual recombination of genes allows beneficial mutations to accumulate faster and that it helps to limit the spread of harmful mutations. Rice and UCSB colleague Adam K. Chippindale tested the idea that sexual reproduction also separates beneficial and harmful mutations from each other, allowing good mutations to break free of bad company. Creating artificial populations of asexual fruit flies, the team studied 17 of these asexual populations and 17 populations of normal, sexually reproducing fruit flies for 10 generations. They salted each of the 34 populations with some individuals that had a gene for red eyes instead of white, and arranged for this trait to exert a positive influence on reproductive success. The red-eyed flies thrived in both populations for a few generations, as expected. But the proportion of flies with red eyes eventually ceased to grow in the asexual population. The advantage of having red eyes was balanced by whatever relatively unfit qualities were present in the original carriers of mutation. In the sexual population, however, the red-eye gene continued to penetrate the population.
The researchers predict that given enough generations the red-eye gene would eventually have been present in every individual in the sexual fruit fly population. Moreover, they predict that it would have disappeared in the asexual population. The flies they used varied greatly in reproductive fitness, so each lineage of asexual red-eyed flies was randomly dropped into a background of genetic material that could help or hinder it. In such circumstances, Rice and Chippindale found, even an extremely beneficial mutation cannot itself guarantee reproductive success. Different parts of the genetic background will, over time, determine the survival of a beneficial mutation in an asexual population. Thus, the great benefit of sex is to free good mutations from the limitations of the larger genome in which they occur. By recombining them with other genomes, beneficial mutations are judged by natural selection on their own merits and can emerge into an entire population.