From The Economist:
Guilt-free intercourse may, as Philip Larkin wrote, have begun in 1963, but sexual reproduction has been around a good deal longer than that. Single-celled organisms began exchanging and mixing up genetic information in ways modern biologists recognise as rudimentary forms of sex about two billion years ago. Yet the question of why sex exists at all remains troublesome. A creature which reproduces asexually passes on all of its genes to each of its progeny. One that mates with another, by contrast, passes on only half of them. On the face of things that is a huge selective disadvantage. There must therefore, evolutionary biologists believe, be equally huge compensating benefits.
Two ideas exist about what these might be. One is that the constantly changing genetic variety sex creates stops parasites and pathogens evolving stable techniques for exploiting a host species. This is the “Red Queen” hypothesis, an allusion to a character in “Through the Looking-Glass” who had to run as fast as she could to stay in the same place. The other idea is that the continual mixing of genes from generation to generation separates good and bad mutations, permitting the bad ones to be purged by natural selection without taking the good ones along for the ride. This process was described by Joel Peck, one of its progenitors, as plucking rubies from rubbish.
“Plucking rubies” and the “Red Queen” are not mutually exclusive. Both could be true. But, while the queen has experimental evidence to back her up, rubies have had little such validation. Until now.