Biologists at Indiana University have figured out that sex between two individuals bodes a lot better for survival than reproduction by self-fertilization.

  The new theory of mating, reported in the July 8 issue of the journal Science, offers a possible explanation for why sex exists. By combining the DNA of two parents, sex allows them to produce genetically diverse offspring that are different from their parents. They’re able to survive by  rebuffing parasites that want to kill them off. Species that self-fertilize, by contrast, are more likely to risk infection by parasites and become extinct: The parasites evolve with both generations.

The widespread existence of sex has been a major problem for evolutionary biology since the time of Charles Darwin,” said Levi T. Morran, a post-doctoral researcher and lead author of the study, funded by a grant from the National Space Foundation.

  The National Science Foundation is a federal agency that supports research and education in all the non-medical fields of science and engineering. With an annual budget of about $6.87 billion in fiscal 2010, the NSF funds approximately 20 percent of all federally supported basic research conducted by U.S. colleges and universities.

 It turns out that sex between individuals in a species does not make evolutionary sense, the authors say, because it often involves the production of males. Males don’t directly produce offspring. 

 Self-fertilization is a lot more efficient way to keep a species alive, so evolutionary theory predicts that this method of reproduction should be should be widespread, and sex rare. But this is not the case, as we all know, and the authors set out to understand why.

The study is titled “Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex.” The Red Queen hypothesis, for literary types, takes its name from Lewis Carroll’s Alice in Wonderland: “It takes all the running you can do, to keep in the same place.”

 Cross-fertilization, or sex, keeps populations one step ahead of the parasites that want to wipe them out. The team of scientists used a microscopic roundworm as a host subject  and a pathogenic bacteria to allow them to conduct more than 70 experiments, genetically manipulating the worm’s mating system and exposing it to the parasite.