Sex may have evolved in part as a defence against parasites, suggests a research article.
Published in the journal American Naturalist, the article highlights the fact that when an asexual creature reproduces, it makes clones - exact genetic copies of itself.
It further point out that each clone has the same genes, and, consequently, the same genetic vulnerabilities to parasites.
Sexual offspring, on the other hand, are genetically unique, often with different parasite vulnerabilities. That is why, says the write-up, a parasite that can destroy some can't necessarily destroy all.
In theory, that should help sexual populations maintain stability, while asexual populations face extinction at the hands of parasites.
These propositions are based on several pieces of research on Potamopyrgus antipodarum, a snail common in fresh water lakes in New Zealand which has both sexual and asexual versions.
Jukka Jokela of the Swiss Federal Institute of Aquatic Science and Technology, Mark Dybdahl of the University of Washington and Curtis Lively of Indian University, Bloomington began observing several populations of these snails for ten years starting in 1994.
The researchers monitored the number of sexuals, the number asexuals, and the rates of parasite infection for both.
They found that clones that were plentiful at the beginning of the study became more susceptible to parasites over time.
As parasite infections increased, the once plentiful clones dwindled dramatically in number. Some clonal types disappeared entirely.
However, sexual snail populations remained much more stable over time. This, the authors say, is exactly the pattern predicted by the parasite hypothesis.
"The rise and fall of these female-only lineages was surprisingly fast and consistent with the prediction of the parasite hypothesis for sex. These results suggest that sexual reproduction provides an evolutionary advantage in parasite rich environments," Jokela said.