The utility of sex in defending against parasites has a variety of evidence. But every time I read of a study like this, what I wonder is why there aren't more hermaphroditic species, with each individual being both male and female, instead of having separate genders. Many, I think most, plants, are hermaphrodites. Earthworms are hermaphrodites. Some fish change genders over their lifetimes. Some reptiles develop into male or female depending on the temperature during the incubation of their eggs. It would seem that hermaphrodism would have the benefits of both increased fertility and genetic variety.
http://www.sciencedaily.com/releases/2009/07/090723142041.htm
ScienceDaily (July 27, 2009) — The coevolutionary struggle between a New Zealand snail and its worm parasite makes sex advantageous for the snail, whose females favor asexual reproduction in the absence of parasites, say Indiana University Bloomington and Swiss Federal Institute of Technology biologists in this week's Current Biology.
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"Asexual females can have a very large reproductive advantage, as all the individuals in the clone can directly produce offspring," said IU evolutionary biologist and coauthor Curt Lively, who has been working on the system for 25 years. "So evolutionary biologists have long wondered why clonal reproduction does not replace sexual reproduction in natural populations. Our studies suggest that interactions with parasites might be part of the answer, because genetically variable sexual individuals might be more likely to escape infection."
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