What's so great about sex? From an evolutionary perspective, the answer is not as obvious as one might think. An article published in the July issue of the American Naturalist suggests that sex may have evolved in part as a defense against parasites.
Despite its central role in biology, sex is a bit of an evolutionary mystery. Reproducing without sexlike microbes, some plants and even a few reptileswould seem like a better way to go. Every individual in an asexual species has the ability to reproduce on its own. But in sexual species, two individuals have to combine in order to reproduce one offspring. That gives each generation of asexuals twice the reproductive capacity of sexuals. Why then is sex the dominant strategy when the do-it-yourself approach is so much more efficient?
One hypothesis is that parasites keep asexual organisms from getting too plentiful. When an asexual creature reproduces, it makes clonesexact genetic copies of itself. Since each clone has the same genes, each has the same genetic vulnerabilities to parasites. If a parasite emerges that can exploit those vulnerabilities, it can wipe out the whole population. On the other hand, sexual offspring are genetically unique, often with different parasite vulnerabilities. So a parasite that can destroy some can't necessarily destroy all. That, in theory, should help sexual populations maintain stability, while asexual populations face extinction at the hands of parasites.
The scenario works on mathematical models, but there have been few attempts to see if it holds in nature.
Enter Potamopyrgus antipodarum, a snail common in fresh water lakes in New Zealand. What makes these snails interesting is that there are sexual and asexual versions. They provide scientists with an opportunity to compare the two versions side-by-side in nature.
Jukka Jokela of the Swiss Federal Institute of Aquatic Science and Technology, Mark Dybdahl of the Uni
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