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Sex Evolved as an Escape From Parasites, Study Suggests

Why is sex the dominant form of reproduction on the planet? Scientists think they know why–and it all has to do with evasion of parasites.

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NGS photo of elephants mating by Michael Nichols

Sex may have evolved in part as a defense against parasites, an article published in the July issue of the academic journal American Naturalist suggests.

“Despite its central role in biology, sex is a bit of an evolutionary mystery,” says a news release about the article.

“Reproducing without sex–like microbes, some plants and even a few reptiles–would 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.

NGS photo of water spiders mating by Robert Sisson

“When an asexual creature reproduces, it makes clones–exact 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.”

There have been few attempts to see if this hypothesis holds in nature, according to the article.

“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.”

NGS photo of Gelaba baboons mating by Michael Nichols

Jukka Jokela of the Swiss Federal Institute of Aquatic Science and Technology, Mark Dybdahl of the University of Washington and Curtis Lively of Indiana University, Bloomington began observing several populations of these snails for ten years starting in 1994. They monitored the number of sexuals, the number asexuals, and the rates of parasite infection for both.


NGS photo of ladybugs mating by Robert Sisson

The team 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.

“Meanwhile, 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,” Jokela said. “These results suggest that sexual reproduction provides an evolutionary advantage in parasite-rich environments.”


NGS photo of giraffe mating by Michael Nichols 

More from National Geographic News:

Prehistoric Undersea Sex Was No Fluke

Study Links Origin of Sexual Reproduction With High Mutation Rates

“First Sex” Found in Australian Fossils?

Sex Speeds Up Evolution, Study Finds


  1. Spencer
    January 1, 2:29 am

    What would constitute an environment that is not parasite rich? Is there such a thing as a place where lie exists, that is not parasite ricb? it seems to me, that wherever there is life, there are parasites hence, it is not possible to find a non-parasite rich environment in which to test, because parasites are everywhere that life can be found

  2. jaylen
    boston ma
    May 4, 2012, 12:06 pm

    what about people? or other animals?