such as "Introduction", "Conclusion"..etc
July 31, 2009 — 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 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. 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.
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 University of Washington and Curtis
Lively of Indian 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.
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
"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."
So we may well have to thank parasites—in spite of their nasty reputation—for the joy of sex.
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