This time from The Scientist :
In 1886, German evolutionary biologist August Weismann proposed that sexual reproduction reshuffles genes to create “individual differences” upon which natural selection acts. Additional ideas have emerged since Weismann’s hypothesis: sex rids the genome of deleterious mutations; sex rapidly introduces beneficial mutations; sex helps organisms dodge parasitic infections. Yet these evolutionary justifications for sex have remained hypotheses because there is not enough evidence to suggest that any of them provide enough of a benefit to surmount the exquisitely high costs of sex, which include the time and energy it takes to find a mate, the passage of only half of one’s genes to the next generation, and the breaking apart of favorable gene combinations. (See “Why Sex?”)
More than 99 percent of multicellular eukaryotes reproduce sexually and have evolved elaborate ways to do so, including behavioral, physiological, and biochemical adaptations. So there must be some enduring benefit. But despite years of observing, theorizing, and experimenting, researchers have been unable to pin down exactly what that might be. “Why sex evolved is very hard to answer,” says Timothy James, who studies sex in fungi at the University of Michigan in Ann Arbor. “[Many] evolutionary biologists are trying to understand why it’s so rarely lost, even though it’s so costly.”
A current trend is to study the few organisms that are capable of reproducing both ways, including yeast and pond (bdelloid) rotifers. The article is most informative about tests done on the various theses but in the end,
And so the paradox of sex lives on. “We still really don’t know the answer to this very most basic question,” says Mark Welch. “We don’t know why sex exists.”
Unless it exists simply because the life forms prefer it. 😉
One problem with studying the few creatures that can reproduce both ways is that the ability exists in such a tiny minority of life forms, it’s not clear that retaining asexual reproduction provides a broad general benefit.
See also: Bdelloid rotifer uses horizontal gene transfer (HGT), dispenses with sex
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