Why does sex exist?

Birds and bees do it, but to scientists who study evolution, sex seems wasteful, since many species that reproduce asexually get along just fine without it. In the evolutionary battle to survive, asexual species, whose members can clone themselves, would seem to have an advantage over sexual animals -- like people -- that take two to reproduce.

In fact, while asexual reproduction is efficient, it also carries risks for survival of the species, according to a report in the March 16th issue of the journal Nature.

To test their theories, the researchers created a model in which hypothetical species -- sexual and asexual -- competed for the same resources. The findings suggest that variety may indeed be the spice of life, according to the lead author of that report, Dr. C. Patrick Doncaster, of the University of Southampton in England.

Asexual organisms, since they reproduce by forming copies of themselves, tend to be very homogeneous, he told Reuters Health. At first, they grab all the available resources due to their ability to reproduce more quickly and reach greater numbers than sexual species.

However, once their numbers reach a certain level, competition within the species appears to keep them from pushing out sexual species, according to Doncaster. He also noted that asexual species are more likely to live off a single source of food -- and therefore have a higher risk of starvation -- than sexual species, which usually rely on several different types of food. Sexual species are more varied in general, which offers advantages in the struggle against the more abundant asexual species.

As an example from nature, Doncaster cited a type of lizard that lives in the Arizona desert. Some of the species of lizard reproduce sexually while others reproduce asexually. Since asexual reproduction is more efficient, allowing a population to double in a generation, it would make sense that the asexual lizards would win out over the sexual ones. But the asexual lizards tend to eat a narrower selection of foods than the sexual ones, so they do not compete with each other, according to Doncaster.

"I don't think our study has any direct consequence for mankind," Doncaster told Reuters Health. "It just helps explain why sex is so ubiquitous in the natural world."

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