For more than 25 years, Robert Vrijenhoek has been returning to the remote hills of the Sonoran Desert in Mexico to study two unique species of minnows: one an asexual reproducer and the other a sexual reproducer. One of the first things Vrijenhoek observed in his studies was that the sexual species outnumbered the asexual species, making up 60 percent to 80 percent of the total fish population. He wanted to know why this was so.

One of Vrijenhoek's first discoveries in his search for an explanation was that 40 percent of all minnows were infected with a parasite that causes black spot disease. Upon closer investigation, he observed an interesting pattern: Most of the parasitized fish were asexual reproducers. Why should they be more parasitized than the sexual reproducers they were living right beside? he wondered. The answer, he thought, might lie in what evolutionary biologists call the Red Queen hypothesis.

In Lewis Carroll's story Through the Looking Glass, Alice and the Red Queen are running as fast as they can, and Alice says, "Isn't this curious, as fast as we run, nothing seems to change. We're staying in the same place." The Red Queen answers, "Yes, you have to run just as fast as you can to stay in the same place."

The process of evolution is a similar kind of race, in that all organisms live in a complex world full of parasites, viruses, bacteria, predators, and competitive species -- all of them evolving. According to the Red Queen hypothesis, at the moment any species stops evolving in response to these challenges and threats, it risks extinction.

In Vrijenhoek's minnow study, the asexual minnows have all but stopped evolving. They are genetic clones of each other and an easy target, especially for a short-lived, quickly evolving parasite. Sexual reproduction creates variability in sexual minnows' offspring so that the parasite cannot easily adapt to them. That is the value of sexual reproduction.

It is this variability in individuals that allows those best adapted to their environment to survive and reproduce to create future populations. Thus, the sexual minnow population, with its variability, was better adapted to resisting the parasite than was the asexual population. This is a clear demonstration of the process of natural selection -- the primary mechanism of evolution in populations.