The phenomenon is known as "negative frequency-dependent selection," and has been widely accepted in theory but has not been widely tested through experimentation. Previous experimentation had been in laboratory situations, not in the wild.

Reznick has co-authored a paper with former post-doctoral researcher Kimberly A Hughes, now an assistant professor at the University of Illinois, Urbana, which shows how mate selection by female guppies for mates with rare color patterns keeps the gene pool deep.

The paper, titled, Frequency-Dependent Survival in Natural Guppy Populations, recently published in the journal Nature, puts the theory to the test and showed that, indeed, preference for males with rare color patterns-- a reflection of a rare phenotype in the population -- maintains broad genetic variation among the fish in any given natural population.

"This paper for the first time shows that this inferred feature, which we've accepted as a matter of faith, has been put through experimentation and replicated and that now there's nothing to take on faith," Reznick said.

This preference for the rare is thought to be self regulating mechanism that keeps uncommon phenotypes from disappearing and maintains genetic variation.

The research was carried out on three natural guppy populations in Trinidad with 34 separate experiments on populations in 19 separate pools along three streams. The work was carried out over four years -- 1996, 1999, 2003 and 2004. It includes the contributions of University of Illinois, Urbana colleague Robert Olendorf; Helen Rodd of the University of Toronto; Anne E. Houde of Lake Forest College, Ill.; and Carla Hurt of the Smithsonian Tropical Research Station, Panama City, Panama.

Reznick's role in the research emerged from his long-standing study of guppy populations in the rivers and streams of Trinidad and his development of the methods used to study wild fish populations.

So what does this say beyond the guppy world?

"It suggests that frequency dependent selection can serve as a general mechanism for sustaining genetic variation in natural populations since it has been experimentally demonstrated to be effective in a new context and has been shown to be effective in a natural setting," Reznick said.

The research was supported by the National Science Foundation's Division of Environmental Biology, Population Biology Program.

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