As singles-bar denizens know, the ratio of males to females can get dramatically skewed in certain populations. The same thing can happen in nature but, as researchers have found, nature has a way of evening things out.

How? According to one controversial theory, when either sex gets in short supply, its per-capita contribution to the next generation's gene pool increases, thus spawning disproportionate numbers of the minority sex until the ratio finally returns to 1-to-1.

It has been difficult to test this theory because many species have gene-transfer systems that always produce a 1-to-1 ratio. But researchers from the State University of New York at Stony Brook's Marine Sciences Research Center recently confirmed the "frequency-dependent" effect by studying a fish called the Atlantic silverside.

Two factors determine sex in that species: inherited genes and water temperature during a critical period of early development. Cooler waters, such as occur early in the spring-summer breeding season, cause more of the newly hatched offspring to become female; later warmer waters turn more into males.

Researchers isolated several groups of the fish and kept each tank at a constant high or low temperature, producing initial sex-ratio imbalances as high as 4 to 1. After each subsequent mating period, the offspring were exposed to the same high or low temperature, which should have continued to have its ratio-skewing effect. But within four to six generations, the authors report in the Dec. 14 Science, something changed in the fish physiology so that both hot and cold groups had returned to sexual parity.