Alas, the male platy fish has no sword. But when biologist Alexandra Basolo attached a black and yellow sword to a little platy's tail fin, the female platy went wild. Time after time, the male platy with a phony sword beat out his swordless rival for the female's affections. Why?

The experiment is an attempt to unravel the forces that drive evolution. Charles Darwin first proposed that sexual selection led males to develop elaborate traits, such as the peacock's feathers, that appear to increase the male's ability to acquire mates while decreasing his ability to survive.

There are several theories about why this occurs. One holds that such traits signify good genes, meaning the female that mates with the male with the grandest feathers will have the healthiest offspring.

Another theory holds that if a few females prefer mates with traits, then those two characteristics -- the preference and the trait -- will evolve together in runaway fashion, eventually making both the norm.

Basolo and others now maintain that there is a third explanation: Females have a pre-existing bias for traits. The female platy had no experience with swords. She just prefers them.

"There appears to be a bias in the sensory system that can direct the course of evolution," said Basolo, of the University of California at Santa Barbara, whose report appears in the current issue of the journal Science.

In the case of the platy, a popular aquarium fish, the bias may have caused the evolution of the species' close cousin, the swordtail.