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Naturalist Charles Darwin's groundbreaking theories turned the Victorian science community on its head and forever changed the way people look at nature. Evolution's most notable proponent came to what even he acknowledged was the improbable conclusion that females were the driving force in mating behavior. Darwin noted that the vivid colors, long tails, plumes, horns and antlers of males of many species did nothing to enhance survival. He theorized that the purpose of male ornamentation was to deter rival males and attract females, whose powers of discrimination were so refined that they chose males based on their species' standard of beauty. The research of Assistant Professor of Behavioral Ecology Molly Morris in the area of sexual selection is proving Darwin's theories correct to an extent he never could. Morris' work with swordtails and platyfish tests Darwin's theory that females are the choosy sex. The researcher is focusing on a pigment pattern of inky, black bars on the pale, silvery male swordtails. The vertical bars darken when the fish are courting females or fighting other males. "It's a signal they use with other individuals of their species to attract females and to deter rival males," the researcher says. "They can turn the pigment on and off, move the pigment around to make the bars really dark or make them disappear." Morris says not all species have the pigment pattern, and, of those that do, some have fat bars, some have skinny bars and some appear to use the bars in different ways. The walls of Morris' lab are lined with fish tanks, the most intriguing of which is one she uses to test the females' sexual preferences. Videotaped images and animations of males with and without bars are shown on monitors at the end of the tank, making it seem as though the males are swimming alongside the females. "I can actually look at their preferences for bars and rule out all the other aspects of male behavior or other characteristics," Morris says. "It's nice because I don't have to have as many fish in the lab, and the videos are always ready to go and do what you want them to do. We tested it and got the same results as we would have with live males." Those results, she says, indicate that females prefer several different components of the bars. Some provide information about male quality and others help females mate with a male of the right species. Morris also has found that the female fish prefer males with symmetrical traits. In that sense, it seems the female swordtail's concept of beauty is not that different from that of humans. Recent investigations of human behaviors indicate facial symmetry and attractiveness influence the selection of mates. So, one might ask, do the mating preferences of swordtail fish have any relevance to the mating habits of humans? "Generalizations can be made across organisms, but you have to be careful when making comparisons from animals to humans," Morris says, "especially when the animals you are studying have a very different mating system from ours. One thing we do know is that both humans and swordtails prefer mates that are symmetrical." Morris acknowledges a longtime fascination with organisms like the swordtails and platyfish that occupy her lab -- and her thoughts. Perhaps, like Darwin before her, she is poised to change the way we think about nature. Contributors to this package: Susan Green is a writer and Rick Fatica is a photographer for University Communications and Marketing. Andrea Gibson, BSJ '94, and Kelli Whitlock, MSJ '01, are writers for the Office of Research Communications.