Biologist Robert Vrijenhoek has been studying the Mexican Poeciliid fish for more than 30 years. Some species of Poeciliopsis reproduce sexually while others reproduce asexually. Vrijenhoek found that the genetic diversity produced by sexual reproduction allowed the sexual fish to survive a parasite more successfully than the asexual fish. Invoking the Red Queen hypothesis, Vrijenhoek suggests the sexual populations are able to keep up or adapt to new selective challenges, while the asexual populations, essentially clonal, are not. From Evolution: Why Sex? Also featured: Meredith Small.