Rapid evolution and plasticity of genitalia

Genital morphology exhibits tremendous variation and is intimately linked with fitness. Sexual selection, nonmating natural selection, and neutral forces have been explored as potential drivers of genital divergence. Though less explored, genitalia may also be plastic in response to the developmental environment. In poeciliid fishes, the length of the male intromittent organ, the gonopodium, may be driven by sexual selection if longer gonopodia attract females or aid in forced copulation attempts, or by nonmating natural selection if shorter gonopodia allow predator evasion. The rearing environment may also affect gonopodium development. Using an experimental introduction of Trinidadian guppies into four replicate streams with reduced predation risk, we tested whether this new environment caused the evolution of genitalia. We  measured gonopodium length after rearing the source and introduced populations for two generations in the laboratory to remove maternal and other environmental ef...