Assortative mating between adjacent populations of threespine stickleback (Gasterosteus aculeatus)

The idea that sexual selection can lead to rapid evolution of premating isolation among independent populations (speciation) has been controversial, but is rapidly gaining acceptance among many evolutionary biologists as empirical examples accumulate. A survey of male signals and female preferences (mate recognition systems, MRSs) across the contact zone between divergent populations of threespine stickleback (Gasterosteus aculeatus) in Conner Creek, WA, revealed a spatial pattern of MRSs that is consistent with speciation via sexual selection. Females from locations that possessed melanic males preferred melanic males whereas females from populations possessing typical mosaic males (red chin, blue iris, and blue-green dorsum) preferred mosaic males. I argue that sensory drive sexual selection, acting through geographically varying spectral properties, is responsible for the observed population differences and premating isolation between the adjacent populations.