By studying systems in their earliest stages of differentiation, we can learn about the evolutionary forces acting within and among populations and how those forces could contribute to reproductive isolation. Such an understanding would help us better discern and predict how selection leads to the maintenance of multiple morphs within a species, rather than speciation. The postglacial adaptive radiation of the threespine stickleback (Gasterosteus aculeatus) is one of the best-studied cases of evolutionary diversification and rapid, repeated speciation. Following deglaciation, marine stickleback have continually invaded freshwater habitats across the northern hemisphere, establishing resident populations that diverged innumerable times from their oceanic ancestors. Independent colonization events have yielded broadly parallel patterns of morphological differences in freshwater and marine stickleback. However, there is much phenotypic diversity within and among freshwater populations. We studied a lesser-known freshwater ‘species pair’ found in southwest Washington, where male stickleback in numerous locations have lost the ancestral red sexual signal and instead develop black nuptial coloration. We measured phenotypic variation in a suite of traits across sites where both red and black stickleback do not overlap in distribution and at one site where they historically co-occurred. We found substantial phenotypic divergence between red and black morphs in non-color traits including shape and lateral plating, and additionally find evidence that supports the hypothesis of sensory drive as the mechanism responsible for the evolutionary switch in color from red to black. A newly described “mixed” morph in Connor Creek, Washington differs in head shape and size from the red and black morphs, and we suggest that their characteristics are most consistent with hybridization between anadromous and freshwater stickleback. These results lay the foundation for future investigation of the underlying genetic basis of this phenotypic divergence as well as the evolutionary processes that drive, maintain, or limit divergence among morphs.

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