Natural selection and the evolution of replicated trophic polymorphisms in pumpkinseed sunfish (Lepomis gibbosus

ABSTRACT Diversifying selection is expected to operate through all phases of adaptive divergence. If selection is not shown to be diversifying at the earliest stages of divergence when phenotypes are minimally differentiated, then this theory can be challenged. We test for evidence of diversifying selection between lake environments in the nascent divergence of pumpkinseed sunfish (Lepomis gibbosus), a divergence that is embedded in the apparent adaptive radiation of North American sunfishes. Pumpkinseed individuals primarily inhabit and appear partially adapted to either inshore littoral or offshore pelagic habitats and resources, the same environmental axes that differentiate various sunfish taxa. We first demonstrate associations between phenotype, lake habitat and diet between ecomorphs in three populations from the Mazinaw region of Ontario. This confirms that the range of pumpkinseed trophic polymorphism extends beyond the Adirondack region of New York State. Second, we show a replicated pattern of divergence between ecomorphs across 26 populations in Ontario and New York, for four of ten external body form traits predicted to influence habitat-specific swimming and foraging performance. Selection is implicated because replicated divergence among populations at this larger regional scale is unlikely to have arisen through random processes. Plastic responses to environmental conditions contribute to body form variation in sunfish, but preliminary evidence from other studies suggests that it is the plastic developmental system that has begun to diverge between ecomorphs