| I found evidence that selection acts on polymorphic pumpkinseed sunfish (Lepomis gibbosus) that inhabit littoral and pelagic habitats within single lakes. Strong parallel patterns of morphological divergence in traits related to habitat and resource use were found among 3 eastern Ontario populations and among 26 populations spanning a wider geographic area, suggesting that selection repeatedly favours habitat-specific forms. To test for selection in one population, I estimated phenotype-related fitness in lake habitats using two measures of fitness that are expected to reflect long-term performance: seasonal reproductive onset and life history performance. Both suggested that more pelagic-like phenotypes had higher fitness than more littoral-like forms regardless of habitat, indicating that selection currently favours planktivorous phenotypes over all others. These findings suggest that the pattern of selection and the phenotypic distribution present in polymorphic populations may be related to the relative abundance of local pelagic and littoral resources, and that trophically related phenotypic polymorphism is itself insufficient evidence to infer disruptive selection is acting in a population. |
