Low connectivity between sympatric populations of sunfish ecotypes suggests ecological opportunity contributes to diversification

Ecological opportunities, such as access to a novel habitat or the extirpation of a key competitor, can lead to adaptive divergence by exposing populations to diversifying selection. Typically, effects of ecological opportunity on adaptive divergence are inferred from macroevolutionary patterns rather than tested in populations undergoing contemporary divergence. This limits our insight on how ecological conditions contribute to adaptive divergence. Pumpkinseed sunfish (Centrarchidae: Lepomis gibbosus) have recently and repeatedly colonized a ‘novel’ pelagic habitat in postglacial lakes, and subsequently undergone phenotypic diversification. We investigated whether ecological opportunity has contributed to diversification in a pumpkinseed population that has diversified between lake habitats. We used a between-year mark-recapture study to evaluate whether (1) the novel pelagic habitat represents an ecological opportunity by supporting a large, high-density population, and (2) connectivity between ecotype populations is restricted by limited adult dispersal. We found that phenotypic variation is spatially structured between habitats, similar to prior studies. Submerged shoals in the pelagic habitat do sustain a large adult population at a density seven times greater than the ancestral littoral habitat. Additionally, body condition and size of pelagic pumpkinseed is similar to littoral pumpkinseed. This suggests the pelagic habitat provides an ecological opportunity to pumpkinseed in the form of abundant, available resources. Furthermore, strong between-year habitat fidelity suggests aspects of the ecological opportunity have reduced adult dispersal and could limit gene flow. In combination with prior evidence indicating diversifying selection between habitats, these results provide an example of how ecological opportunity might contribute to contemporary adaptive divergence.