Evidence for divergence and adaptive isolation in post-glacially derived bimodal allopatric and sympatric rainbow smelt populations

Abstract

The roles of ecological speciation and reinforcement in the formation of contemporary diversity remain contentious. In the present study, we contrast phenotypic and molecular divergence within morphologically diverged bimodal sympatric and allopatric pairs of rainbow smelt, Osmerus mordax. We hypothesize that, in sympatry, evidence of selection associated with resource partitioning will be visible through strong divergence, reinforcement, and greater character displacement. Parallel morphological divergence was observed between the two trophic forms (macrophagous and microphagous), with several examples of greater trait divergence in sympatry than allopatry. Mitochondrial DNA sequence analysis indicated no association between historical clades and morphology; however, Bayesian clustering using microsatellites supported the isolation of these morphs under both allopatry and sympatry. Estimates of genetic isolation were one order of magnitude lower than measures of morphological divergence, consistent with a hypothesis of strong contemporary selection. Using experimental crosses, we obtained similar rates of fertilization success among the allopatric hybrid and pure crosses; whereas, in the sympatric hybrid crosses, fertilization rates dropped by 30-50%, suggesting a clear role for reinforcement through prezygotic incompatibilities. The present study supports the hypothesis that processes of post-glacial radiation and diversification differ between sympatry and allopatry, and indicates a role for reinforcement and ecological processes in recent sympatric diversification.