The blue mussels Mytilus edulis and M. galloprovincialis hybridize in southwestern England. Within this hybrid zone environmentally based directional selection favors individuals with alleles specific to M. galloprovincialis. What forces are countering this directional selection and allowing for the maintenance of a stable hybrid population are unknown. We used both the genetics of recently settled larvae and a fine-scale model of the physical oceanography of the region to determine the patterns of larval dispersal throughout the hybrid zone and the bordering parental populations. Evidence from both the model and the genetics suggests that the hybrid zone lies between two barriers to dispersal. Start Point separates the M. edulis population from the hybrid zone and allows minimal dispersal from the hybrid zone into the M. edulis population, but none in the other direction. Likewise, the M. galloprovincialis populations along the northern coast of Cornwall regularly receive immigrating larvae from the hybrid zone, but larvae from the M. galloprovincialis population do not enter the hybrid zone. However, larvae settling at hybrid zone sites have high frequencies of alleles specific to M. edulis, suggesting that reproductive barriers, selection in the larval stage, or gene flow from an undetermined source is effectively balancing the directional selection observed in the adults.
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Vol. 57 • No. 5