Biophysical and Behavioral Agents of Natural Selection in a Hybrid Zone

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The structure of hybrid zones depends on the interaction of natural selection and dispersal. Simple models using different forms of natural selection have been generated to explain the structure and maintenance of hybrid zones. However, field studies have shown that natural selection can be very complex and that an understanding of hybrid zones cannot be obtained through simple models. It is necessary to determine the agent of natural selection and the ecological circumstances under which it operates to understand how hybrid zones are formed and maintained. This study proposes to determine the selective forces operating in a hybrid zone formed between two species of marine mussels. Results from a previous grant demonstrated that the two species differed in their physiological capacity to tolerate elevated silt. One possible adaptation to elevated silt is increased mobility so that the animal can move and avoid smothering. This adaptation may be detrimental in open coast environments where silt concentrations are low and mussels may be dislodged by exposure to increased wave action. Mussels that are more mobile may be exposed to greater wave action if they migrate to the top of mussel beds or into other locations where they are more likely to be dislodged. This study will test the hypothesis that these two species of mussels differ in their behavioral adaptations to silt and that these differences result in strong natural selection against the more mobile species in open coast environments.