Gene flow across a changing environment can counter natural selection and prevent adaptation; this may have profound consequences for the way that different species are distributed in space. The proposed research will examine the effects of gene flow on adaptation to, and distribution across, a strong environmental gradient, using the alga Silvetia compressa and its distribution in the intertidal zone as a model system. This work will include (1) mapping the gradient in emersion time spanned by S. compressa populations and examining how genetic structure and demographic rates vary across this gradient; (2) measuring dispersal distance by mimicking colonization events; (3) assessing local adaptation to position in the intertidal zone with reciprocal transplant experiments; and (4) directly testing the consequences of gene flow across the gradient by conducting controlled crosses and outplanting zygotes into the field. Recent theoretical advances have shown that gene flow and local adaptation may jointly determine species distributions, yet for no organism do we currently understand how these processes interact at the edges of a range. Thus the results of this study will provide a key empirical test of current thinking on the antagonistic effects of gene flow among different selective environments. The role of gene flow in the maintenance of species distributions has implications both for ecological theory (e.g. understanding patterns of biodiversity across gradients and patchy landscapes), as well as for important applied problems such as species invasions, and population-level response to environmental change. Additional broader impacts of this study stem from its geographic scale, and the opportunity for involvement and training of undergraduates from multiple universities.
