Effects of gene flow and local adaptation on the distribution of an intertidal alga

Gene flow may have dramatically different effects on adaptation at the edges of species’ distributions.  Low dispersal and gene flow may prevent range expansion if edge populations lack genetic variation in ecologically limiting traits; however, relatively high gene flow may also constrain distributions by countering the effects of natural selection in edge habitats.  Previous work on the seaweed Silvetia compressa found evidence of adaptive differentiation across the intertidal gradient, within sites:  progeny of upper-limit lineages survive exposure to air significantly better than lower-limit lineages.  Here I investigate the potential for gene flow to both disrupt and promote adaptation and spread in this species, depending on the spatial scale of dispersal.  Breeding experiments indicate that emersion tolerance in S. compressa is under partial genetic control, one of relatively few examples of local genetic adaptation in a marine system; thus within-site gene flow across the intertidal zone should act to constrain adaptation.  Surprisingly, the greatest emersion tolerance is found in progeny from the center of the intertidal distribution.  This mid-zone advantage is inherited only through female function, and thus is thought to reflect a maternal environmental effect, possibly via differential egg provisioning:  eggs produced by mid-zone mothers are significantly larger than upper- or lower-limit eggs.  At larger spatial scales (among sites), dispersal and gene flow in S. compressa can promote adaptation and spread:  the emersion tolerance of upper-limit lineages are not the same across sites, and long-distance dispersal events can increase the phenotypic variance within a population.