Evidence of adaptive divergence in plasticity: Density- and site-dependentselection on shade-avoidance responses in Impatiens capensis

We investigated the conditions under which plastic responses to density are adaptive in natural populations of Impatiens capensis and determined wheth er plasticity has evolved differently in different selective environments. Previous studies showed that a population that evolved in a sunny site exhi bited greater plasticity in response to density than did a population that evolved in a woodland site. Using replicate inbred lines in a reciprocal tr ansplant that included a density manipulation, we asked whether such popula tion differentiation was consistent with the hypothesis of adaptive diverge nce. We hypothesized that plasticity would be more strongly favored in the sunny site than in the woodland site; consequently, we predicted that selec tion would be more strongly density dependent in the sunny site, favoring t he phenotype that was expressed at each density. Selection on internode len gth and flowering date was consistent with the hypothesis of adaptive diver gence in plasticity. Few costs or benefits of plasticity were detected inde pendently from the expressed phenotype, so plasticity was selected primaril y through selection on the phenotype. Correlations between phenotypes and t heir plasticity varied with the environment and would cause indirect select ion on plasticity to be environment dependent. We showed that an appropriat e plastic response even to a rare environment can greatly increase genotypi c fitness when that environment is favorable. Selection on the measured cha racters contributed to local adaptation and fully accounted for fitness dif ferences between populations in all treatments except the woodland site at natural density.