Adaptive divergence in plasticity in natural populations of Impatiens capensis and its consequences for performance in novel habitats

We tested for adaptive differentiation between two natural populations of I mpatiens capensis from sites known to differ in selection on plasticity to density. We also determined the degree to which plasticity to density withi n a site was correlated with plastic responses of experimental immigrants t o foreign sites. Inbred lines, derived from natural populations in an open- canopy site and a woodland site, were planted reciprocally in both original sites at naturally occurring high densities and at Low density. The densit y manipulation represents environmental variation typically experienced wit hin the site of a given population, and the transplant manipulation represe nts environmental differences between sites of different populations. Inter node elongation, meristem allocation, leaf length, flowering date, and tota l lifetime fitness were measured. Genotypes originating in the open site, w here selection favored plasticity of first internode length and flowering t ime (Donohue et al. 2000a), were more plastic in those characters than geno types originating from the woodland site, where plasticity was maladaptive. Therefore, these two populations appear to have responded to divergent sel ection on plasticity. Plasticity to density strongly resembled plasticity t o site differences for many characters, suggesting that similar environment al factors elicit plasticity both to density and to overhead canopy. Thus, plasticity that evolved in response to density variation within a site infl uenced phenotypic expression in the foreign site. Plastic responses to site caused immigrants from foreign populations to resemble native genotypes mo re closely. In particular, immigrants from the open site converged toward t he selectively favored early-flowering phenotype of native genotypes in the woodland site, thereby reducing potential fitness differences between fore ign and native genotypes, However, because genotypes from the woods populat ion were less plastic than genotypes from the sun population, phenotypic di fferences between populations were greatest in the open site at low density . Therefore, population differences in plasticity can cause genotypes from foreign populations to be more strongly selected against in some environmen ts than in others. However, genetic constraints and limits to plasticity pr evented complete convergence of immigrants to the native phenotype in any e nvironment.