Density dependence and population differentiation of genetic architecture in Impatiens capensis in natural environments

We identified environment-dependent constraints on the evolution of plastic ity to density under natural conditions in two natural populations of Impat iens capensis. We also examined the expression of population divergence in genetic variance-covariance matrices in these natural environments. Inbred lines, originally collected from a sunny site with high seedling densities and a woodland site with low seedling densities, were planted in both origi nal sites at natural high densities and at low density. Morphological and l ife-history characters were measured. More genetic variation for plastic re sponses to density was expressed in the sun site than in the woodland site, so the evolutionary potential of plasticity was greater in the sun site. S trong genetic correlations between the same character expressed at differen t densities and correlations among different characters could constrain the evolution of plasticity in both sites. Genetically based trade-offs in mer istem allocation to vegetative growth and reproduction were apparent only i n the high-resource environment with no overhead canopy and no intraspecifi c competition. Therefore, genetic constraints on the evolution of plasticit y depended on the site and density in which plants were grown, and correlat ed responses to selection on plastic characters are also expected to differ between sites and densities. Population differentiation in genetic varianc e-covariance matrices was detected, but matrix structural differences, as o pposed to proportional differences, were detected between populations only in the sun site at natural high density. Thus, population divergence in gen etic architecture can occur rapidly and on a fine spatial scale, but the ex pression of such divergence may depend on the environment.