Natural Selection and Ecotypic Differentiation in Impatiens Pallida

Abstract

One possible response of plant populations to heterogeneous environments is genetic adaptation resulting in the formation of distinct ecotypes. Genetic adaptation to stressful environments may affect both the limits to species boundaries and the potential for response to a changing environment. Reciprocal transplant experiments have frequently been used to describe ecotypic differentiation and to infer the role of natural selection when there is evidence for home site advantage. The demonstration of a home site advantage, however, does not reveal which plant characters are responsible for conferring increased fitness on populations planted in their native site. Here, we combine the classic reciprocal transplant experiment with multivariate regression analysis of selection to ask a series of questions relevant to understanding adaptive genetic differentiation in natural plant populations. Impatiens pallida plants from a mesic floodplain and a dry hillside site were reciprocally transplanted. We initially presumed the hillside to be a stressful site for Impatiens given its sparser population of consistently smaller individuals. This study describes the two environments from the perspective of the plant to ask whether it is stressful. In addition, we investigate genetic differentiation between populations and ask whether the two populations are distinctly adapted to their home sites. To identify traits that may be important for conferring home site advantage, we quantify present—day natural selection in these sites and ask whether the observed selective forces can explain genetic differences. Finally, because phenotypic correlations may play an important role in a population's response to its environment, we investigate relationships among traits to determine the extent to which they are genetically and/or environmentally controlled. The large reduction in total seed production when plants from both populations were grown on the hillside supported our initial bias that this site was stressful to Impatiens. In addition, the higher relative fitness of each population planted in its native site demonstrated that these populations represent distinct ecotypes. Genetic differences between populations were observed for several life history and morphological characters. In particular, plants from the hillside population were smaller and produced cleistogamous flowers earlier than floodplain plants. Selection analysis revealed that, while there is strong selection favoring early flowering on the hillside, there is no advantage to early flowering for plants grown on the floodplain. An increased developmental rate, which allows plants to produce seeds before they succumb to drought stress, appears to be the most important mechanism responsible for the greater relative fitness of the hillside population in its native site. While greater total plant leaf area is favored by selection on the floodplain, there is no evidence for selection on this trait on the hillside. Phenotypic covariances among traits differed between sites and populations, resulting in differences in the action of indirect selection. There is evidence that indirect selection on correlated traits is responsible for some of the observed genetic differences.