9629086 Geber A very common evolutionary occurrence in flowering plants is the evolution of a self-pollinating breeding system from an outcrossing breeding system. Unlike its outcrossing ancestor, the self-pollinating species, subspecies or variety does not depend on pollinator visitation for successful seed set. This project addresses two questions. First, what changes in the development of structures (such as flowers and leaves) and of whole plants have been responsible for the differences in morphology and flowering time between a selfing taxon and its related outcrossing ancestor? In particular, is small flower size and early flowering in the selfing taxon achieved by means of an accelerated rate development or a shorter duration of development. The second question is whether early maturation of structures and whole plants is adaptive in the marginal environments occupied by the selfing taxon. Early maturation is postulated to be advantageous because environmental stress is often associated with a shorter growing season. The proposed study focuses on a comparison of the outcrossing and self-pollinating subspecies of Clarkia xantiana (Onagraceae), a winter annual in which the selfing subspecies occurs in arid environments at the margin of the outcrosser's range. Floral and vegetative plant development in the two subspecies will be examined. Natural selection on developmental and morphological characters in experimental gardens will be measured along a rainfall gradient that spans a portion of the geographic range of the two subspecies. Physiological differences between the subspecies and selection on physiology will also be examined, because physiology is an important determinant of plant growth under stress. The significance of the project is that it addresses a little-studied aspect of a major evolutionary transition in plants, including the origin of new species. In addition, by analyzing the roles of development and selection in the evolution of suites of characters, the project addresses a fundamental issue in evolutionary biology: why do traits evolve together? Lastly, the project also addresses the nature of evolution in stressful environments, a topic that has acquired particular significance as habitats are facing the impacts of anthropogenic stresses and climate change.
