Dissertation Research: Mechanisms Defining Ecological Range Limits in a Plant Hybrid Zone

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Different species are usually reproductively isolated from one another. However, in some cases hybrid zones form where species cross-fertilize and produce viable offspring. This project focuses on how physiological and pollinator differences maintain separate elevational ranges of two plant species, and thereby the species differences, despite some hybridization. The study system consists of two species of Penstemon wildflowers that grow at different elevations in the Sierra Nevada Mountains. Previous experiments indicated that neither complete separation of pollinators nor genetic barriers prevent hybridization, suggesting the importance of environmental factors. In the proposed experiment, survival and growth rate of pure parents and hybrids will be measured in common gardens at three elevations. Physiological traits (photosynthetic rate and water use) will be measured to determine whether they are responsible for determining the elevational ranges of pure parents and hybrids. Pollinators will be studied to determine whether hybridization is limited by behavioral choices of pollinators. This project will clarify mechanisms responsible for shaping elevational ranges of different species and hybrids. It will contribute to knowledge about the production and maintenance of new species, a fundamental issue in evolutionary biology. It will also provide useful information for conservation of rare plant species threatened by hybridization, and about risks of hybridization between genetically modified crops and native species. This project will provide training to one graduate student and several undergraduates per year of funding.