The plant genus Calochortus (Liliaceae family, with ca 60 species) has a center of diversity in California and has undergone striking diversification in flower form and habitat. Several species are restricted to small areas, and 25% occur on serpentine soils. An analysis of mutational differences in chloroplast DNA indicates that Calochortus is composed of seven geographically cohesive groups, with closely related species often having adjoining ranges. Three of the major floral forms ("cat's ears," "star tulips," for example) have arisen two to six times each, and serpentine tolerance has evolved at least seven times. It is hypothesized that such geographic cohesion and parallel radiations are the products of restricted gene flow, which should promote genetic differentiation and speciation at small spatial scales; different lineages may evolve similar adaptations as they invade different areas independently. To test these ideas, Prof. Givnish and his students will analyze the relationship of Calochortus species in the San Francisco Bay Area to each other and to landforms they occupy, using nuclear DNA variation, studies of incipient mating barriers as a function of geographic and genetic distance, and reconstruction of past patterns of migration, hybridization, and gene flow within and among species. Their objectives are to (1) reconstruct the history of geographic differentiation within the Bay Area Calochortus and determine whether the species restricted to serpentine are each other's closest relatives or have instead been derived independently from one or more of the wide-ranging species; (2) examine the role of hybridization and introgression in this complex group; and (3) analyze the pattern of genetic differentiation within a "ring-species" complex that surrounds California's Central Valley, to identify suture zones (areas of abrupt genetic differentiation) where expanding and gradually differentiating populations finally abutted each other. The proposed research will bridge traditional concerns of taxonomy, systematics, population genetics, and historical biogeography by quantifying the spatial scales at which genetic differentiation and incipient mating barriers arise within species, and relating these to the geographic and morphological patterns of divergence among species. Findings will provide the basis for comparative analyses involving other plant and animal groups in California, including ongoing research on several groups of plants and amphibians, and a ring-species complex involving the California valley oak. Data on patterns of genetic variation and differentiation will be of value to conservation. This project will train two female Ph.D. students in molecular phylogenetics, phylogeography, population genetics, and plant systematics. Prof. Givnish will recruit two undergraduates from underrepresented minorities to assist in data collection and develop senior theses. A web site on Calochortus evolution will be developed to bring results and new insights to a broad audience of amateur naturalists, conservationists, and professional botanists and evolutionary biologists.
