Evolution and dynamics of hybridization in Penstemon subgenus Dasanthera (Scrophulariaceae S.L.)

Penstemon subgenus Dasanthera is a small group of 16 taxa distributed at high elevations in western North America. I examined evolutionary trends and the importance of hybridization and gene flow on the evolution of the subgenus. Phylogenetic relationships among members of Penstemon subgenus Dasanthera were assessed to examine biogeographic relationships and morphological trends in the subgenus. These data support previous hypotheses suggesting that the Cascade-Sierra Nevada lineage is derived from the northern Rocky Mountain lineage. These results also indicate a shift in growth form concurrent with migration to the Cascade/Sierra Nevada cordillera and a series of floral shifts related to hummingbird pollination in two species in the Cascade-Sierra lineage.; To examine potential outcomes of hybridization, ISSR and morphological markers were used to assess hybrid zone structure and patterns of gene flow on Wizard Island, Crater Lake National Park, Oregon. I surveyed three hybrid subpopulations to examine population structure in the hybrid zone. Plants were categorized based on diagnostic morphological characters for P. davidsonii and P. rupicola, and proportion of species-typical bands was calculated for each hybrid index category. Comparisons were made between hybrid categories and unintrogressed populations of P. davidsonii and P. rupicola to test for directionality of gene flow. These results indicate asymmetrical gene flow and potential for introgressive hybridization in these species.; Asymmetry in patterns of gene flow could be the result of either endogenous or exogenous selection pressures. I tested for the role of endogenous selection, in terms of pollinator visitation and differential seed siring ability of P. davidsonii and P. rupicola. Although the same suites of pollinators visit both P. davidsonii and P. rupicola, frequency of visitation varies between species. It is not clear how pollinator visitation affects gene flow in hybrid zones. Seed set data demonstrate that P. rupicola has higher fruit set and seed set, but P. davidsonii has higher seed germination. These data suggest that factors influencing hybrid zone structure on Wizard Island are complex, and include a combination of pre-pollination and post-pollination barriers, and selection for fitness traits in the hybrid zone.