Hybridization has played an important role in the evolutionary history of many plant groups. Potential for hybrid speciation or genetic assimilation may be more extreme in spatially isolated or island populations. Studies of hybrid swarms in isolated populations may lead to a better understanding of processes of gene flow, hybrid speciation and potential for genetic assimilation. Penstemon subgenus Dasanthera (Scrophulariaceae) has long been noted for extensive hybridization in areas where species overlap. Two species of Penstemon, P. rupicola and P. davidsonii, and their hybrids occur on Wizard Island in Crater Lake National Park, Oregon. Collections of morphological data and tissue for molecular analyses were made from morphologically “pure” populations of P. davidsonii and P. rupicola and a large P. davidsonii X rupicola hybrid swarm on Wizard Island. The Wizard Island population was subsampled based on morphological measurements to include plants which were similar to parental species, intermediate, and morphologically unique. Inter-simple sequence repeat (ISSR) markers were used to assess levels of gene flow and the extent of hybridization on Wizard Island. Of 20 floral and vegetative characters measured, nine characters which displayed significant differences between P. davidsonii and P. rupicola were chosen for analysis. Morphological measurements of hybrids suggest extensive hybridization on Wizard Island. Penstemon davidsonii and P. rupicolashared most ISSR bands, however both species exhibited several species-specific bands. Despite extensive hybridization, hybrid speciation seems unlikely based on the ecological preferences of each species. However, the potential for interspecific gene flow is high within this population as well as other populations within the Penstemon subg. Dasanthera species complex.

Key words: hybridization, inter simple sequence repeats (ISSR), interspecific gene flow, Penstemon, Scrophulariaceae