Floral Isolation in Penstemon

Four species of Penstemon section Peltanthera in cismontane southern California are very different in appearance and occur in several sympatric combinations. Despite the fact that they are largely interfertile, hybridization is relatively rare. Field studies have shown that each of the species has its own normal range of pollinators, and it is concluded that floral isolation adequately restricts the hybridization that might lead to swamping or merging of the species.