The large and diverse genus Penstemon (ca. 271 species) is endemic to North America and has been divided into six subgenera primarily based on anther dehiscence patterns. Species of Penstemon are known to be pollinated by a variety of insects (hymenopterans, lepidopterans, dipterans) and hummingbirds. Nucleotide sequence data from ITS and two noncoding regions of chloroplast DNA were used to reconstruct the phylogeny of Penstemon. Trees generated from nuclear and chloroplast DNA sequences are incongruent, which is probably the result of hybridization, and not fully resolved, which is likely due to a rapid evolutionary radiation. Penstemon represents a recent continental radiation where speciation has resulted primarily from evolutionary adaptations to ecological niches such as pollinator specialization. The results from these analyses show that the current circumscription of subgenera and sections needs revision to reflect more closely the evolutionary relationships of species. Specifically, species in subgenera Saccanthera, Habroanthus, and Penstemon are polyphyletic. These results also confirm the independent origin of hummingbird floral morphology in 10 clades.

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