Results of phylogenetic analyses of 18S-26S nuclear ribosomal DNA ITS (Internal Transcribed Spacer) and ETS (External Transcribed Spacer) sequences lead us to reject the previously suggested hypothesis of monophyly for the annual species of Sidalcea (Malvaceae), a western North American genus comprising ca. 25 species of annuals and perennials. Based on the rDNA data, we conclude that the annual habit arose at least three times, probably as an adaptation to seasonally dry habitats. Among the perennials, S. oregana and S. malviflora are the most heterogeneous species (with numerous subspecific taxa) and each appears to be polyphyletic. Comparisons of evolutionary rates between perennial and annual lineages in Sidalcea establish that both ITS and ETS have evolved significantly faster in the annuals than in the perennials. Roush's (1931) hypothesis that the perennial species S. hickmanii and S. malachroides represent basally divergent groups within Sidalcea is upheld by our rDNA trees (although S. stipularis, discovered subsequent to Roush's study, represents an additional basally divergent lineage). Roush's (1931) suggestion that the genus spread northward from Mexico along two major routes (through the Rocky Mountains and the Sierra Nevada foothills) is not congruent with our results. Of ca. 20 rare or endangered taxa in Sidalcea, those corresponding to basally divergent lineages (i.e., S. hickmanii, S. keckii, S. malachroides, and S. stipularis) are well diagnosed by rDNA mutations. Others, e.g., the endangered S. nelsoniana (in the S. oregana clade), cannot be distinguished from more common, closely related taxa using ITS and ETS sequences.

Key words: evolutionary rates, Malvaceae, nuclear ribosomal DNA, phylogeny, Sidalcea