The phylogeny of 20 Thalictrum species and four outgroups was reconstructed using two molecular loci, the nrDNA ITS region (internal transcribed spacers of nuclear ribosomal DNA) and the chloroplast DNA rpl16 intron, and 37 morphological characters. The 20 species represent 10 of 14 sections in this genus of 190 species. The nuclear data provided better-resolved trees than the morphological data. The combined analysis consensus tree had bootstrap support above 70% for the majority of clades. Thalictrum species vary in their breeding system and pollination mechanisms within a breeding system. Species can be hermaphroditic (all flowers have both male and female organs), dioecious (male and female individuals), or andromonoecious (male and hermaphroditic flowers on an individual) and are pollinated by wind, insect, or both. While wind-pollination is common in dioecious and andromonoecious species, and insect-pollination in hermaphroditic species, both insect- and wind-pollination occur in some dioecious and some hermaphroditic species. The evolution of the breeding system and the pollination mechanism were examined using the methods of character optimization. Two separate origins of dioecy were detected in North America, and no reversal back to hermaphroditism was observed. There was one origin of andromonoecy, and it was separated from the origins of the dioecious clades. Of the known ecological correlates of dioecy (woodiness, fleshy fruits, islands, tropical habitats, and wind-pollination), only wind-pollination is correlated with dioecy in this genus. Preliminary results suggest that wind-pollination preceded the evolution of dioecy and andromonoecy in this genus.

Key words: breeding system evolution, character optimization, phylogeny, pollination, Ranunculaceae, Thalictrum