XVI International Botanical Congess

The ivesioid Potentilleae consists of a group of species that have sometimes been placed in the segregate genera Ivesia, Horkelia, and Purpusia, among others. Phylogenetic analysis of ITS sequences supports the interpretation of this group as a monophyletic lineage nested within Potentilla s.s., with P. biennis and P. rivalis as closely related species. The ivesioid clade represents an interesting radiation within western North America, characterized by trends toward increased glandularity, reduced leaflet size, reduced flower size and number of parts, and restriction to unusual habitats. These trends can collectively be interpreted as adaptations to the increasingly xeric climate that developed in the late Tertiary. The resultant pattern of island biogeography in a continental setting includes numerous highly localized endemic taxa.