XVI International Botanical Congess

Molecular phylogenetic methods were used to assess relationships within Callandrena, a North American subgenus of the genus Andrena, and between Callandrena and related subgenera. Callandrena are oligolectic, specializing on groups within the Asteraceae for pollen collection. Floral host choice and degree of host specificity among these bees were evaluated in light of the phylogeny. Parsimony analysis of COII sequence data suggests that: (1) Callandrena is not monophyletic, but comprises at least two major clades which are not one another's closest relatives, (2) host shifting is far more common than previously thought, (3) shifts are often to distantly related host plants, and (4) although there is a general trend toward increasing specificity, shifts to more generalist behavior have occurred.