Callandrena is a subgenus of 80 described species in the bee genus Andrena (Hymenoptera: Andrenidae). The group is delimited mainly by branched scopal (pollen-carrying) hairs and shortened mouthparts. All are specialists to varying degrees on pollen of the Asteraceae, with females collecting pollen from plants in at least five composite tribes. Some species are narrow specialists, using pollen from one genus or a few related genera; others collect pollen from plants in more than one tribe. Why bees should specialize on flowers that are easily manipulable and morphologically similar is unknown. We generate an hypothesis of phylogenetic relationships based on both mitochondrial and nuclear DNA markers for 55 species of Callandrena (eight of which are undescribed) and 40 additional Andrena species, representing 23 subgenera, to investigate the evolution of floral host choice in these bees. Host preference was ascertained by analysis of pollen loads on museum specimens. Both molecular data sets dispute the monophyly of Callandrena; there are at least three distinct and strongly supported clades. They have independently evolved a preference for pollen of the Asteraceae and have also evolved convergent morphological traits to accompany this preference. Within monophyletic groups, host shifts are generally among plants within a tribe, however, shifts to other tribes have occurred and may lead to adaptive radiation.

Key words: Asteraceae, Callandrena (Hymenoptera), floral specificity, phylogeny