Bees collect pollen mainly to provision their nests. Like phytophagous insects, bee species display a range of specificity in diet breadth, from broadly generalist (polylectic) to narrowly specialist (oligolectic). As in the study of phytophagous insects, the direction of the evolution of specificity in bees has been the subject of debate, with some workers postulating that specialization confers some benefit and is derived, with others arguing that generalist behavior requires more sophisticated neural or physiological adaptations and is the derived trait. Unlike in phytophagous insects, however, the evolution of diet breadth in bees has been but poorly studied in a phylogenetic context. Here, we reconstruct the phylogeny of 96 species of North American Andrena, a large Holarctic genus which includes both polylectic and oligolectic members. Our phylogeny is based on both mitochondrial and nuclear markers, and is reconstructed using parsimony, likelihood and Bayesian methods. We also study the evolution of host choice within 23 species of an oligolectic clade currently placed in the subgenus Callandrena, whose members specialize on plants in the four tribes of the Asteraceae. Microscopic examination of pollen loads confirm host identity to the tribal level. Overall, oligolecty is found to be the ancestral state in the Andrena studied. Within the Callandrena clade, the ancestral host was a member of the Heliantheae, and hosts shifts are phylogenetically constrained, although the ability to use the ancestral host is retained in many species. These findings parallel those for phytophagous insects, despite the putatively mutualistic nature of pollination interactions.

Key words: Andrena, Asteraceae, bees, Callandrena, evolution of specificity, pollen