Nest architecture and use of floral oil in the oil-collecting South African solitary beeRediviva intermixta(Cockerell) (Hymenoptera: Apoidea: Melittidae)

Our understanding of bee phylogeny has improved over the past fifteen years as a result of new data, primarily nucleotide sequence data, and new methods, primarily model-based methods of phylogeny reconstruction. Phylogenetic studies based on single or, more commonly, multilocus data sets have helped resolve the placement of bees within the superfamily Apoidea; the relationships among the seven families of bees; and the relationships among bee subfamilies, tribes, genera, and species. In addition, molecular phylogenies have played an important role in inferring evolutionary patterns and processes in bees. Phylogenies have provided the comparative framework for understanding the evolution of host-plant associations and pollen specialization, the evolution of social behavior, and the evolution of parasitism. In this paper, we present an overview of significant discoveries in bee phylogeny based primarily on the application of molecular data. We review the phylogenetic hypotheses family-by-family and then describe how the new phylogenetic insights have altered our understanding of bee biology.

Bees, the largest (>16,000 species) and most important radiation of pollinating insects, originated in early to mid-Cretaceous, roughly in synchrony with the angiosperms (flowering plants). Understanding the diversification of the bees and the coevolutionary history of bees and angiosperms requires a well supported phylogeny of bees (as well as angiosperms). We reconstructed a robust phylogeny of bees at the family and subfamily levels using a data set of five genes (4,299 nucleotide sites) plus morphology (109 characters). The molecular data set included protein coding (elongation factor-1alpha, RNA polymerase II, and LW rhodopsin), as well as ribosomal (28S and 18S) nuclear gene data. Analyses of both the DNA data set and the DNA+morphology data set by parsimony and Bayesian methods yielded a single well supported family-level tree topology that places Melittidae as a paraphyletic group at the base of the phylogeny of bees. This topology ("Melittidae-LT basal") is significantly better than a previously proposed alternative topology ("Colletidae basal") based both on likelihood and Bayesian methods. Our results have important implications for understanding the early diversification, historical biogeography, host-plant evolution, and fossil record of bees. The earliest branches of bee phylogeny include lineages that are predominantly host-plant specialists, suggesting that host-plant specificity is an ancestral trait in bees. Our results suggest an African origin for bees, because the earliest branches of the tree include predominantly African lineages. These results also help explain the predominance of Melittidae, Apidae, and Megachilidae among the earliest fossil bees.