Maximum-likelihood estimate of relationships among bees. Tree estimated using one randomly-selected DNA sequence per gene per genus. Numbers at nodes represent bootstrap percentage after 100 replicates. To enhance visualization of relationships, wasp outgroups were removed, and clades of tribes and subfamilies containing more than one genus have been collapsed. The tree with all genera is in Additional file 3. When taxonomic groups are not monophyletic, clades are labelled with the taxonomic name and either “(part)” if more than one genus is included, or “(Genus)” if only one terminal taxon is in the taxonomic group.