Recents works has recovered robust phylogenetic relationships among most major Bombus lineages based on three nuclear genes, long-wavelength rhodopsin (LW Rh), arginine kinase (ArgK), and elongation factor-1a F2 copy (EF-1a). Relating this data with historical biogeography and evolution of behavioral and morphological attributes in bumble bees, it is shown that early diversification of bumble bees took place primarily in the Palearctic, followed by repeated intercontinental faunal interchanges. The study describe major evolutionary trends in brood-rearing behavior and proboscis length, characters of potential importance for shaping the diversification of bumble bees.
The two basal bumble bee subgenera, Mendacibombus and Bombias, occur in the Palearctic and the Nearctic, respectively, suggesting a rather ancient Holarctic distribution followed by Palearctic/Nearctic vicariance.
Early diversification of bumble bees was suggested to have occurred primarily in the Palearctic a result consistent with the highest diversity of extant bumble bee species in this region. Subsequent migrations to the Nearctic regions and less frequent re-colonizations of the Palearctic have occurred repeatedly during the evolution of bumble bees. Both ancient and more recent faunal interchanges can be inferred. Older dispersals include immigration to the Nearctic by ancestors of Fervidobombus, Pyrobombus, and the clade consisting of several exclusively New World subgenera, and re-colonization of the Palearctic within Pyrobombus. More recent interchanges are characteristic of Pyrobombus, Bombus s. str., and Psithyrus, which include many boreal, arctic, and high montane species. Those relatively recent dispersals are responsible for Nearctic/Palearctic pairs of closely related species such as B. perplexus and B. hypnorum, B. terricola and B. patagiatus, and B. ashtoni and B. bohemicus. The most recent dispersals involve the few boreal/arctic species with natural Holarctic distributions. Both recent and more ancient faunal interchanges between the Palearctic and the Nearctic are paralleled by several other naturally Holarctic bee groups such as Colletes, Hylaeus, Andrena, Lasioglossum, Halictus, Osmiini, and Anthophora. Colonization of Central/South America involves species of Fervidobombus and Robustobombus, although one species of Pyrobombus also occurs south to Panama. The potentially non-monophyletic subgenus Fervidobombus has ca. 20 species distributed from southern Canada through the Neotropics to southern South America. Most Fervidobombus occur at low to moderate elevations, while a single atypical species, Bombus dahlbomii, occurs south to southern Chile and Argentina. The remaining five exclusively Neotropical subgenera not included in our analysis are closely related to Robustobombus and have radiated in Andean South America and montane Central America, as have a few other primarily Holarctic bee groups such as Anthophora, Anthidium, and weak-veined Lasioglossum.
