Phylogenetic analyses of life history traits in allodapine bees and social evolution

Allodapine bees provide enormous material for comparative approaches to social evolution. Sociality varies widely among both genera and species. Some of this variation appears to be associated with ecological features, but some variation appears to shows signs of strong phylogenetic constraint. Importantly, several key life history traits also vary widely among allodapines and these have the potential to strongly influence the kinds of sociality that can evolve. DNA sequence data accumulated over the last few years now allow us to begin exploring transitions in key life history and social traits from a phylogenetic perspective. One major finding from sequence based analyses overturns the prior hypotheses that the evolution of progressive provisioning of brood, and the origin of sociality, occurred within lineages leading to the extant taxa. Instead, both sociality and progressive provisioning are plesiomorphic for the tribe. Mass provisioning and solitary behaviour in the genus Halterapis, which was previously regarded as the sister clade to the remainder of the allodapine clade, is apomorphic. Although allodapines have traditionally been regarded as displaying recent origins of sociality, so that they might provide insights into early stages in social evolution, it appears that social behaviour in this group has an ancient origin, and that extant forms of social organization result from selection operating on forms of alloparental care that evolved at least 30 Mya. However, allodapines do provide extensive material for investigating both the elaboration and perhaps the loss of social organization. Cofounding has arisen only once, but size related reproductive castes have evolved at least several times. Voltinism, egg-laying schedules, patterns of brood rearing, and colony phenology show strong phylogenetic patterns, and in at least some circumstances major transitions in these traits seem to have evolved very rapidly and correspond to generic-level divergences. However, current data do not suggest that any particular life history or ecological factors are critical to the evolution of eusociality, or its loss. This could mean that we are yet to identify key factors underlying complex sociality, but it could also mean that for this tribe at least, evolutionary transitions in social organization are capricious.