A single origin of large colony size in allodapine bees suggests a threshold event among 50 million years of evolutionary tinkering

Evolutionary origins of highly eusocial organization involving morphological castes have been very rare, yet these origins have often led to enormous diversification and ecological success. This suggests that once an apparently severe selective barrier to highly eusocial behaviour is overcome, major new adaptive landscapes open up. One would therefore expect a discontinuity in patterns of evolutionary change across this barrier. However, we do not know if highly eusocial organization has evolved incrementally from less complex societies, or if it has involved some kind of evolutionary leap. Our study examines this issue using colony size data from 33 allodapine bee species, with a crown age of ca. 47 Mya. Our species cover all major allodapine clades, and include Exoneurella tridentata, the only known allodapine with morphologically discrete castes. Phylogenetic analyses indicate a strong effect of phylogeny on the evolution of maximum brood size, but the effect of phylogeny on maximum colony size (number of adults) depends on whether E. tridentata is excluded or included in analyses. We found no evidence of punctuational change in maximum colony or brood sizes over the phylogeny as a whole, but colony and brood sizes in E. tridentata fall well beyond variation among the other allodapines. Colony size in E. tridentata therefore represents an evolutionary outcome that does not fit within the kinds of incremental changes found in other allodapines. We propose that E. tridentata indicates the crossing of an important threshold, and this has entailed some very unusual ecological circumstances.