Only a single origin of morphological castes in a bee subfamily despite ancient origins of preadaptations to eusociality

Understanding how advanced social organization has evolved comprises one of the key puzzles in evolutionary biology. We know that once this kind of social complexity evolves it can be enormously successful, but we also know that it has evolved very, very rarely. The paradox of why such an enormously successful strategy has evolved so rarely is one of the major questions facing evolutionary biology. Resolving this critical paradox is very difficult because most of the socially complex species have few surviving relatives that are solitary, so we are unable to examine the evolutionary stages leading from simple to complex organization. The bee subfamily Xylocopinae contains four extant tribes. Three of these, the Allodapini, Xylocopini and Ceratinini, have been extensively used to investigate social evolution. The fourth tribe, Manueliini contains species whose biology has been largely unknown until very recently. It is important to understand the phylogenetic position of Manueliini so we can explore, whether sociality is plesiomorphic for the Xylocopinae or whether it evolved multiple times in this subfamily. Using phylogenetic inference and nesting biology studies on 34 species across all four tribes reveals that a weak form of sociality is ancestral for the Xylocopinae, and was in place for over 90 million years. In this subfamily the earliest societies did not entail a foraging worker caste, but instead comprised females sharing a nest with supersedure of dominance. The only lineage where morphologically distinct foraging worker castes evolved represents a shift towards assured fitness gains by subordinates, mediated by environmental harshness.