The most common image that comes to mind with the term “bee” is the honey bee, Apis mellifera, with its highly derived and well-studied eusocial organization. The diversity of social organization among bees and near relatives (Apoidea), however, is much richer and extensive. The “bees” (Anthophila, within the superfamily Apoidea) include approximately 20,000 species forming a monophyletic clade with a likely single ancestral origin (Danforth, et al., 2006). Ecologically, all bees rely on floral resources to survive, and share a relatively common body plan, related to their reliance on flight as their primary form of locomotion. These commonalities belie their incredible social diversity. The basal (plesiomorphic) and most common life history form in this taxon is actually solitary living. Across the bee phylogeny, however, there are multiple and diverse taxa in which adult females form social alliances without reproductive castes (primarily communal sociality), and diverse taxa have independently evolved reproductive caste-based eusocial societies. Beyond these general categories, bee species vary considerably in how they form social groups and in the specific behaviors they perform cooperatively. Sociality has appeared and been lost multiple times in different lineages, and the expression of sociality can vary within a single species or even within populations. This diversity simultaneously makes the bees one of the most complex and most rewarding of taxa with which to explore questions of social evolution. SOCIAL DIVERSITY How Common is Sociality in Bees? In his nearly forgotten paper, “The application of Darwin’s theory to the flowers and flower-visiting insects,” Müller (1872) laid out the arguments that bees are essentially vegetarian digger wasps, most of which are solitary ground-nesters, with some species showing tolerance for social interactions and nest cohabitation (reviewed in Wcislo & Tierney, 2009). The majority of bee species are solitary, or live in communal social groups consisting of adult females that independently engage in nest construction, defense, and offspring care, but without reproductive division of labor (O’Neil, 2001; Wcislo & Tierney, 2009). In a much more limited number of taxa, family groups (i.e. a mother and her female offspring) have evolved into eusocial societies, in which morphologically or functionally sterile female offspring remain at the nest to help their parent queen rear offspring.
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