Evolution of sociality in the allodapine bees: a review of sex allocation,ecology and evolution

Comparative studies provide one of the most powerful means of assessing the relative roles of selective agents underlying social evolution in insects. Because of the wide Variation in social organisation, sex allocation and e cological traits within and between species of allodapine bees, this group provides a wealth of material for such comparative work. Recent studies on Australian allodapine bees are reviewed here and their consequences for und erstanding social evolution are discussed. Studies to date suggest the foll owing trends: (i) benefits of group living appear to be linked to preventin g brood failure rather than to increased brood rearing efficiency; (ii) fem ale-biased sex allocation, when it occurs, is linked to benefits of group l iving and kinship among nestmates, and is probably mediated via local fitne ss enhancement; (iii) female biased sex allocation patterns do not usually coincide with opportunities for sib-rearing and are therefore unlikely to f acilitate eusociality; (iv) relatedness within colonies is usually high, bu t in some species females will nest with unrelated females if kin are not a vailable; and (v) phylogenetic studies suggest that opportunities for sib-r earing, arising from brood development patterns and colony phenology, are p lesiomorphic for the exoneurine group, but in at least one phylogenetically distal clade, Exoneura sensu stricto, the evolution of large group size an d social complexity coincides with the loss or reduction of opportunities f or sib-rearing. Assured fitness return models may be applicable to weakly s ocial allodapine species, but do not predict patterns of eusociality. Inste ad, Australian studies suggest that the evolution of large group size and m arked reproductive skew is linked with the need to defend against enemies a t the nest, rather than high levels of relatedness, female biased sex alloc ation or opportunities to rear siblings.