Origin and evolution of eusociality: a perspective from studying primitively eusocial wasps

Eusocial insects are those that show overlap of generations, cooperative brood care and reproductive caste differentiation. Of these, primitively eusocial insects show no morphological differences between reproductive and worker castes and exhibit considerable flexibility in the social roles that adult females may adopt. This makes them attractive model systems for investigations concerning the origin of eusociality. The rapidly accumulating information on primitively eusocial wasps suggests that haplodiploidy is unlikely to have an important role in the origin of eusociality. General kin selection (without help from haplodiploidy) could however have been an important factor due to the many advantages of group living. Pre-imaginal caste bias leading to variations in fertility is also likely to have some role. Because workers often have some chance of becoming reproductives in future, mutualism and other individual selection models suggest themselves as important factors. A hypothesis for the route to eusociality which focuses on the factors selecting for group living at different stages in social evolution is presented. It is argued that group living originates owing to the benefit of mutualism (the ‘Gambling Stage’) but parental manipulation and subfertility soon become important (the ‘Manipulation Stage’) and finally the highly eusocial state is maintained because genetic asymmetries created by haplodiploidy are exploited by kin recognition (the ‘Recognition Stage’).