The evolution of morphological castes under decoupled control

Abstract

AbstractEusociality, where units that previously reproduced independently function as one complex entity, is a phenomenon of major interest in evolutionary biology. Obligately eusocial societies are characterised by morphologically differentiated castes and reduced conflict. We explore conditions under which morphological castes may arise in the social Hymenoptera and the factors constraining their evolution. Control over offspring morphology and behaviour may often be decoupled. Queens and provisioners can influence offspring morphology directly, through the nutrition they provide,while offspring control their own behaviour as adults. Queens and provisioners may, however, influence worker behaviour indirectly, if offspring modify their behaviour in response to their morphology. Our results suggest that the evolution of a morphologically differentiated worker caste depends on the prior presence of a behavioural caste: specialist worker morphology will be mismatched with behaviour unless some offspring already choose to work. A mother’s certainty about her offspring’s behaviour is also critical – less certainty results in greater mismatch. We then show how under decoupled control, morphologically differentiated castes should be less and less likely to be lost through evolution as they become more specialized. We show how worker productivity in the absence of a morphological trait can affect the likelihood of that trait being favoured by natural selection. We also comment on the evolutionary irreversibility of eusociality.