The evolution of morphological castes under decoupled control

Abstract

The evolution of eusociality is regarded as a major evolutionary transition, where units that previously reproduced independently function as one complex entity. Advanced eusocial societies are characterised by morphologically differentiated castes and reduced conflict. We explore conditions under which morphological castes may arise and the factors constraining their evolution. Control over offspring morphology and behaviour may often be decoupled. Queens and provisioners can influence 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 should also be critical -less certainty results in greater mismatch. Decoupled control is important in maintaining a worker caste, and may result in reduced or no conflict between offspring and provisioners. We also show how worker productivity in the absence of a morphological trait can affect the likelihood of that trait being favoured by natural selection.