The evolution of cooperation and division of labour in insects

Why have some insect lineages evolved highly cooperative societies with specialised castes, while others remain solitary or only facultatively social? In this thesis, I explore this question by creating and analysing large-scale comparative datasets across hundreds of insect species, focusing on the evolution of cooperation and division of labour. I begin by outlining key concepts in the study of cooperation (Chapter 1) and reviewing the diversity of insect social systems (Chapter 2), highlighting the need for more precise, lineage-specific definitions of social traits to better understand their evolutionary dynamics. In Chapter 3, I investigate morphological variation among workers in 152 ant species, testing whether life history traits can predict the degree of worker size variation – a proxy for task specialisation and division of labour. In Chapter 4, I expand the dataset to 546 ant species, examining reproductive division of labour by assessing queen-worker size dimorphism and worker sterility. I test whether life history can explain variation in reproductive specialisation, and I reconstruct the ancestral ant to estimate how often these forms of specialisation evolved independently. Finally, in Chapter 5, I broaden the scope to 582 species of ants, bees, and wasps, testing whether the way groups are formed affects the evolution of cooperation and sterility. Together, these studies provide new insights into the conditions that favour the emergence of reproductive division of labour and major evolutionary transitions.