Functional explanations for division of labour

Division of labour, when cooperating individuals specialise in complementary tasks, is a pervasive and diverse biological phenomenon. Understanding the reasons it ultimately evolves and the conditions that promote it are fundamental to evolutionary biology. Previous research identifies three broad reasons division of labour is favourable: between-individual differences, sufficiently strong performance trade-offs, and interactions between performers. Focusing on groups with aligned inclusive fitness interests, we show these principles are robust across a wider range of biological scenarios, including groups with multiple types of individuals, performing multiple tasks, with discrete or continuous units of performance, and where selection favours a balance of tasks rather than simply more of each. Division of labour can be favoured at the individual level without a resource trade-off, and even when some tasks exhibit diminishing returns. Larger groups are more likely to divide labour, and we discuss when complete specialisation is expected. Our framework provides a general classification of division of labour, highlighting the common thread underlying different proximate mechanisms, and can be tailored to specific biological systems.