Group transformation: life history trade-offs, division of labor, and evolutionary transitions in individuality

Reproductive division of labor has been proposed to play a key role in evolutionary transitions in individuality (ETIs). This chapter provides a guide to a theoretical model that addresses the role of a trade-off between life history traits in selecting for a reproductive division of labor during the transition from unicellular to multicellular organisms. In particular, it focuses on the five key assumptions of the model: (1) fitness is viability times fecundity; (2) collective traits are linear functions of their cellular counterparts; (3) there is a trade-off between cell viability and fecundity; (4) cell contribution to the collective is optimal; and (5) there is an initial reproductive cost in large collectives. Thereafter, the chapter contrasts two interpretations of the model in the context of ETIs. Originally, the model was interpreted as showing that, during the transition to multicellularity, the fitness of the lower level (the cells) is “transferred” to the higher level (the collective). However, despite its apparent intuitiveness, fitness transfer may obscure actual mechanisms in metaphorical language. Therefore, this chapter advocates an alternative and more conservative interpretation of the model that focuses on cell traits and the evolutionary constraints that link them. In addition, it allows for pursuing subsequent questions, such as the evolution of development.