Evolution of division of labor in self-organizing spatially structured groups

Abstract

In an evolutionary transition in individuality, a collection of individuals organizes into a group that becomes an individual in its own right. Division of labor among individuals in a group is frequently used as an indicator of individuality on the level of the group. Despite extensive research, the processes and circumstances that contribute to an evolutionary transition in individuality are largely unknown. Notably, many models of evolutionary transitions in individuality take a group-structured population as their starting point. In contrast, we ask whether emergent rather than pre-defined groups can be the onset of an evolutionary transition in individuality. To that end, we use an existing model where individuals evolve a gene for altruism, and thereupon self-organize into spatially structured groups that are units of selection in their own right. An individual that behaves altruistically pays a reproductive cost, but provides a reproductive benefit to others. Here, the model is extended to enable phenotypic differentiation between individuals that display altruistic behavior and individuals that do not. We find that a reproductive division of labor can be selected upon the emergence of spatially distinct groups. Moreover, we show that sterile helpers evolve in the system, and that a small fraction of altruists is sufficient to maintain the emergent group structure. Thus, a pre-defined group structure is not required for selection of division of labor. However, we stress that division of labor is not necessarily selected on the level of the group, but might also increase the fitness of individuals within a group. Future work is needed to determine the roles of within- and among-group selection in the evolution of division of labor. Therefore, we question whether division of labor is a good indicator of group-level individuality.