Evolution of multicellularity driven by emergent collective migration

The evolution of multicellularity is a major evolutionary transition: individual cells give up their reproductive autonomy to form aggregates. Aggregation evolves because it can confer a fitness advantage over unicellularity, e.g. because of protection from predators, functional specialisation or because aggregates can respond to environmental cues unavailable to single cells. These aggregate-level properties arise from cell-cell interactions, and determine the evolutionary course of the cells by imposing novel selection pressures. Thus, the evolutionary feedback between cell interactions and group-level properties is at the root of the evolution of multicellularity. We explore the emergence of multicellular aggregates in a computational model where a population of cells searches for resources by chemotaxis in a spatially and temporally noisy gradient. Cells can evolve their adhesion to one another, and are selected on a cell's distance from the source of the gradient as a proxy for the availability of resources. We show that undifferentiated multicellularity evolves because cell aggregates perform collective chemotaxis more efficiently than single cells. A unicellular strategy based on efficient dispersal (rather than collective movement) can also evolve when environmental changes occur frequently. We find that both strategies prevent the invasion of the other through interference competition. We conclude that collective behaviour can be an emergent driver of the evolution of adhesion - and therewith undifferentiated multicellularity.