Abstract
The evolutionary transition to multicellularity has occurred on numerous occasions, but transitions to complex life forms are rare. While the reasons are unclear, relevant factors include the intensity of within versus between group selection that are likely to shape the course of life cycle evolution. A highly structured environment eliminates the possibility of mixing between evolving lineages thus ensuring strong competition between groups. Less structure intensifies competition within groups decreasing opportunity for group-level evolution. Here we present experiments that contrast two ecological frameworks that differ in the way in which nascent multicellular groups, and their constituent cells, compete. Groups of the bacterium Pseudomonas fluorescens were propagated under a regime requiring reproduction via a life cycle with developmental and dispersal phases. By controlling the extent of mixing during the dispersal phase it was possible to alter the relative emphasis on the two phases. While all groups possessed ‘paradigmatic’ features of multicellular individuals (e.g., bottleneck and germ line), the mode of group interaction substantially affected the strength and direction of selection operating at both group and cell levels. Constraints on meta-population structure may therefore explain the observation that multicellular aggregates rarely complete the transition to individuality.
