Growth dynamics and the evolution of cooperation in microbial populations

Microbes providing public goods are widespread in nature despite
running the risk of being exploited by free-riders. However, the
precise ecological factors supporting cooperation are still
puzzling. Following recent experiments, we consider the role of
population growth and the repetitive fragmentation of populations
into new colonies mimicking simple microbial life-cycles.
Individual-based modeling reveals that demographic fluctuations,
which lead to a large variance in the composition of colonies,
promote cooperation. Biased by population dynamics these
fluctuations result in two qualitatively distinct regimes of robust
cooperation under repetitive fragmentation into groups. First, if
the level of cooperation exceeds a threshold, cooperators will take
over the whole population. Second, cooperators can also emerge from
a single mutant leading to a robust coexistence between cooperators
and free-riders. We find frequency and size of population
bottlenecks, and growth dynamics to be the major ecological factors
determining the regimes and thereby the evolutionary pathway
towards cooperation.