Cooperation mitigates diversity loss during spatial range expansions

Spatially expanding populations are known to experience rapid diversity loss because of the serial founder effect - the small number of individuals at the tip of the expansion wave, that seed the population in a new territory, makes them susceptible to demographic fluctuations, leading to loss of diversity. However, this is only true in competitively growing populations, in which the per capita growth rate of the population decreases monontonically with increasing density. For cooperative populations, which grow the fastest at intermediate optimal densities, the founder effect is mitigated because of migration from the fast-growing high density bulk of the expansion wave. In previous work, we have demonstrated that yeast populations undergo a transition from pulled to pushed expansion waves as cooperation is increased. Here, we extend the experimental system to demonstrate that pushed waves, caused by increased cooperation, indeed mitigate the rate of diversity loss. We also experimentally quantify the effective population size during expansions and its scaling relationship with the bulk population size.