Adaptation in pushed waves

Spatial range expansions are ubiquitous in nature, ranging from surface-dwelling biofilms to mammalian species migrating due to climate change and habitat loss. Most of our understanding of evolution during range expansions stems from theoretical studies on pulled traveling waves, in which pioneer individuals living at the edge of the wave have the highest chance of colonizing new territory and reproducing. The corresponding small effective population size leads to inefficient selection, which can cause the accumulation of deleterious mutations at the front with consequent decrease in fitness.

Using theory and simulations, we find that introducing cooperative effects in the population, which changes the dynamics of the wave from pulled to pushed, increases the effect of selection and allows beneficial mutations to establish more readily while purging deleterious mutations. At low mutation rate, in the sequential fixation regime, strong cooperation thus leads to faster adaptation. By contrast, at high mutation rate, clonal interference dominates and the adaptation rate exhibits a non-monotonic trend as a function of cooperation.