Environmental disorder can tip the population genetics of range expansions

Evolutionary dynamics is fundamentally shaped by stochastic processes: spontaneous mutations enter populations randomly, and the fate of a mutant lineage is determined by the competition between (random) genetic drift and (deterministic) selection. In populations undergoing range expansions, fluctuations in the reproductive process and the local motion of individuals are enhanced within a small subpopulation at the edge of the population. Range expansions are typically studied in homogeneous environments, but we argue here that the fluctuations at the range edge are susceptible to small-scale environmental heterogeneities that may have a strong effect on the evolutionary dynamics of the expanding population. To show this, we tracked the dynamics of the clones of spontaneous mutations with a tunable fitness effect in bacterial colonies grown on randomly disordered surfaces. We find that environmental heterogeneity on scales much larger than an individual, but much smaller than the total population, can dramatically reduce the efficacy of selection. Time lapse microscopy and computer simulations suggest that this effect is a general consequence of a local "pinning" of the expansion front, whereby stretches of the front are slowed down on a length scale that depends on the structure of the environmental heterogeneity. This pinning focuses the range expansion into a small number of individuals with access to expansion paths, increasing the importance of chance and thus limiting the efficacy of selection.