Environmental heterogeneity limits the action of selection

Evolutionary dynamics is fundamentally shaped by stochastic processes: 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. Geographical heterogeneities could therefore have a dramatic impact on evolutionary dynamics if they shape the local advance of the population front.

To test this, we track the dynamics of spontaneous mutations with a tunable fitness effect in colonies of E. coli grown on randomly disordered surfaces and find that environmental heterogeneity 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.