Clonal interference patterns shift during range expansion

The movement of a population through space can have profound impacts on its evolution, as observed theoretically, experimentally, and clinically. Furthermore, it has been observed that mutants emerging at the spreading front develop higher frequencies in the population than their counterparts further from the front. Here we use fundamental arguments from population genetics regarding expected time scales of beneficial mutant establishment and fixation in a population undergoing range expansion to characterize the degree of clonal interference expected in various regions while the population is migrating. By quantifying the degree of clonal interference along the wave front of a population undergoing range expansion using a measure we term the Clonal Interference Index, we show that evolution is increasingly mutation-limited toward the wave tip. In addition, we predict that the degree of clonal interference varies non-monotonically with respect to position along the wave front. The work presented here extends a powerful framework in population genetics to a canonical physical model of range expansion, which we hope allows for continued development of these models in both fields.