Environmental vs Demographic Noise in the Genetic Structures of Range Expansions

A key question in population genetics is how spatial variability in the environment influences the evolution of expanding populations competing for space, which may have far-reaching consequences, e.g., for the emergence of antibiotic resistance in bacterial colonies. In the context of neutral evolution, we examine the interplay between intrinsic noise caused by stochastic variation in growth processes, e.g. genetic drift, and extrinsic noise caused by spatially heterogeneous environments. Using a multi-species Eden model, we simulate a growing population expanding over a landscape with a quenched-random distribution of nutrients, and we measure how genealogical tree structure is affected by the distribution of landscape features. Our results reveal an environmental 'pinning' effect on the genetic lineages, which we can understand as random walks that are highly biased toward certain fastest paths. We also demonstrate non-monotonic behavior of measures relevant to population genetics as the density of nutrient "hot-spots" increases.