Microbial range expansions in time-varying environments

Classical population genetics studies assume that populations are well-mixed and environments are constant in time, a situation that is rarely encountered in nature. Here, we investigate how temporal fluctuations of the environment affect the interaction of different genotypes at the front of microbial range expansions. We perform experiments with different strains of the budding yeast, S. cerevisiae, whose relative fitness switches following changes in the environmental state. Our experiments show that physical interactions such as growth-induced pressure gradients can significantly enhance the survival of less fit genotypes at the front of range expansions, thus aiding their recovery following environmental changes that increase their selective advantage. We investigate the physical causes of such an enhanced survival using continuum models inspired by overdamped fluid dynamics and use them to investigate spatial population genetics in fluctuating environments.