Impact of crowding on the diversity of expanding populations

Growing cell populations become densely packed as cells proliferate and fill space. Crowding prevents spatial mixing of individuals, significantly altering the evolutionary outcome from established results for well-mixed populations. Despite the fundamental differences between spatial and well-mixed populations, little is known about the impact of crowding on genetic diversity. With microbial colonies on plates, we show that the allele frequency spectrum is characterized by a power law for low frequencies. Using cell-based simulations and microfluidic experiments, we identify the origin of this distribution in the volume-exclusion interactions within the crowded cellular environment, enabling us to extend these findings to a broad range of dense populations. This study highlights the importance of cellular crowding for the emergence of rare genetic variants.