Dr. Kirill Korolev, 2010–2013 Pappalardo Fellow (Soft Condensed Matter Theory & Biophysics) The evolution of natural populations involves more than mutations followed by natural selection—the reshuffling of genes during genetic recombination, stochasticity, and spatial migrations are also important. The effects of fluctuations and spatial structure become especially pronounced when organisms spread to new territories because the number of organisms at the front of the expansion is typically small. The interplay of number fluctuations and migration causes spatial genetic demixing, i.e., spatial segregation of different genotypes. We formulated a simple phenomenological model that describes genetic demixing, and tested its predictions in bacterial colonies growing on the surface of a Petri dish. In addition, we developed a set of experimental methods to estimate important biological parameters of microbial colonies (migration rates, selective advantage, and effective population size responsible for the fluctuations). These methods could be useful for future studies of evolutionary dynamics in populations with spatial structure. More importantly, genetic demixing affects many evolutionary processes. In particular, we showed that mutualism between two species can evolve only when it is sufficiently strong and fair, i.e., both species benefit equally.