Evolution is the process by which organisms are modified over time. The fundamental mechanisms underlying evolution are mutations, natural selection and genetic drift. While natural selection favors fitter individuals, genetic drift, which arises from finite size effects, yields random changes in allele frequencies. A key factor impacting the amplitude of these fluctuations, and more broadly evolutionary dynamics, is the spatial structure of populations. When a population is subdivided into demes of finite size connected by migrations, the way it explores its fitness landscape differs from the one of well-mixed populations, in a way that depends on the specific characteristics of the spatial structure. Indeed, the fixation probability of the lineage of a mutant is structure- dependent. In addition, spatial structure affects ecological interactions between individuals, which ultimately impacts evolutionary dynamics. Typically, the more subdivided a population, the sparser the physical interactions between individuals, which tends to reduce competition between them.
In this thesis, we aim to assess the influence of spatial structure and finite size effects on the evolution and ecology of asexual microbial populations. We first examine the impact of population size on the exploration of rugged fitness landscapes by well-mixed populations, using a biased random walk model. Then, we turn our attention to the evolution of structured populations in rugged fitness landscapes and investigate the impact of spatial structure on the way bacterial populations explore their fitness landscapes. Finally, we develop a lattice-gas agent-based model to describe a system of Vibrio cholerae bacteria in liquid conditions. These bacteria interact with each other through type IV pili (appendages used by cells to bind to each other) and the type VI secretion system (a kin-discriminatory molecular weapon). In this system, the spatial organization is key to understanding the extent of non-kin depletion caused by cells carrying a functioning type VI secretion system. Therefore, spatial structure can strongly impact the ecology and evolution of V. cholerae.
Regarding the evolution of well-mixed populations, we find that adaptation is often more efficient at finite population size. Specifically, there is often a finite population size optimizing the search for high fitness peaks. This result highlights the significant role of finite size effects in evolution. In the case of spatially structured populations, we find that adaptation is often more efficient when there is a migration asymmetry associated with the presence of suppression of selection within the structure. We further find that the more suppression of selection, the smaller the effective population size for steady-state properties which matter for long-term evolution. Finally, the experimental observations about the interplay between type IV pili and the type VI secretion system, which yields a predator-prey dynamics in the system considered, are well reproduced by simulations from our model. Our results suggest that the spatial organization of the population, which is shaped by type-IV-pili-mediated interactions, is key to understanding the extent of the killing of prey by predators. Typically, if predators do not bind to prey, killing is largely prevented.