The effect of population bottleneck size and selective regime on genetic diversity and adaptability in bacteria

Population bottlenecks are sudden and often drastic reduction in population size and are ubiquitous in natural ecosystems. Bottleneck events reduce the genetic diversity, which leads to a decreased potential of rapid adaptation. We were interested in how the interplay of genetic drift (population size reduction) and adaptive evolution (selective regime) affects Escherichia coli genome evolution. Therefore, we conducted an evolution experiment with E. coli populations in two temperature regimes and three different bottleneck sizes. After 1000, we observed differences across bottleneck sizes regarding genetic diversity, parallel evolution and adaptability that were highest in the large (weak) bottleneck size. Nevertheless, the selection applied had a stronger effect on most parameters. Thus, we suggest that random genetic drift may had less impact than selection on the genomes evolution of E. coli regardless of the strength of the population bottleneck (i.e., drift). Our results can be of great assistance in interpreting the results of natural (dynamic) ecosystems.