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

Population bottlenecks leading to a drastic reduction of the population size are common in the evolutionary dynamics of natural populations; their occurrence is known to have implications for genome evolution due to genetic drift, the consequent reduction in genetic diversity and the rate of adaptation. Nevertheless, an empirical characterization of the effect of population bottleneck size on evolutionary dynamics of bacteria is currently lacking. Here we show that selective conditions have a stronger effect on the evolutionary history of bacteria in comparison to genetic drift following population bottlenecks. We evolved Escherichia coli populations under three different population bottlenecks (small, medium, large) in two temperature regimes (37°C and 20°C). We find a high genetic diversity in the large in comparison to the small bottleneck size. Nonetheless, the cold temperature led to reduced genetic diversity in all bottleneck sizes, hence, the temperature has a stronger effect on the genetic diversity in comparison to the bottleneck size. A comparison of the fitness gain among the evolved populations reveals a similar pattern where the temperature has a significant effect on the fitness. Our study demonstrates that population bottlenecks are an important determinant of the evolvability in bacteria; their consequences depend on the selective conditions and are best understood via their effect on the standing genetic variation.