The Eco-Evo dynamics of asexual microbial populations with fluctuating population dynamics

Abstract

Population size is one of the major factors in determining evolutionary trajectories in any given environment. It is known that larger populations are more likely to adapt to stressful environmental conditions because of a stronger effect of selection, whereas smaller populations, due to the greater effect of genetic drift, show reduced fitness changes. Most of these empirical studies have been conducted with populations that were maintained at constant sizes throughout. However, in nature, fluctuations in population sizes are fairly common, e.g. in microbial pathogens that are transmitted across hosts or from host to external environments. To study the effects of various regimes of population size fluctuations on evolutionary dynamics under resource-limited conditions both in the presence or absence of a novel niche, we conducted experimental evolution using Escherichia coli populations for 640 generations. We observed that larger populations generally perform better. Smaller populations show better collateral adaptation to novel niches when selected in absence of any such factor. Large populations demonstrate an increased effect of drift when evolving in the presence of novel environments. Finally, faster-fluctuating populations are observed to have higher fitness than slower fluctuating populations in the early stages, however, they evolve to have similar fitness by the end of 640 generations. Further experiments are needed to elucidate the factors that cause such differences in evolutionary trajectories of the fluctuating populations.