The effects of stochasticity at the single-cell level and cell size control on the population growth*

Establishing a quantitative connection between the population growth rate and the generation time distribution of single cells is a prerequisite for understanding evolutionary dynamics of microbes. However, existing theories fail to account for the experimentally observed correlations between mother-daughter generation times that are unavoidable when cell size is controlled for - which is typically the case. Here, we study population-level growth in the presence of cell size control. We derive a closed formula for the population growth rate and demonstrate that it only depends on the single-cell growth rate variability, not other sources of stochasticity or the details of the size control mechanism. We corroborate our theory using experimental data on E. coli. Our work provides an evolutionary rationale for the narrow growth rate distributions often observed in nature: when single-cell growth rates are less variable but have a fixed mean, the population will exhibit an enhanced population growth rate.