  The observation that phenotypic variability is ubiquitous in isogenic
populations has led to a multitude of experimental and theoretical studies
seeking to probe the causes and consequences of this variability. Whether it be
in the context of antibiotic treatments or exponential growth in constant
environments, non-genetic variability has shown to have significant effects on
population dynamics. Here, we review research that elucidates the relationship
between cell-to-cell variability and population dynamics. After summarizing the
relevant experimental observations, we discuss models of bet-hedging and
phenotypic switching. In the context of these models, we discuss how switching
between phenotypes at the single-cell level can help populations survive in
uncertain environments. Next, we review more fine-grained models of phenotypic
variability where the relationship between single-cell growth rates, generation
times and cell sizes is explicitly considered. Variability in these traits can
have significant effects on the population dynamics, even in a constant
environment. We show how these effects can be highly sensitive to the
underlying model assumptions. We close by discussing a number of open
questions, such as how environmental and intrinsic variability interact and
what the role of non-genetic variability in evolutionary dynamics is.
