The impact of nonheritable variation in division rates on population growth across environments

Abstract

Phenotypic heterogeneity is a pervasive feature of biological populations, yet its impact on population growth is often interpreted through changes in mean individual fitness alone. In this paper, we investigate how nonheritable variability in division rates influences the asymptotic growth of a population. Using a class of linear models with phenotypic structure, we show that variability modifies the dominant eigenvalue of the system in a nonlinear manner, leading to an intrinsic tradeoff: while variability reduces population growth under favourable conditions, it mitigates population decline under stress. These results provide a simple mechanistic framework for understanding how heterogeneity influences population-level dynamics. In particular, they suggest that stress-dependent amplification of mutational effects may arise from changes in phenotypic variability, rather than from changes in mean fitness alone. We illustrate this mechanism using mutation accumulation data in Chlamydomonas reinhardtii, where the observed patterns of relative fitness under increasing stress are consistent with increased variability within genotypes. More broadly, our analysis highlights the importance of variability as a determinant of population growth, and shows that some effects commonly attributed to changes in mean fitness may instead reflect the nonlinear consequences of phenotypic heterogeneity.