Biophysical constraints determine the selection of phenotypic fluctuations during directed evolution*

We address the question of the role of selection in evolution and its relationship with phenotypic fluctuations. Phenotypic fluctuations have been conjectured to be beneficial characteristics to protect against fluctuating selection from environmental changes, the so-called “bet-hedging” strategy. However, it is not well-understood how phenotypic fluctuations shape the evolutionary trajectories of organisms. To address these questions we have performed directed evolution experiments and modeling on the speed of migration phenotype of chemotactic bacteria. We present a theoretical model that recapitulates the observed reduction of phenotypic fluctuations in experiment. Our stochastic modeling on the evolution of migration fronts suggests that whether or not phenotypic fluctuations grow or shrink during successive rounds of selection and growth is determined by both strength of selection and the existence of physical constraints.