Breaking through evolutionary constraint by environmental fluctuations

Abstract

Epistatic interactions can frustrate and shape evolutionary change 1, 2 3-7. Indeed, phenotypes may fail to evolve because essential mutations can only be selected positively if fixed simultaneously 5,8,9. How environmental variability affects such constraints is poorly understood. Here we studied genetic constraints in fixed and fluctuating environments, using the Escherichia coli lac operon as a model system for genotype-environment interactions. The data indicated an apparent paradox: in different fixed environments, mutational trajectories became trapped at sub-optima where no further improvements were possible, while repeated switching between these same environments allowed unconstrained adaptation by continuous improvements. Pervasive cross-environmental trade-offs transformed peaks into valleys upon environmental change, thus enabling escape from entrapment. This study shows that environmental variability can lift genetic constraint, and that trade-offs not only impede but can also facilitate adaptive evolution.