Breaking through evolutionary constraint by environmental fluctuations

Epistatic interactions can frustrate and shape evolutionary change 1,2 . Indeed, phenotypes may fail to evolve because essential mutations can only be selected positively if fixed simultaneously . 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 genotypeenvironment 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. . CC-BY-NC-ND 4.0 International license not peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/016790 doi: bioRxiv preprint first posted online Mar. 20, 2015;