The evolution of robustness and fragility during long-term bacterial adaptation

Most mutations affecting fitness are harmful, and their inevitable occurrence reduces mean population fitness. Theory predicts that well-adapted populations may evolve mechanisms to minimize this deleterious load. Direct selection to increase mutational robustness can be achieved in the laboratory. However, its spontaneous evolution during general adaptation remains uncertain, with mixed evidence across model systems. Here, we studied the effects of highly pleiotropic point mutations in Escherichia coli over a 15,000-generation adaptive trajectory. The fitness effects of both beneficial and deleterious mutations were attenuated with increased adaptation over time. In contrast, pleiotropic effects in new environments became more severe and widespread with greater adaptation. These results show that trade-offs between robustness and fragility can rapidly evolve in regulatory networks, regardless of whether driven by adaptive or non-adaptive processes. More broadly, these results show that adaptation can generate a hidden potential for phenotypic diversity, unpredictably shaping evolutionary prospects in new environments.