Evolutionary Capacitance Emerges Spontaneously during Adaptation to Environmental Changes

All biological populations are to a greater or lesser degree evolvable, but the forces that shape evolvability, especially the evolution of evolvability as an adaptive response to pressure to evolve rapidly, are a source of controversy. One determinant of evolvability is the set of possible mutations available to a genotype, or “mutational neighborhood.” A benign (instead of deleterious) mutational neighborhood can enhance evolvability. Whether selection for evolvability itself can result in a more benign mutational neighborhood remains an open question. We document the evolution of increased evolvability in a fluctuating environment despite using a regime that precludes, by design, the adaptive evolution of evolvability. Instead, a benign mutational neighborhood arises as a byproduct of transiently elevated error rates via a mechanism that we call emergent evolutionary capacitance. This is evolutionary capacitance similar to that seen in a model system for capacitance, the yeast prion [PSI+], but without a capacitor. Unexpressed genetic sequences are an important reservoir of genetic variation, but because they are invisible to selection, they are expected to be mostly deleterious. Nelson and Masel describe how environmental change can result in unmasking of unexpressed genetic variation, improving the quality of these sequences and thus increasing evolvability.