Unifying (simple) models of genetic robustness and evolvability

Here, I consolidate and extend simple models of how genetic robustness affects the evolvability of phenotypes with discrete states. I share three main insights. (1) What we already know about the effects of mutational robustness on evolvability can be readily transferred to environmental robustness. (2) Even without a hard genotype-level trade-off between robustness and evolvability, the optimal level of phenotypic robustness depends on the rate of environmental change. But counter-intuitively, when adaptive landscapes are complex, an increase in environmental stability can increase the frequency of environmentally-robust but mutationally-sensitive genotypes. (3) Even with a fixed probability that a mutation is neutral, populations can evolve along the spectrum of robustness and evolvability by evolving the genotype-determined neighborhood of mutationally-accessible phenotypes. Indeed, because it allows for the evolution of increased evolvability without a concomitant increase in genetic load, selection should favor changes in the phenotypic neighborhood over changes in mutational sensitivity. Teaser TextModels of the evolution of discrete phenotypes show that their evolvability is highest when environments fluctuate at intermediate rates, and with intermediate levels of mutational robustness. By merging previous models and relaxing previous assumptions, I show that previous insights are robust. I also show that the same models tell us very different things about the evolvability of discrete traits, namely, that what applies to robustness against mutation is readily transferable to robustness against environmental perturbation, and that selection for evolvability will preferentially work on the neighborhood of mutationally-accessible phenotypes rather than the probability of non-neutral mutation.