SUMMARY
Mutations are critical for evolutionary change. Although mutation spectra (mutational biases) vary greatly across organisms, we lack direct evidence of their evolutionary consequences. Here, we show experimentally that a biased spectrum alters the distribution of fitness effects in a bacterium, and should facilitate adaptation by increasing beneficial mutations and reducing genetic load. Adaptive walk simulations show that this advantage arises via better sampling of mutational classes that were poorly explored by the ancestor. Hence, reversing the spectrum of a biased ancestor is generally advantageous, while introducing bias in an unbiased ancestor is selectively neutral. Indeed, across bacterial lineages, evolutionary transitions in DNA repair enzymes – which shape spectra – typically reverse ancestral bias. These broad consequences of mutation spectra imply a major role in shaping evolutionary dynamics.
Competing Interest Statement
The authors have declared no competing interest.