Abstract: Evolutionary analyses have focused on natural selection as the major driver of evolutionary change, with mutations thought to play a relatively small role. Analyses of mutation typically focus on mutation rate, because it constraints the supply of beneficial mutations during adaptation. However, a growing body of work now shows that mutation bias – whereby some types of mutations occur more frequently than others – is also important during evolution. Because mutation biases determine the diversity of genetic variants available for selection, they can constrain the genetic paths to adaptation. In this talk, I will discuss our recent and ongoing work to understand how shifts in mutation bias shape evolutionary trajectories in microbial populations. Our experimental and theoretical results demonstrate that a large change in the direction of mutation bias can influence the distribution of fitness effects of mutations, in turn affecting pleiotropy and the repeatability of evolutionary outcomes. Hence, to fully understand and predict evolution in laboratory, clinical, and natural contexts, we must account for the type – not just the rate – of mutation supply.