What is mutation bias and how can scientists study it? How does changing a population’s mutation bias influence its evolutionary trajectory? In this episode, we talk with Deepa Agashe, an Associate Professor at the National Centre for Biological Sciences in Bangalore, India. We first talk with Deepa about mutation bias and how she uses  E. coli to understand it. We then focus on a 2023 PNAS paper about the fitness effects of experimentally changing the mutation bias in E. coli. In this research, Deepa and her team used a strain (MutY) of bacteria containing a mutation that knocks out an important DNA repair enzyme. They then isolated subsequent single mutations produced within both MutY and wildtype lines and studied the fitness effects of those mutations. Surprisingly, more than a third of mutations in the mutant lines were beneficial, and often across several different environments. Zooming out, the big picture is that shifts in mutation bias seem to generate new kinds of mutations that weren’t previously accessible to lineages, and a greater fraction of those may be beneficial in some circumstances. Art by Keating Shahmehri. Find a transcript of this episode on our website. --- Support this podcast: https://podcasters.spotify.com/pod/show/bigbiology/support