Matti Gralka1, Diana Fusco1, Oskar Hallatschek2. 1. Biophysics and Evolutionary Dynamics Group, Departments of Physics and Integrative Biology, University of California, Berkeley, California. 2. Biophysics and Evolutionary Dynamics Group, Departments of Physics and Integrative Biology, University of California, Berkeley, California. Electronic address: ohallats@berkeley.edu.