Bottlenecks inhibited mutator fixation across all parameter combinations simulated. The probability of fixation (circles) declined while the average time to fixation (squares) increased faster in populations with smaller mutations. Beneficial mutations were drawn from exponential distributions with means 0.05 (solid), 0.02 (dashed), or 0.01 (dotted). All populations were propagated at N = 106, while Ub varied between 10−6 (A), 10−7 (B), and 10−8 (C). The mutator increased the mutation rate 50-fold. Error bars on the time to fixation represent 1 standard deviation. Results are averaged over 1000 runs of the simulation.