  In an adapted population of mutators in which most mutations are deleterious,
a nonmutator that lowers the mutation rate is under indirect selection and can
sweep to fixation. Using a multitype branching process, we calculate the
fixation probability of a rare nonmutator in a large population of asexual
mutators. We show that when beneficial mutations are absent, the fixation
probability is a nonmonotonic function of the mutation rate of the mutator: it
first increases sublinearly and then decreases exponentially. We also find that
beneficial mutations can enhance the fixation probability of a nonmutator. Our
analysis is relevant to an understanding of recent experiments in which a
reduction in the mutation rates has been observed.
