The evolution of mutation rates.

 Deleterious mutations occur in many loci.  Mutator genes will usually be associated sooner or later with a deleterious mutation they have caused and will decline in frequency.  Therefore, natural selection in sexual populations will tend to eliminate any allele that increases mutation rates, and mutation rates should evolve toward the minimal achievable level.  Mutation rates are expected to evolve to the optimal achievable level because selection is not strong enough to favor investment in a more efficient system.