Frequency of changes in genome sequence.
This can be measured in terms of number of mutations per genome per generation, number of mutations per nucleotide per genome replication episode, etc. Importantly, mutation rates can vary between as a function of genotype and therefore can be subject to .
Generally the greater the mutation rate then the faster variation upon which natural selection can act can enter a population, though note that equivalent increases in number of mutations entering a population can be accomplished instead by increasing .
Note too that with too high rates, along with the preponderance of deleterious mutations among mutations, the fitness of individuals within populations can be reduced. Ultimately what mutation rates will be optimal will be dependent on to what extent adaptation is limited by mutation rates, the extent to which adaptation is important in terms of maintaining fitness, and the cost of acquisition by progeny of deleterious mutations.