Measurements Of Mutation Rate
Mutation rate is often difficult to measure. The frequency of existing mutations in a population is not a good indication of the mutation rate, since a single mutation may be passed on to many offspring. In addition, there are often selective pressures that increase or decrease the frequency of a mutation in a population.
Mutation rates differ widely from one gene to another within an organism and between organisms. Generally mutation rates in bacteria are about one mutation per one hundred million genes per generation. While this sounds quite low, consider that the Escherichia coli bacteria in our intestines produce more than 20 billion new bacteria every day, each of which has approximately four thousand genes. This works out to about ten million new mutations in the population every day. In mice, the rate is about one mutation per ten thousand genes per generation. While this is much larger than the rate for bacteria, the mouse generation time is also much greater.