The mutation rate is the rate at which organisms experience genetic mutations, which can include simple genetic variations or more complex mutations which result in a considerable divergence from other organisms of the same species. The mutation rate is usually expressed as the chance of mutations within a single generation. People sometimes confuse mutation rates with mutation frequency: the rate refers to a probability of mutation, while the frequency actually involves counting the number of organisms with a particular genetic mutation.
A species that has a high mutation rate can find it easier to adapt to a changing environment. However, high mutation rates can be bad for individuals, because many mutations are not beneficial. As a result, the high rate of mutation would cause many individuals to die, while a few developed to respond to environmental changes and perpetuate the species. This makes high rates good for species as a whole, but not good for individual representatives.
A low mutation rate, on the other hand, is good for individuals, because their chances of being born with a mutation are relatively low. Low rates can be bad for species, though, because they make it hard for organisms to adapt favorable mutations which help the organism respond to changes in the natural environment.
One of the classic examples of a high mutation rate can be found in many bacteria and viruses. These organisms find it beneficial to mutate rapidly, because they reproduce in huge numbers, so losing numerous individuals doesn't hurt the species as a whole. The high rate of mutation also allows them to adapt to situations which can include the need to incubate for an extended period of time, or the introduction of new drugs which kill off many individuals within the species.
High mutation rates can pose a serious problem when they occur in organisms which cause disease, because it is difficult to eradicate these organisms from the body. HIV, for example, has an infamously high mutation rate which allows it to constantly evade new medications introduced to combat it.
In addition to having an impact on the development of a species over time, mutation rates can also be used to gather information about the history of a species. Researchers have tracked specific changes to determine when organisms diverged genetically, utilizing mutations much like clocks. By rolling back the mutations over time, using data about an organism's mutation rate, a researcher can estimate when two related species might have been the same species, or genetic research can find a common ancestor.