spontaneous mutation

A spontaneous mutation arises on its own, from the cell's ordinary chemistry and machinery, with no outside agent prodding it. It is the genetic equivalent of a typo made by a careful typist simply because no one and nothing is perfect.

These mutations come mainly from two sources. The DNA-copying machinery, though remarkably accurate, occasionally inserts the wrong base or slips on repeats. And DNA is a chemical molecule that decays even when undisturbed: bases can lose a part of themselves or change form through reactions with water and the cell's own metabolites. Most such errors are caught and fixed by repair, but a small fraction become permanent.

Spontaneous mutations set a baseline rate of new variation that occurs in every generation regardless of environment, in contrast to induced mutations caused by a mutagen. This steady trickle is what supplies populations with fresh raw material over the long run, and it is measurable: the per-generation mutation rate in humans is roughly on the order of one in a hundred million bases per site.