A
mutation which occurs by itself without first being affected by a
mutagen, for example during the process of
DNA replication. Spontaneous mutations arise at a remarkably
constant rate. The rate that spontaneous mutations arise has been used as an
evolutionary clock to estimate how closely related two (or more) separate
species are to each other.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.