A "molecular clock" is any technique that may be used to calibrate the age of a species / taxonomic group by using the mutation rate of one genomic sequence.
The mutation rate is the time required for one sequence to mutate (change) in its nucleotide bases.
The mutation rate is used to estimate the age of one species by molecular clocks that estimate nucleotide differences in a given period of time.
In conclusion, a "molecular clock" allows estimating the age of a species by using the mutation rate.
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