Answer
A molecular clock is steady accumulation of mutations over time, allowing the dated fossil (or other) record to be connected to the molecular genetic record. It assumes that mutations (marked by base pair changes) are accumulated at steady, unchanging rate.
Work Step by Step
The molecular clock is a useful tool, but it must be used with care. We now know that the mutations we find today have not always accumulated at a uniform rate, meaning that the molecular clock is sometimes inaccurate.
