Question

When researchers use a molecular clock what are they measuring?

Answer 1

Measures time from random changes (mutations) 

Answer 2

They are measuring the time that two species have been evolving independently.

A molecular clock is a method that is used by scientists to estimate the time that two species evolve separately using the knowledge of the DNA differences. To carry out this method, scientists use (i) an estimate of the number of genetic mutations that occur between two closely related species and (ii) the average genetic mutation rate to estimate the time in prehistory when the two species split.