Final answer:
Molecular clocks estimate how long it has been since two species diverged from a common ancestor by comparing DNA sequence differences. They rely on the assumption that genetic mutations accumulate at a steady rate over time.
Step-by-step explanation:
The question "One of their methods to measure molecular differences between species relied on?" refers to the utilization of molecular clocks, which are tools used to measure the molecular differences between species by analyzing DNA or protein sequences. This method estimates the divergence time of species from a common ancestor, based on the molecular evidence such as the rate of genetic mutations. Molecular clocks assume that these mutations accumulate at a constant average rate for a given DNA region.
Species that show greater differences in their DNA sequences are assumed to have diverged earlier in geologic history from their shared ancestor. To refine the method, different molecular clock estimations based on various DNA regions may be used together to increase accuracy. Furthermore, the time of divergence based on molecular data can be compared with those estimated from the fossil record to get a better understanding of the species' evolutionary timeline.