“MOLECULAR CLOCK” is a name describing methods and models that assess how rates of genetic evolution vary across the tree of life.This information is used to put timescale on this tree. DNA holds clues about the timing of key events in evolution
By comparing DNA sequences, geneticists can not only reconstruct relationships between different populations or species but also infer evolutionary history over deep timescales. Molecular clocks can trace the branches of a family tree back to the original root. As two species arose from a common ancestor, changes occurred in their genomes. The more the species diverged, the greater the number of genetic differences. Counting these differences, and dividing them by the rate of genetic mutation, takes us back in time to the ancestor.
For example, in 2012, researchers concluded that the AIDS epidemic in India had a common ancestor 40 years ago, and from this they could infer the spread of the disease. This informed medical policy, helped to predict where the disease would spread, and allowed the pre-emptive and effective deployment of resources, saving countless lives because of it