1. 26.5 Molecular Clocks Help Track Evolutionary Time

2. Molecular Clocks
Molecular clocks use an approach for measuring the absolute time for evolutionary changes based on the observation that some genes and other regions of genomes appear to evolve at constant rates.
The molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving independently.

3. Applying a Molecular Clock: Dating the Origin of HIV
The comparison of various samples of HIV are used to determine when HIV was actually first introduced into the human population.