A metaphor where information is passed through time can illustrate the concept.
Imagine an isolated monastery where an ancient book is copied out by generation after generation of monks.
The complexity of biology, however, makes the cogs of the molecular clock more fiddly than hoped.
Three aspects of evolution can mislead the molecular clock.
And on the smallest scale, epidemiologists are able to trace the spread of disease over mere decades.
The molecular clock has provided a narrative for evolution across short and long timescales.
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.
One year, a monk decides to venture forth into the world and he founds a new monastery, taking a copy of the book with him.
He continues copying the book out to preserve it through time and minor errors inevitably slip in.