What is a molecular clock and why is it important when using molecular techniques to create a phylogeny?

Question: What is a molecular clock and why is it important when using molecular techniques to create a phylogeny?

Solution: Molecular clock is a hypothesis that predicts a constant rate of molecular evolution among species. It is also a method of genetic analysis that can be used to estimate evolutionary rates and timescales using data from DNA or proteins.
Molecular clocks are typically used in phylogenetic analyses, which aim to reconstruct evolutionary trees that show the relationships among species of interest. Internal nodes in the tree represent evolutionary divergence events. The timing of these events can be estimated using molecular clocks. A number of statistical methods are available for testing the molecular-clock hypothesis for a given set of DNA or protein sequences. When the molecular clock is rejected for a data set, one can use a statistical model to account for rate variation when estimating evolutionary timescales. When estimating evolutionary timescales in a phylogenetic analysis, the molecular clock needs to be calibrated.

In a phylogenetic tree, the species or groups of interest are found at the tips of lines referred to as the tree's branches. For example, the phylogenetic tree below represents relationships between five species, A, B, C, D, and E, which are positioned at the ends of the branches: