The molecular clock

Observations about amino acid changes that occurred during the divergence between species, show that molecular evolution takes place at a constant rate.

This suggests that molecular evolution is constant enough to provide a molecular clock of evolution. This means that the amount of molecular change between two species measures how long ago they shared a common ancestor. Molecular differences between species are therefore used to infer phylogenetic relations.

The geneticist Linda Partridge discusses the molecular clock.

Figure: the rate of evolution of hemoglobin. Each point on the graph is for a pair of species, or groups of species. Some of the points are for a-globin, others for -globin. From Kimura (1983).