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It has been one billion years since plants and animals separated, yet of the 103 amino acids in this protein, there is only one difference between a pea and a cow (Zihlman 2001)!
This is due to the enormous amount of functional constraint on this molecule as a result of its essential function.
Some genes have been shown to evolve at disparate rates across genes and lineages and over time(Ayala et al.1998).
A great deal of data exists that shows this variation.
One of the most widely used examples are the copper (Cu), zinc (Zn) superoxide dismutase (SOD) and glycerol-3-phosphate dehydrogenase (GPDH) proteins.
If this rate is reliable, the gene could be used as a molecular clock.
When a stretch of DNA does indeed behave like a molecular clock, it becomes a powerful tool for estimating the dates of lineage-splitting events.