Evolutionary clocks are genetic sequences within genes that can help determine when in the past species diverged from a common ancestor. There are certain patterns of nucleotide sequences that are common among related species that seem to change at a regular time interval. Knowing when these sequences changed in relation to the Geologic Time Scale can help determine the age of the species' origin and when speciation occurred.
History of Evolutionary Clocks
Evolutionary clocks were discovered in 1962 by Linus Pauling and Emile Zuckerkandl. While studying the amino acid sequence in hemoglobin of various species. They noticed that there seemed to be a change in the hemoglobin sequence at regular time intervals throughout the fossil record. This led to the assertion that the evolutionary change of proteins was constant throughout geologic time.
Using this knowledge, scientists can predict when two species diverged on the phylogenetic tree of life. The number of differences in the nucleotide sequence of the hemoglobin protein signifies a certain amount of time that has passed since the two species split from the common ancestor. Identifying these differences and calculating the time can help place organisms in the correct place on the phylogenetic tree in respect to closely related species and the common ancestor.
There are also limits to how much information an evolutionary clock can give about any species. Most of the time, it cannot give an exact age or time when it was split off of the phylogenetic tree. It can only approximate the time relative to other species on the same tree. Often, the evolutionary clock is set according to concrete evidence from the fossil record. Radiometric dating of fossils can then be compared to the evolutionary clock to get a good estimation of the age of the divergence.
A study in 1999 by FJ Ayala came up with five factors that combine to limit the functioning of the evolutionary clock. Those factors are as follows:
- Changing the amount of time between generations
- Population size
- Differences specific to a certain species only
- Change in the function of the protein
- Changes in the mechanism of natural selection
Even though these factors are limiting in most cases, there are ways to account for them statistically when calculating times. If these factors do come in to play, however, the evolutionary clock is not constant like in other cases but is variable in its times.
Studying the evolutionary clock can give scientists a better idea of when and why speciation occurred for some parts of the phylogenetic tree of life. These divergences may be able to give clues as to when major events in history happened, such as mass extinctions.