The purpose of molecular clocks is to help scientists estimate evolutionary time. With this, they can determine when different species diverged from each other in the past. The concept of a molecular clock is based upon the idea in the neutral theory of evolution (Sid talked about this) and that neutral mutations occur at a relatively constant rate. The theory also says that most genetic variation is due to the accumulation of neutral mutations. Therefore, if you know the rate at which neutral mutations occur, then you could find when two species diverged. In order to calibrate the clock and find the rate, scientists must use a known, like the fossil record. The fossil record helps them to determine the date when the last common ancestor between two species was alive.
Unfortunately, this hypothesis is not completely correct. Not all DNA everywhere is every organism mutates at the exact same rate. Instead, there are certain genes and protein sequences that do seem to mutate at the same rate across taxa. Even with this, there are still exceptions, but generally, it is true. . Broadly speaking, the evolution of important genes occurs more slowly than that of genes with less vital functions. More rapidly changing genes are used to date more recent evolutionary events, and slower evolving genes are used to map more ancient divergences . One example is SSU rRNA, a gene that encodes an RNA found in the small ribosomal subunit. This gene was established very early on in the evolution of life because it is found in all living organisms. Its sequence has changed very slowly. This makes it useful for evaluating distant evolutionary relationships. On the other hand, mitochondrial genome and DNA sequences change very frequently during evolution. These would be better used for recent evolutionary relationships.
Molecular clocks do not look like typical clocks. They are displayed as linear graphs. The y-axis is a measure of the number of nucleotide differences in a homologous gene between different pairs of species. The x-axis plots the amount of time that has elapsed since each pair of species shared a common ancestor. Over a defined period of time, the graph is linear, however over a larger span, evolutionary biologists believe that the line would not be as straight. This could be a result of differences in the generation times of the species being analyzed or variation in mutation rates between different species.
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This is a Yangtze River Dolphin. It is now extinct. In the article, they used the molecular clock method to date when it evolved from other Cetacea. |
This is a bottle nose dolphin. It is evolutionary related to the Yangtze River Dolphin. |
Molecular Clocks This is another short passage about molecular clocks. It is even shorter than the one above, but it gives a good example of how molecular clocks are used by showing pictures of nucleotide sequences and neutral mutations in them.