Relative rate test

The relative rate test is a genetic comparative test between two ingroups (somewhat closely related species) and an outgroup or “reference species” to compare mutation and evolutionary rates between the species.^[1] Each ingroup species is compared independently to the outgroup to determine how closely related the two species are without knowing the exact time of divergence from their closest common ancestor.^[2] If more change has occurred on one lineage relative to another lineage since their shared common ancestor, then the outgroup species will be more different from the faster-evolving lineage's species than it is from the slower-evolving lineage's species. This is because the faster-evolving lineage will, by definition, have accumulated more differences since the common ancestor than the slower-evolving lineage. This method can be applied to averaged data (i.e., groups of molecules), or individual molecules. It is possible for individual molecules to show evidence of approximately constant rates of change in different lineages even while the rates differ between different molecules. The relative rate test is a direct internal test of the molecular clock, for a given molecule and a given set of species, and shows that the molecular clock does not need to be (and should never be) assumed: It can be directly assessed from the data itself. Note that the logic can also be applied to any kind of data for which a distance measure can be defined (e.g., even morphological features).

Uses

The initial use of this method was to assess whether or not there was evidence for different rates of molecular change in different lineages for particular molecules. If there was no evidence of significantly different rates, this would be direct evidence of a

The other use of the test is to determine the effect of metabolic processes. It had previously been believed that birds have a much slower molecular evolutionary rate than other animals, such as mammals, but that was based solely on the small genetic differences between birds, which relied on the fossil record. This was later confirmed with the relative rate test, however the theory was that this was because of metabolic rate and a lower body temperature in birds. Mindell’s paper explains that there was no direct correlation found between these and molecular evolution in the test taxa of birds based on mitochondrial evolution, but birds as a whole do have a lower mutation rate. There are still many hypotheses in this area of study that are being tested, but the relative rate test is proving crucial in order to overcome the fossil record bias.^[5]

Although these are specific instances of the relative rate test, it may also be used to compare species for phylogenetic purposes. For example, Easteal wanted to compare nucleotide substitution rates in four genes of four

It is generally agreed that the relative rate test has many strengths that make it invaluable for experimenters. For example, using this test, the date of divergence between two species is not needed.[2] Also, a generalized test minimizes sampling bias^[8] and the bias of the fossil record. However, the relative rate test is very poor in some areas, such as detecting major differences compared to rate constancy if it is being used as a test for the

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