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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. 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.〔Wu, Chung-I, and Wen-Hsiung Li. 1985. Evidence for higher rates of nucleotide substitution in rodents than in man. Proceedings of the National Academy of Sciences of the United States of America 82(6): 1741-1745. http://www.pnas.org/content/82/6/1741.full.pdf+html〕
== Uses ==
Two main uses for the relative rate test are to determine if and how generation time and metabolic processes affect mutational rate. Firstly is generation time. The most famous experiment testing this hypothesis compared eleven genes between mice or rats to humans, with pig, cow, goat, dog, and rabbits acting as a reference. Rodents have a much shorter generation time than humans, and so the theory was that they would have much faster mutation rates, and so evolve faster. This theory was supported through testing coding regions and untranslated regions with the relative rate test (which showed that rodents had a mutation rate much higher than humans) and backed up by comparing paralogous genes because they are homologous via gene duplication and not speciation and so the comparison is independent of the time of divergence.〔
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.〔Mindell, D.P., A. Knight, C. Baer, and C.J. Huddleston. 1995. Slow rates of molecular evolution in birds and metabolic rate and body temperature hypotheses. Letter to the Editor. http://vertebrates.si.edu/other_vz_staff/pdfs/MBE_1996.pdf.〕
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 eutherian mammals. He did this via the relative rate test and then, using this data, he was able to construct a phylogeny using various methods, including parsimony and maximum likelihood.〔Easteal, S. 1990. The pattern of mammalian evolution and the relative rate of molecular evolution. Genetics 124(1): 165-173. http://www.genetics.org/content/124/1/165.full.pdf+html.〕 He took the same approach in another experiment to compare humans to other primates, and found no significant difference in evolutionary rates.〔Easteal, S. The relative rate of DNA evolution in primates. Molecular Biology and Evolution 8(1): 115-127. http://mbe.oxfordjournals.org/content/8/1/115.full.pdf+html.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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