- McDonald–Kreitman test
The McDonald–Kreitman Test looks for ancient selection over long periods, as opposed to the steady accumulation of mutations that confer no selective advantage predicted by the Neutral theory. It was first devised by John McDonald and Martin Kreitman in 1991, based on an investigation of differences in amino acid sequence of the alcohol dehydrogenase gene of Drosophila species. Nucleotide differences between the coding regions of homologous genes of related species are enumerated and sorted into four categories, as shown below:
Fixed Polymorphic Synonymous Ds Ps Nonsynonymous Dn Pn
Sites are classed as polymorphic if they show any variation within species, while they are classed as fixed if they differ between species but not within them. If the ratio of fixed differences to polymorphisms is much higher for nonsynonymous changes (i.e. Dn/Pn >> Ds/Ps), this indicates that genetic changes have been subject to positive selection.
The McDonald-Kreitman test itself consists of the G-test performed on the numbers in the table above, which would indicate whether the two ratios are significantly different.
As an extension of this, the proportion of base substitutions fixed by natural selection, α, can be estimated:
45% of amino acid differences between Drosophila simulans and D. yakuba are estimated to have been fixed by selection, while 35% of amino acid differences between primates are estimated to be fixed by selection.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Martin Kreitman — is an American geneticist, and Professor at the University of Chicago. He graduated from Stony Brook University with a B.S. Biology, in 1975, and from University of Florida with a M.S. Zoology, in 1977, and from Harvard University with a Ph … Wikipedia
Neutral theory of molecular evolution — The neutral theory of molecular evolution states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants (not affecting fitness). The theory was introduced by Motoo Kimura … Wikipedia
Stabilizing selection — value. Put another way, extreme values of the character are selected against. This is probably the most common mechanism of action for natural selection.A classic example of this is human birth weight. Babies of low weight lose heat more quickly… … Wikipedia
Evolution — This article is about evolution in biology. For other uses, see Evolution (disambiguation). For a generally accessible and less technical introduction to the topic, see Introduction to evolution. Part of a series on … Wikipedia