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Unformatted text preview: 2/10/10 1 244 244 Unusually rapidly evolving protein categories may be under directional selection (or low constraint) Chimp genome paper, Nature 2005 GO = Gene Ontology 245 245 What is the contribution of directional selection to protein divergence? The McDonald-Kreitman test 1) Under the null hypothesis (neutral evolution), polymorphism and divergence result from genetic drift 2) Therefore, rapidly evolving proteins should also show high levels of protein polymorphism within species 3) Polymorphic and Fxed amino acid variants are compared to polymorphic and Fxed synonymous variants 4) Can only detect recurrent adaptive Fxations and can only reject if there are sufFcient numbers of variants in a gene 2/10/10 2 246 246 An example of a dataset compatible with neutral protein variation Synonymous Nonsynonymous Fixed 10 5 Polymorphic 20 10 Expectation of polymorphic and Fxed variants under neutrality Synonymous Nonsynonymous Fixed a b Polymorphic c d b/a = d/c Logic of the MK test 247 247...
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