February_4_2013_NKW

Substitution mutation rate per unit time or

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Unformatted text preview: tation rate for population * probability of fixation of each mutation •  Remember the distinction we made between mutation vs. substitution? – Mutation rate per unit time or generation per copy = µ (per site, gene, or genome, in one genome copy) •  This next result merges the two concepts for neutral mutations. ‒  Fixation probability of a neutral mutation: 1/N (N=size of haploid population) – Mutation rate in population of N genomes (haploid) = N µ K = N µ * 1/N = µ Rate of substitution equals mutation rate! 4 •  For neutral sites K=µ •  No effect of Ne –  More mutations in a large population, correspondingly smaller chance of fixation for each –  Small populations and large populations should evolve at same rate for neutral changes if they have the same mutation rate. •  Predicts clock-like change if most changes are neutral, similar rate independent of population size •  Allows measurement of mutation rate over long time scale •  assumption of neutrality •  need to know date of ancestor in order to estimate rates •  Alternatively, if rate of change is known, divergence date can be used to estimate date of ancestral sequence – •  idea of molecular clock NEARLY NEUTRAL THEORY: Exactly how neutral is neutral? IT DEPENDS ON Ne s = selec&on coefficient of a new muta&on fitness without new muta&on = 1 fitness with new muta&on= 1 ­s therefore: s = 0 NO SELECTION AT ALL s = 0.5  new muta&on reduces fitness by half But what about when the nega&ve effect on fitness (s) of the new muta&on is TINY=say 10 ­6 ? Will allele with a &ny effect on fitness be governed by Selec&on or by Gene&c Dri8? 5 Kimura showed that an allele will be mainly governed by driP if: |s| < (1/2Ne) For a popula&on of Ne = 50: absolute value of s must be > 0.01 for selec&on to govern its fate, if it is lower than that DRIFT WILL GO...
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This note was uploaded on 02/02/2014 for the course ECOL 335 taught by Professor Reinthal during the Spring '10 term at Arizona.

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