February_4_2013_NKW

February_4_2013_NKW - Ecol-335 4 February 2013(Monday...

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1 Ecol-335 4 February 2013 (Monday) Neutral Theory Reading: Futuyma Chapter 10 Next week only M-W sections (review for exam) Today at 4pm in Marley 230, seminar on plant evolution My ofFce hours today, 1-3 - Neutral theory and nearly neutral theory Neutral alleles contribute more to polymorphism within populations, due to their slow fixation time neutral t=4N the larger the population size, the longer it takes for fixation a mutation. Looking backwards: the larger the population size, the longer ago was the shared ancestor of all alleles. **Coalescence Time : time to common ancestor of all current alleles. Average coalescence time for neutral allele is t = 4N (diploids) or 2N (haploids).
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2 • Kimura showed that mutation alone could maintain high levels of polymorphism, especially in large populations – Gave an explanation for the mounting evidence that populations had high levels of polymorphism – alternative to selection-based explanations that required balancing selection to be common. Equilibrium level heterozygosity at a locus increases as a func7on of the product of N e & u 0 (muta7on rate) for neutral alleles (Kimura) Muta&on rate and popula&on size determine level of polymorphism If N e is larger: more genome copies to mutate and alleles are lost less o8en by sampling If u is larger: more muta&ons per genome 4 x N e x u 0 (Fg 10.13)
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3 • Kimura also showed that mutation and drift could result in constant rates of evolution , among lineages with no effect of population size or varying environments. • In other words, the neutral theory predicted a molecular clock, just like Zuckerkandl and Pauling’s independent idea. Constancy and inconstancy in the rate of sequence evolu&on nonsynonymous more dependent on protein, speciFc posi&on synonymous changes are more clock-like,
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4 • Remember the distinction we made between mutation vs. substitution? • This next result merges the two concepts for neutral mutations. –Mutation rate per unit time or generation per copy = μ (per site, gene, or genome, in one genome copy) –Mutation rate in population of N genomes (haploid) = N r Fixation probability of a neutral mutation: 1/N (N=size of haploid population) K = N ± 1/N = μ Rate of substitution equals mutation rate!
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February_4_2013_NKW - Ecol-335 4 February 2013(Monday...

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