Lec05.drift.mut.mig - affected by selection or only very...

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1 Drift, Mutation, Migration Three factors that can also cause evolution in populations We’ll consider each in turn and some of their most important effects on a population Is all evolution due to selection? ± Can gene frequencies change without selection being the cause, and, if so, under what circumstances? Genetic Drift ± Definition: changes in gene frequencies caused by stochastic factors (chance) ± Operates most strongly in small populations
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2 Genetic Drift ± In the simplest case, the probability of an allele going to fixation (100%) equals the frequency of the allele in the population ± The time to fixation is proportional to the population size ± In small populations drift can cause evolution contrary to selection. Genetic Drift ± Drift always decreases genetic variation in a population ± Without proof, the rate of neutral allele substitution due to drift is only a function of the neutral mutation rate of new alleles not population size! ± Neutral mutations are ones that are not
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Unformatted text preview: affected by selection or only very weakly affected E.g. third positions in many codons. Mutations The stuff that ultimately makes evolution possible Lets consider the fate of mutations as a function of population size, and the strength of selection. Also consider the effect of sex on the rate of fixing multiple favorable mutations in a population. 3 Mutation Rate of mutation per base in DNA is much lower than selection pressures in most cases Selection usually about 10-2 to 10-4 Mutation usually about 10-8 to 10-10 Occasionally higher, e.g., HIV is 10-2 Selection has probably operated on the mutation rate Most mutations neutral or deleterious Very few are favorable (increase fitness) Some measured mutation rates Sex and Mutations 4 Migration Migration is essentially gene flow due to movement of individuals from one population to another. Effect of Migration...
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Lec05.drift.mut.mig - affected by selection or only very...

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