MUTATION AND MIGRATION - (u); backward mutation changes a...

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MUTATION AND MIGRATION We have learned how selection can change the frequencies of alleles and genotypes in populations. Selection typically eliminates variation from within populations. (The general exception to this claim is with the class selection models we have called "balancing" selection where alleles are maintained in the population by overdominance, habitat-specific selection, or frequency dependent selection). If selection removes variation, soon there will be no more variation for selection to act on, and evolution will grind to a halt, right? This would be true if it were not for the reality of Mutation which will restore genetic variation eliminated by selection. Thus, mutations are the fundamental raw material of evolution. We will be gin by considering what mutation will do as an evolutionary force acting by itself. Simply, mutation will change allele frequencies, and hence, genotype frequencies. Lets consider a "fight" between forward and backward mutation. Forward mutation changes the A allele to the a allele at a rate
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Unformatted text preview: (u); backward mutation changes a to A at a rate (v). We can express the frequency (p) of the A allele in the next generation (p t+1 ) in terms of these opposing forward and reverse mutations, much like forward and reverse chemical equations: (p t+1 ) = p t (1-u) + q t (v). The first part on the right is accounts for alleles not mutated (1-u), and the second part accounts for the increase in p due to mutation from a to A (the frequency of a times the mutation rate to A). We can also describe the change in allele frequency between generations ( p) as: p = (p t+1 ) - (p t ) . This is useful because it lets us calculate a theoretical equilibrium frequency which is defined as the point at which there is no more change in allele frequencies, i.e. when p = 0 which is when (p t+1 ) = (p t ) ; from above: p t (1-u) + (1-p) t (v) = p t [remember, q=(1-p)]. Now solve for p and convince yourself that the equilibrium frequency = p = v/(u+v) . Similarly the equilibrium frequency of q = u/(u+v)....
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This note was uploaded on 11/05/2011 for the course BIOLOGY MCB2010 taught by Professor Jessicadigirolamo during the Fall '10 term at Broward College.

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