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Lecture 7

Lecture 7: "Population Genetics V"

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Biology 3466B
Yolanda Morbey

Evolution Lecture No. 7: Population Genetics V th Monday September 24 , 2012 Genetic Drift & The Fixation Of Alleles: -The fixation of alleles is an inevitable event that happens with genetic drift. Assuming a population size of N and the number of copies of a unique allele to be 2N (diploid individuals), the probability that a unique allele A will be fixed yields the following equation: prob(A) = 1 / 2N -The probability that an allele A with multiple x copies will become fixed yields the following equation: 1 prob(A 1 = x / 2N Genetic Drift & The Loss Of Heterozygosity: -The equation for determining the loss of heterozygosity due to genetic drift is as follows: H g + 1 Hg[1 – ½ N] -As the term in parentheses ranges from 0.5 - 1, H is always decreasing in the next generation. Genetic Drift In Action: -In an experiment focusing on the gene for eye colour in Drosophila, the generation 0 population consisted almost exclusively of heterozygote individuals (at 0.5). Then due to the effect of genetic drift on the small population, the level of heterozygosity diverges from 0.5 to almost 0 by generation 10. At generation 19, the genotype frequencies develop into homozygote extremes with rarely any heterozygotes. Decline In Heterozygosity: -In the Drosophila populations, a steady decline in heterozygosity was predicted (as it is expected for genetic drift), but the decline went faster then was actually predicted. This is now found in many populations to be a common phenomenon. Effective Population Size: -The effective population size (N ) es the actual number of randomly breeding adults and differs from the N oobserved population size). N is less than N sinceosome flies did not reproduce or did not survive to reproduce (It is pretty common for organisms to go unmated in life). The equation for the estimation of N es as follows: N e 4(N m(N) f (N +mN) f Ne= N ooly when N = N m f -In general, there may be a probability that the starting frequency of the beneficial allele will eventually become fixed due to genetic drift. The Neutral Theory: -Neutral theory contrasts with the selectionist theory as it proposes that most of evolution is due to genetic drift as opposed to selection. In general, neutral theory states that the rate of evolution at a given locus may be predominantly due to genetic drift alone with some negative selection (where most mutations are deleterious). -Neutral theory also describes the rate of evolution as the substitution rate (the rate at which a new allele created by mutation becomes fixed within a population). Note that the neutral theory is not an absolutist hypothesis, but rather an alternative to selectionist theory. Substitution & Mutation: -According to the main points of neutral theory: Most mutations are neutral, deleterious alleles are eliminated by selection and substitutions occur at a clock-like rate. The Neutral Theory Applied To Influenza: - While examining a gene that supports the premise of neutral theory, it was observed that the rate of nucleotide substitutions follows a linear relationship and is constant. There seems to be a higher rate of silent (synonymous) substitutions than non-synonymous, which is undergoing negative selection. This is all the result of genetic diversity and since the quotient of non-synonymous to
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