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UIC BIOS 101 - GENETIC DRIFT & NEUTRAL THEORY of EVOLUTION

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GENETIC DRIFT & NEUTRAL THEORY of EVOLUTIONReading AssignmentNatural Selection view of EvolutionAmounts of Genetic VariationNew IdeaSampling VariationGenetic driftAllele frequency in finite populationsGENETIC DRIFT in populations differing by 2 orders of magnitude Note fixation of both allelesExperimental Study of DriftThe Neutral Theory Fixation (of alleles)Rate of loss of genetic variation in one generationGenetic Variability Loss over timeLosses and gains of polymorphismBottlenecksProbability of eventual fixation of neutral allelesGENETIC DRIFT recapitulationNeutral Theory recapitulationDynamics of Genetic VariationGeographic structure & migrationVocabularyExam 2 Lecture 8Exam 2 Lecture 8UIC UIC BioSBioS 101 Nyberg101 Nyberg11GENETIC DRIFT & GENETIC DRIFT & NEUTRAL THEORY of NEUTRAL THEORY of EVOLUTIONEVOLUTIONThere is much genetic variation within There is much genetic variation within almost all species. The amount of almost all species. The amount of genetic variation is too much to be genetic variation is too much to be maintained by selection. maintained by selection.Exam 2 Lecture 8 UIC BioS 101 Nyberg 2Reading AssignmentReading Assignment►►The sections relevant to this lecture are 25.3 The sections relevant to this lecture are 25.3 Genetic Drift, Genetic Drift, ►►Your lab on Population Genetics.Your lab on Population Genetics.►►A current major activity of biologists A current major activity of biologists generating phylogenies which aregenerating phylogenies which are based on based on the fact that allelic substitutions, presumably the fact that allelic substitutions, presumably neutral, accumulate proportional to time since neutral, accumulate proportional to time since last common ancestor.last common ancestor.Exam 2 Lecture 8 UIC BioS 101 Nyberg 3Natural Selection Natural Selection view of Evolutionview of Evolution►►Mutations arise by chance Mutations arise by chance (meaning the mutations (meaning the mutations are not directed to match environmental needs).are not directed to match environmental needs).►►Favorable (= higher fitness) mutations increase in Favorable (= higher fitness) mutations increase in frequency via selection (changed fitness often frequency via selection (changed fitness often associated with changed environment). associated with changed environment). ►►Deleterious (= lower fitness) mutations are Deleterious (= lower fitness) mutations are reduced in frequency (many resistance mutations reduced in frequency (many resistance mutations are deleterious unless toxic agent present).are deleterious unless toxic agent present).Exam 2 Lecture 8 UIC BioS 101 Nyberg 4Amounts of Genetic VariationAmounts of Genetic Variation►►When molecular techniques allowed scientists When molecular techniques allowed scientists to measure genetic variation in proteins they to measure genetic variation in proteins they found 1) many found 1) many polymorphic loci, polymorphic loci, and 2) and 2) including some with many alleles.including some with many alleles.►►The amount of genetic variation was much The amount of genetic variation was much greater than scientists expected from the greater than scientists expected from the population genetic models that existed.population genetic models that existed.►►Selection models were Selection models were ‘‘tweakedtweaked’’, but , but ……..Exam 2 Lecture 8 UIC BioS 101 Nyberg 5New IdeaNew Idea►►The The neutral theory neutral theory of evolution developed of evolution developed by by MotooMotoo Kimura (1Kimura (1stst publication 1968).publication 1968).►►The neutral theory departed from all The neutral theory departed from all existing models by using existing models by using N, the population , the population size, as the most important population size, as the most important population parameter.parameter.►►What is the neutral theory of evolution?What is the neutral theory of evolution?Sampling VariationSampling Variation►►Real populations have a finite number of Real populations have a finite number of members, N.members, N.►►Whenever you have a finite number of Whenever you have a finite number of individuals you have sampling variation.individuals you have sampling variation.►►Sampling leads to changes in frequency due Sampling leads to changes in frequency due to chance.to chance.►►Allele frequencies are not expected to stay Allele frequencies are not expected to stay the same in a real population.the same in a real population.Exam 2 Lecture 8 UIC BioS 101 Nyberg 6Exam 2 Lecture 8 UIC BioS 101 Nyberg 7Genetic driftGenetic drift►►Let the frequency of in next generation = pLet the frequency of in next generation = p’’►►p will not = pp will not = p’’ if the population is finite, if the population is finite, because of chance (sampling).because of chance (sampling).►►The proportion of deviation from one The proportion of deviation from one generation to next shouldgeneration to next shouldGo up sometimes and down sometimesGo up sometimes and down sometimesBe inversely proportional to N, pop sizeBe inversely proportional to N, pop sizeEND when one allele reaches 100% of END when one allele reaches 100% of population, a state called fixation.population, a state called fixation.Exam 2 Lecture 8 UIC BioS 101 Nyberg 8Allele frequency in finite populationsAllele frequency in finite populationsOne way to see that the infinite population size One way to see that the infinite population size (=Hardy(=Hardy--Weinberg model) is not compatible with Weinberg model) is not compatible with finite populations is to calculate expected frequencies finite populations is to calculate expected frequencies and realize that the calculated value is impossible in and realize that the calculated value is impossible in real population. If 1 of 10 snails have the recessive real population. If 1 of 10 snails have the recessive phenotype, the estimated recessive allele frequency is phenotype, the estimated recessive allele frequency is 0.316, but 0.30 & 0.35 are the closest possible actual 0.316, but 0.30 & 0.35 are the closest possible actual frequencies in population with only 20 (=2N) genes in frequencies in population with only 20 (=2N) genes in ten diploid snails.ten diploid snails.GENETIC DRIFT GENETIC DRIFT in populations in populations differing by 2 orders of magnitudediffering by 2 orders of magnitude Note fixation of both


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UIC BIOS 101 - GENETIC DRIFT & NEUTRAL THEORY of EVOLUTION

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