Cutter_Lecture_05 - GeneticDrift 1...

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Genetic Drift 1
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What does “chance” mean in evolution? Randomness in physical processes Two or more outcomes are possible… Can’t predict with certainty which outcome will manifest in any particular case Can predict what the possible outcomes are Can predict the probability of alternative outcomes Stochastic (unpredictable) evolutionary forces: Mutation Recombination Migration Genetic Drift Deterministic (predictable) evolutionary force: Natural Selection 2
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What does chance in evolution mean? Evolution = heritable change over time Changes in allele frequency in a population from one generation to the next Chance results in evolution that is non adaptive NOT mal adaptive or counter adaptive Changes that occur independently of individuals’ ability to survive and reproduce Provides a null hypothesis for evolutionary change Compare observed pattern to that expected due simply to chance A test for the action of natural selection Do not assume adaptation by selection…need to prove it! 3
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Are stochastic process sufficient to explain the characteristic? Comparison to null distribution Characteristic of interest Expected distribution due to chance processes 4
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Genetic drift is an inevitable consequence of “sampling error” The evolutionary fate of a new mutation Selectively neutral Fixation (frequency = 1) or loss (frequency = 0) Changes in allele frequency due to drift 5 p A1 = 1 p A1 = (2N 1)/2N q A2 = 1 p A1 = 1/2N
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In a stable population, each mating pair leaves average of 2 progeny that survive to reproduce New mutation is initially in a heterozygous individual New mutation is initially only a single copy Fate of a new mutation 6 × Probability = ½ Probability = ½×½ = ¼ Therefore, the probability is ¼ that the next generation will have ZERO A2 alleles! Accounting for variation in offspring number
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Cutter_Lecture_05 - GeneticDrift 1...

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