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352_Outline141_S11 - Biology 352 Lecture 14 LECTURE 14...

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Biology 352 Lecture 14 Spring 2011 1 LECTURE 14 : Genetic Drift I. Last Time. A. Mutation. 1. Forward mutation model. 2. Backward and forward mutation model. B. Migration. II. Genetic Drift: General background. A. Hardy-Weinberg model: populations are infinitely large. 1. Real populations are not infinite. 2. Real populations may be very small. B. Real populations are subject to genetic drift . 1. Definition: the random sampling error of gametes resulting in fluctuations in allelic frequencies from one generation to the next. 2. Also called the "Sewell Wright effect." 3. Controversy: how important is genetic drift to evolution? C. Random sampling error. 1. Events may have expected outcomes based on probabilities. 2. Observed outcomes often differ from expected outcomes due to chance (=random sampling error). 3. Example: flip a coin 10 times and record the outcomes. a. Theoretical expectation: 5 heads and 5 tails.
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Biology 352 Lecture 14 Spring 2011 2 b. Observed outcome: 6 heads, 4 tails; 3 heads, 7 tails, etc. c. Deviation due to random sampling error . 4. Magnitude of sampling error is a function of sample size . a. 6 heads, 4 tails is highly likely and represents a 20% deviation. b. Flip coin 500 times: How likely is 20% deviation now? D. The binomial distribution. 1. A probabilistic event with 2 outcomes (e.g. heads or tails; alleles A or a ).
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