Genetic drift B09 notes

Genetic drift B09 notes - BB3140 B09 Genetic drift Chapter...

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BB3140 B09 Genetic drift Chapter 10 These notes are based on a lecture given by Professor Lauren Mathews. I. Random genetic drift- this is potentially an extremely important evolutionary force. Many evolutionary biologists feel that genetic drift plays a very large role in generating biological diversity Definition of Genetic Drift : whenever there are two or more alleles for a gene present in a finite population, the relative frequencies of the alleles change from generation to generation in accordance with random sampling. > Hardy-Weinberg (Chapter 9) suggests that there will be no change in allele frequencies through the generations. >BUT, even in the absence of selection, these alleles are subject to random sampling error that can alter allele frequencies over time II. How does genetic drift happen? a. The importance of genetic drift in evolution is dependent on population size, or the size of the gamete pool (the size of the sample that is drawn from the total pool of alleles). Why is this? (This section is covered in Chapter 10 pp. 256-257) Example: finite population with 2 alleles, A1 and A2. p=0.5, q=0.5 Question: What is the chance that a random sample of 4 gametes from a pool that has p=0.5 are all A1? = (chance of each allele being the same) = (½ x ½ x ½ x ½ ) x 2= 1/16 Question: What is the chance that a random sample of 8 gametes from a pool that has p=0.5 are all A1? = (½ x ½ x ½ x ½ x ½ x ½ x ½ x ½) = 1/256 Another way to express this idea is to observe that in general, probability theory says that variance in the sampling procedure is greater the smaller the number of trials in the sample - so the smaller the number of genes sampled, the greater the chance that one allele will be overrepresented by chance. The textbook covers this idea in two different ways: coalescence and random walk. See Figs. 10.1, 10.2, and 10.3 and text, pp. 257-260) This means that changes in allele frequencies driven by genetic drift are more likely to be an important (perhaps the most important)
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evolutionary force in small populations. Though genetic drift also
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Genetic drift B09 notes - BB3140 B09 Genetic drift Chapter...

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