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Unformatted text preview: 1 Evolutionary effects of genetic drift and gene ow Lecture 17 Populations are finite in size and random sampling of gametes each generation will cause allele frequencies to change from one generation to the next. Random genetic drift 2 Allele frequencies in parental generation fr(red) = 0.58 fr(yellow) = 0.42 Chance survivorship: Allele frequencies in mating individuals fr(red) = 0.61 fr(yellow) = 0.49 3 Allele frequencies in offspring fr(red) = 0.61 fr(yellow) = 0.49 Chance events in sampling gametes Three colors of marbles are in a barrel. Different colors represent different alleles. If you take a small sample, the frequencies of the three colors may be quite different from those in the barrel. 4 Changes in allele frequencies in gametes Gametes of generation t+1. Gametes of generation t. Adults Think about tossing a coin With 1000 tosses, what chance of 700 heads and 300 tails? With 10 tosses, what chance of 7 heads and 3 tails? Coin tosses might be thought of as a model for sampling gametes in a population. These gametes will provide the genetic material for the next generation. 5 Incorporating genetic drift into our thinking about allele frequencies Cannot adopt a deterministic view: that p t+1 (the allele frequency in generation t+1) can be predicted from p t . Instead, adopt a more probabilistic view: that there is some distribution of p t+1 given some value of p t . Each population could have a different of p t+1 . p t p t+1 p t+1 Change over time in allele frequency Drift in allele frequency occurs because of accidental differences in reproductive success or gamete transmission. With small populations, increased chance of changes in allele frequencies over time. For any single case, changes in allele frequency due to random drift cannot be predicted. BUT, the average behavior of allele frequencies can be predicted ....
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 Fall '07
 GEBER

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