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Unformatted text preview: 1 Evolutionary effects of genetic drift and gene ﬂow 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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This note was uploaded on 09/28/2008 for the course BIOEE 2780 taught by Professor Geber during the Fall '07 term at Cornell.
 Fall '07
 GEBER

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