Ch23 part2 - Chapter 23 The Evolution of Populations Three...

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Unformatted text preview: Chapter 23 The Evolution of Populations Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next Genetic drift tends to reduce genetic variation through losses of alleles Animation: Causes of Evolutionary Change Animation: Causes of Evolutionary Change Fig. 23-8-3 Generation 1 C W C W C R C R C R C W C R C R C R C R C R C R C R C R C R C W C R C W C R C W p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 C R C W C R C W C R C W C R C W C W C W C W C W C W C W C R C R C R C R C R C R p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 C R C R C R C R C R C R C R C R C R C R C R C R C R C R C R C R C R C R C R C R The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population Allele frequencies in the small founder population can be different from those in the larger parent population The Bottleneck Effect The bottleneck effect is a sudden reduction in population size due to a change in the environment The resulting gene pool may no longer be reflective of the original populations gene pool If the population remains small, it may be further affected by genetic drift Fig. 23-9 Original population Bottlenecking event Surviving population Understanding the bottleneck effect can increase understanding of how human activity affects other species Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Fig. 23-10 Number of alleles per locus Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Minnesota, 1998 (no bottleneck) Nebraska, 1998 (no bottleneck) Kansas, 1998 (no bottleneck) Illinois 19301960s 1993 Location Population size Percentage of eggs hatched 1,00025,000 <50 750,000 75,000 200,000 4,000 5.2 3.7 93 <50 5.8 5.8 5.3 85 96 99 (a) (b)...
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This note was uploaded on 10/21/2009 for the course BIOL 6C taught by Professor Sundram during the Spring '09 term at DeAnza College.

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Ch23 part2 - Chapter 23 The Evolution of Populations Three...

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