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2005_campbl23 - Chapter 23 The Evolution of Populations...

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Chapter 23: The Evolution of Populations
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Important Point:
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Gene Pools One species, but members are more likely to mate within their herd than the other A gene pool is the sum of alleles at all loci within a population
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Polymorphism A polymorphism is more than one allele present at a given locus within a single population of organisms Population genetics is essentially the study of allele and genotype frequencies within populations of organisms
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Mendel meets H.W. Recall Mendelian genetics Hardy-Weinberg Equilibrium means genotype frequencies stay the same
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Hardy-Weinberg Theorem 320 20 x2
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Hardy-Weinberg Theorem
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Hardy-Weinberg Theorem
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Hardy-Weinberg Theorem Note same The triumph of Darwinism occurred with the ‘Modern synthesis’, the integration of the mechanics of Darwinian evolution with those of Mendelian genetics (1930s)
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H.W. Equilibrium Hardy- Weinberg means that both genotype and allele frequencies stay the same over time
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H.-W. Frequencies (2 alleles) Note how genotype frequencies are 100% a function of previous-generation allele frequencies. This is precisely what the H.W. equation tells us. It is the default evolutionary assumption (i.e., no evolution is occurring) Calculated H.W. frequencies,1 locus, 2 alleles “Fixed” allele
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H.-W. Assumptions To assume Hardy-Weinberg equilibrium all of the following must be true: 1. The population must be very large (no sampling error/genetic drift) 2. There must be no net mutation 3. There must be no natural selection (though as we will see that this assumption can be temporarily suspended in the course of using the Hardy- Weinberg theorem) 4. No migration between populations 5. Random mating (equivalent to mixing all sperm and eggs in population into a common bucket to foster fertilization) In other words, no mechanisms that can affect genetic structure—i.e., allele or genotype frequencies—may be operating
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Eggs & Milt (Sperm) in Bucket http://wdfw.wa.gov/wildwatch /salmoncam/hatchery.html
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Non-Random Mating Anything that interferes with the random mating between individuals is nonrandom mating Nonrandom mating results in deviations from a Hardy-Weinberg generation of genotypes from a given frequency of alleles
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H.-W. Equilibrium If no mechanisms that can affect genetic structure are operating, then Hardy-Weinberg genotype frequencies will be established in a single generation… And these frequencies will persist indefinitely (I.e., so long as there are no mechanisms operating that can affect genetic structure) Remember that an organism can be homozygous for a given allele even if within the population is polymorphic (meaning that more than one allele exists) Indeed, three alleles can exist within a population, even if only at best two can exist within a single individual
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Chalk discussion of H.W.
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