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Ch23 part1 - Chapter 23 The Evolution of Populations...

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Chapter 23 Chapter 23 The Evolution of Populations
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Overview: The Smallest Unit of Evolution One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only populations evolve Genetic variations in populations contribute to evolution Microevolution is a change in allele frequencies in a population over generations
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Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
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Genetic Variation Variation in individual genotype leads to variation in individual phenotype Not all phenotypic variation is heritable Natural selection can only act on variation with a genetic component
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Variation Within a Population Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis Quantitative characters vary along a continuum within a population
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Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels Average heterozygosity measures the average percent of loci that are heterozygous in a population Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals
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Variation Between Populations Most species exhibit geographic variation , differences between gene pools of separate populations or population subgroups
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Fig. 23-3 13.17 19 XX 10.16 9.12 8.11 1 2.4 3.14 5.18 6 7.15 9.10 1 2.19 11.12 13.17 15.18 3.8 4.16 5.14 6.7 XX
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Some examples of geographic variation occur as a cline , which is a graded change in a trait along a geographic axis
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Fig. 23-4 1.0 0.8 0.6 0.4 0.2 0 46 44 42 40 38 36 34 32 30 Georgia Warm (21°C) Latitude (°N) Maine Cold (6°C) Ldh-B   b allele frequency
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Mutatio n Mutations are changes in the nucleotide sequence of DNA Mutations cause new genes and alleles to arise Only mutations in cells that produce gametes can be passed to offspring Animation: Genetic Variation from Sexual Recombination Animation: Genetic Variation from Sexual Recombination
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Point Mutations A point mutation is a change in one base in a gene
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The effects of point mutations can vary: Mutations in noncoding regions of DNA are often harmless Mutations in a gene might not affect protein production because of redundancy in the genetic code
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