CHAPTER 21 The Mechanisms of Evolution

1. CHAPTER 21The Mechanisms of Evolution

2. Chapter 21: The Mechanisms of Evolution Charles Darwin and Adaptation Genetic Variation within Populations The Hardy–Weinberg Equilibrium Microevolution: Changes in the Genetic Structure of Populations

3. Chapter 21: The Mechanisms of Evolution Studying Microevolution Maintaining Genetic Variation How Do Genotypes Determine Phenotypes? Constraints on Evolution Short-Term versus Long-Term Evolution

4. Charles Darwin and Adaptation • Darwin developed his theory of evolution by natural selection by carefully observing nature, especially during his voyage around the world. 4

5. Charles Darwin and Adaptation • Darwin based his theory on well-known facts and some key inferences. 5

6. Charles Darwin and Adaptation • Darwin had no examples of the action of natural selection, so he based his arguments on artificial selection by plant and animal breeders. 6

7. Charles Darwin and Adaptation • Modern genetics has elucidated the mechanisms of heredity, which have provided the solid base that supports and substantiates Darwin’s theory. 7

8. Genetic Variation within Populations • A single individual has only some of the alleles found in the population of which it is a member. Review Figure 21.3 8

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10. Genetic Variation within Populations • Genetic variation characterizes nearly all natural populations. Review Figures 21.4, 21.5 10

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13. Genetic Variation within Populations • Allele frequencies measure the amount of genetic variation in a population. • Genotype frequencies show how a population’s genetic variation is distributed among its members. 13

14. Genetic Variation within Populations • Biologists estimate allele frequencies by measuring a sample of individuals from a population. • The sum of all allele frequencies at a locus is equal to 1. Review Figure 21.6 14

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16. Genetic Variation within Populations • Populations that have the same allele frequencies may have different genotype frequencies. 16

17. The Hardy–Weinberg Equilibrium • A population that is not changing genetically is said to be at Hardy–Weinberg equilibrium. 17

18. The Hardy–Weinberg Equilibrium • The assumptions that underlie the Hardy–Weinberg equilibrium are: • population is large • mating is random • no migration • mutation can be ignored • natural selection is not acting on the population. 18

19. The Hardy–Weinberg Equilibrium • In a population at Hardy–Weinberg equilibrium, allele frequencies remain the same from generation to generation, and genotype frequencies remain in the proportions p2 + 2pq + q2 = 1. Review Figure 21.7 19

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21. The Hardy–Weinberg Equilibrium • Biologists can determine whether an agent of evolution is acting on a population by comparing the population’s genotype frequencies with Hardy–Weinberg equilibrium frequencies. 21

22. Microevolution: Changes in the Genetic Structure of Populations • Changes in allele frequencies and genotype frequencies within populations are caused by several evolutionary agents: • mutation • gene flow • random genetic drift • assortative mating • natural selection. 22

23. Microevolution: Changes in the Genetic Structure of Populations • The origin of genetic variation is mutation. • Most are harmful or neutral to bearers, but some are advantageous, particularly if the environment changes. 23

24. Microevolution: Changes in the Genetic Structure of Populations • Migration of individuals among populations followed by breeding produces gene flow; immigrants may add new alleles or change the frequencies of alleles already present. 24

25. Microevolution: Changes in the Genetic Structure of Populations • Random genetic drift alters allele frequencies in all populations, but overrides natural selection only in small ones. • Organisms of normally large populations may pass through periods (bottlenecks) when only a small number of individuals survive. Review Figure 21.8 25

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27. Microevolution: Changes in the Genetic Structure of Populations • New populations established by a few founding individuals also have gene frequencies that differ from those in the parent population. Review Figure 21.10 27

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29. Microevolution: Changes in the Genetic Structure of Populations • If individuals mate more often with individuals bearing the same or different genotypes than would be expected on a random basis, frequencies of homozygous and heterozygous genotypes differ from Hardy–Weinberg expectations. Review Figure 21.11 29

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31. Microevolution: Changes in the Genetic Structure of Populations • Self-fertilization reduces the frequencies of heterozygous individuals below Hardy–Weinberg expectations without changing allele frequencies. 31

32. Microevolution: Changes in the Genetic Structure of Populations • Natural selection is the only evolutionary agent that adapts populations to their environments, and may preserve allele frequencies or cause them to change with time. 32

33. Microevolution: Changes in the Genetic Structure of Populations • Stabilizing, directional, and disruptive selection change the distributions of phenotypes governed by more than one locus. Review Figures 21.12, 21.13, 21.14 33

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37. Studying Microevolution • Biologists study microevolution by measuring natural selection in the field, experimentally altering organisms, and building computer models. Review Figures 21.15, 21.16 37

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40. Maintaining Genetic Variation • Random genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation, but most populations are genetically highly variable. 40

41. Maintaining Genetic Variation • Sexual reproduction generates an endless variety of genotypic combinations that increases evolutionary potential of populations, but does not influence frequencies of alleles. • Rather, it generates new combinations of genetic material on which natural selection can act. 41

42. Maintaining Genetic Variation • Much genetic variation within many species is maintained in distinct subpopulations. Review Figure 21.17

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44. Maintaining Genetic Variation • Genetic variation within a population may be maintained by frequency-dependent selection. Review Figure 21.18 44

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46. How Do Genotypes Determine Phenotypes? • Genotypes do not uniquely determine phenotypes. • A given phenotype can be produced by more than one genotype. 46

47. How Do Genotypes Determine Phenotypes? • An organism’s phenotype is the result of a complex series of developmental processes influenced by environmental factors and genes. Review Figures 21.19 47

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49. Constraints on Evolution • Natural selection acts by modifying what already exists. • A population cannot get temporarily worse in order to achieve some long-term advantage. 49

50. Short-Term versus Long-Term Evolution • Patterns of macroevolutionary change can be strongly influenced by infrequent or slowly occuring events unlikely to be observed during microevolutionary studies. • Additional evidence is needed to understand why evolution took a particular course. 50