Table of Contents

1
CHAPTER 21The Mechanisms of Evolution
2
Chapter 21 The Mechanisms of Evolution

• Charles Darwin and Adaptation
• Genetic Variation within Populations
• The HardyWeinberg 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.
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5
Charles Darwin and Adaptation

• Darwin based his theory on well-known facts and
  some key inferences.
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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.
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7
Charles Darwin and Adaptation

• Modern genetics has elucidated the mechanisms of
  heredity, which have provided the solid base that
  supports and substantiates Darwins theory.
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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
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9
Figure 21.3
figure 21-03.jpg

• Figure 21.3

10
Genetic Variation within Populations

• Genetic variation characterizes nearly all
  natural populations.
• Review Figures 21.4, 21.5
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Figure 21.4
figure 21-04.jpg

• Figure 21.4

12
Figure 21.5
figure 21-05.jpg

• Figure 21.5

13
Genetic Variation within Populations

• Allele frequencies measure the amount of genetic
  variation in a population.
• Genotype frequencies show how a populations
  genetic variation is distributed among its
  members.
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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
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15
Figure 21.6
figure 21-06.jpg

• Figure 21.6

16
Genetic Variation within Populations

• Populations that have the same allele frequencies
  may have different genotype frequencies.
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17
The HardyWeinberg Equilibrium

• A population that is not changing genetically is
  said to be at HardyWeinberg equilibrium.
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18
The HardyWeinberg Equilibrium

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

• In a population at HardyWeinberg 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
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Figure 21.7
figure 21-07.jpg

• Figure 21.7

21
The HardyWeinberg Equilibrium

• Biologists can determine whether an agent of
  evolution is acting on a population by comparing
  the populations genotype frequencies with
  HardyWeinberg equilibrium frequencies.
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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.
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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.
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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.
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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
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26
Figure 21.8
figure 21-08.jpg

• Figure 21.8

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
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Figure 21.10
figure 21-10.jpg

• Figure 21.10

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 HardyWeinberg expectations.
• Review Figure 21.11
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Figure 21.11
figure 21-11.jpg

• Figure 21.11

31
Microevolution Changes in the Genetic Structure
of Populations

• Self-fertilization reduces the frequencies of
  heterozygous individuals below HardyWeinberg
  expectations without changing allele frequencies.
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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.
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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
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Figure 21.12
figure 21-12.jpg

• Figure 21.12

35
Figure 21.13
figure 21-13.jpg

• Figure 21.13

36
Figure 21.14
figure 21-14.jpg

• Figure 21.14

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
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Figure 21.15
figure 21-15.jpg

• Figure 21.15

39
Figure 21.16
figure 21-16.jpg

• Figure 21.16

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.
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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.
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42
Maintaining Genetic Variation

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

43
Figure 21.17
figure 21-17.jpg

• Figure 21.17

44
Maintaining Genetic Variation

• Genetic variation within a population may be
  maintained by frequency-dependent selection.
• Review Figure 21.18
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Figure 21.18
figure 21-18.jpg

• Figure 21.18

46
How Do Genotypes Determine Phenotypes?

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

• An organisms phenotype is the result of a
  complex series of developmental processes
  influenced by environmental factors and genes.
• Review Figures 21.19
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Figure 21.19
figure 21-19.jpg

• Figure 21.19

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.
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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.
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