Transcript Chapter 21: The Mechanisms of Evolution

Chapter 21: The Mechanisms of Evolution
CHAPTER 21
The Mechanisms of Evolution
Chapter 21: The Mechanisms of Evolution
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
Chapter 21: The Mechanisms of Evolution
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
Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Charles Darwin and
Adaptation
• Darwin based his theory on well-known
facts and some key inferences.
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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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.
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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Figure 21.3
Figure 21.3
figure 21-03.jpg
Chapter 21: The Mechanisms of Evolution
Genetic Variation within
Populations
• Genetic variation characterizes nearly all
natural populations.
Review Figures 21.4, 21.5
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Chapter 21: The Mechanisms of Evolution
Figure 21.4
Figure 21.4
figure 21-04.jpg
Chapter 21: The Mechanisms of Evolution
Figure 21.5
Figure 21.5
figure 21-05.jpg
Chapter 21: The Mechanisms of Evolution
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.
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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Figure 21.6
Figure 21.6
figure 21-06.jpg
Chapter 21: The Mechanisms of Evolution
Genetic Variation within
Populations
• Populations that have the same allele
frequencies may have different genotype
frequencies.
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Chapter 21: The Mechanisms of Evolution
The Hardy–Weinberg
Equilibrium
• A population that is not changing genetically
is said to be at Hardy–Weinberg equilibrium.
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Chapter 21: The Mechanisms of Evolution
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.

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Chapter 21: The Mechanisms of Evolution
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
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Chapter 21: The Mechanisms of Evolution
Figure 21.7
Figure 21.7
figure 21-07.jpg
Chapter 21: The Mechanisms of Evolution
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.
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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Figure 21.8
Figure 21.8
figure 21-08.jpg
Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Figure
21.10
Figure 21.10
figure 21-10.jpg
Chapter 21: The Mechanisms of Evolution
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
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Chapter 21: The Mechanisms of Evolution
Figure 21.11
Figure 21.11
figure 21-11.jpg
Chapter 21: The Mechanisms of Evolution
Microevolution: Changes in the
Genetic Structure of Populations
• Self-fertilization reduces the frequencies of
heterozygous individuals below Hardy–
Weinberg expectations without changing
allele frequencies.
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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Figure 21.12
Figure 21.12
figure 21-12.jpg
Chapter 21: The Mechanisms of Evolution
Figure
21.13
Figure 21.13
figure 21-13.jpg
Chapter 21: The Mechanisms of Evolution
figure 21-14.jpg
Figure 21.14
Figure 21.14
Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Figure 21.15
Figure 21.15
figure 21-15.jpg
Chapter 21: The Mechanisms of Evolution
Figure 21.16
Figure 21.16
figure 21-16.jpg
Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
Maintaining Genetic Variation
• Much genetic variation within many species
is maintained in distinct subpopulations.
Review Figure 21.17
Chapter 21: The Mechanisms of Evolution
Figure 21.17
Figure 21.17
figure 21-17.jpg
Chapter 21: The Mechanisms of Evolution
Maintaining Genetic Variation
• Genetic variation within a population may be
maintained by frequency-dependent
selection.
Review Figure 21.18
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Chapter 21: The Mechanisms of Evolution
Figure
21.18
Figure 21.18
figure 21-18.jpg
Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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
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Chapter 21: The Mechanisms of Evolution
Figure 21.19
Figure 21.19
figure 21-19.jpg
Chapter 21: The Mechanisms of Evolution
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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Chapter 21: The Mechanisms of Evolution
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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