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Chapter 23
The Evolution of Populations
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: The Smallest Unit of Evolution
• Microevolution is a change in allele
frequencies (gene pool) in a population over
generations (time)
• 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
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Genetic Variation
• Variation (of alleles) in individual genotype leads
to variation in individual phenotype
• Not all phenotypic variation is heritable (fig 23.2)
– Body builders do not pass their huge muscles to the next
generation
• Natural selection can only act on variation with a
genetic component
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Fig. 23-2
Nonheritable Variation
(a)
(b)
a. Caterpillars raised on a diet of oak flowers resembled the flowers
b. Siblings raised on oak leaves resembled oak twigs
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
– Mendel’s flowers: Purple or white
• Quantitative characters vary along a continuum
within a population
– Skin color
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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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Concept 23.1: Mutation and sexual reproduction
produce the genetic variation that makes evolution
possible
• Two processes that produce the variation in
gene pools that contributes to differences
among individuals
– mutation
• Point Mutations
• Chromosome mutations
– sexual reproduction
• Crossing Over
• Independent Assortment
• Fertilization
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Mutation
• Mutations are changes in the nucleotide
sequence of DNA
• Mutations cause new genes and alleles to arise
– Original source of new alleles
• Only mutations in cells that produce gametes
can be passed to offspring
Animation: Genetic Variation from Sexual Recombination
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Point Mutations
• A point mutation is a change in one base in a
gene
• The effects of point mutations can vary:
– Mutations in noncoding regions of DNA are often
harmless
– Mutations that result in a change in protein production
are often harmful
– Mutations that result in a change in protein production
can sometimes increase the fit between organism and
environment
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Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
– Duplication of large chromosome segments is
usually harmful
– Duplication of small pieces of DNA is
sometimes less harmful and increases the
genome size
– Duplicated genes can take on new functions
by further mutation
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Sexual Reproduction
• Sexual reproduction can shuffle existing alleles
into new combinations in every generation.
– Crossing over during Prophase I of meiosis
– Independent Assortment of chromosomes
during meiosis
– Fertilization
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Concept 23.2: The Hardy-Weinberg equation can
be used to test whether a population is evolving
• The first step in testing whether evolution is
occurring in a population is to clarify what we
mean by a population
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Gene Pools and Allele Frequencies
• A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
• A gene pool consists of all the alleles for all
loci in a population
– In diploid species each individual has two alleles for a
particular gene and the individual may be either
heterozygous or homozygous
– If all the members of a population are homozygous for
the same allele , the allele is fixed.
• The greater the number of fixed alleles the lower
the species diversity
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Hardy-Weinberg Equilibrium
• The Hardy-Weinberg principle describes a
population that is not evolving
• The Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a
population remain constant from generation to
generation unless they are acted upon by
forces other than Mendelian segregation and
the recombination of alleles.
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• Hardy-Weinberg equilibrium describes the
constant frequency of alleles in such a gene
pool
• If p and q represent the relative frequencies of
the only two possible alleles in a population at
a particular locus, then
– p2 + 2pq + q2 = 1
p+q=1
– where p2 and q2 represent the frequencies of
the homozygous genotypes and 2pq
represents the frequency of the heterozygous
genotype
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-7-4
20% CW (q = 0.2)
80% CR ( p = 0.8)
Sperm
CR
(80%)
64% ( p2)
CR CR
p2 + 2pq + q2 = 1
p+q=1
CW
(20%)
16% ( pq)
CR CW
16% (qp)
CR CW
4% (q2)
CW CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this generation:
With random mating, these gametes
will result in the same mix of
genotypes in the next generation
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW
= 20% CW = 0.2 = q
+ 16% CW
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants
Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem describes a
hypothetical population
• In real populations, allele and genotype
frequencies do change over time
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• The five conditions for nonevolving populations
are rarely met in nature:
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Applying the Hardy-Weinberg Principle
• We can assume the locus that causes
phenylketonuria (PKU) is in Hardy-Weinberg
equilibrium given that:
– The PKU gene mutation rate is low (1)
– Mate selection is random with respect to whether or
not an individual is a carrier for the PKU allele (2)
– Natural selection can only act on rare homozygous
individuals who do not follow dietary restrictions (3)
– The population is large (4)
– Migration has no effect as many other populations
have similar allele frequencies (5)
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• The occurrence of PKU is 1 per 10,000 births
– q2 = 0.0001
– q = 0.01
p2 + 2pq + q2 = 1
p+q=1
• The frequency of normal alleles is
– p = 1 – q = 1 – 0.01 = 0.99
• The frequency of carriers is
– 2pq = 2 x 0.99 x 0.01 = 0.0198
– or approximately 2% of the U.S. population
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 23.3: Natural selection, genetic drift, and
gene flow can alter allele frequencies in a
population
• Three major factors alter allele frequencies and
bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Natural Selection
• Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Genetic Drift
• The smaller a sample, the greater the chance
of deviation from a predicted result
• Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to
the next
– Genetic drift tends to reduce genetic
variation through losses of alleles
– Founder Effect
– Bottleneck Effect
Animation: Causes of Evolutionary Change
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Less green alleles
available
The Founder Effect
• The founder effect occurs when a few
individuals become isolated from a larger
population
• Allele frequencies in the small founder
population can be different from those in the
larger parent population
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The Bottleneck Effect
• The bottleneck effect is a sudden reduction in
population size due to a change in the
environment (fire or flood)
– The resulting gene pool may no longer be
reflective of the original population’s gene pool
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Effects of Genetic Drift: A Summary
1. Genetic drift is significant in small populations
2. Genetic drift causes allele frequencies to
change at random
3. Genetic drift can lead to a loss of genetic
variation within populations
4. Genetic drift can cause harmful alleles to
become fixed
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Gene Flow
• Gene flow occurs when a population gains or
loses alleles by genetic additions or subtractions
from the population.
• Gene flow tends to reduce differences between
populations over time
• Gene flow occurs at a higher rate than
mutations and is more likely to alter allele
frequencies directly
–
Once introduced to a population, natural selection may then
cause the new allele to increase or decrease in frequency
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Gene flow and human evolution
Fig. 23-11
Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
• Only natural selection consistently results in
adaptive evolution (organism is becomes better suited to its environment)
by acting on an organism’s phenotype
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Relative Fitness
• The phrases “struggle for existence” and
“survival of the fittest” are misleading as they
imply direct competition among individuals
• Reproductive success is generally more subtle
and depends on many factors other than battle
– Relative fitness is the contribution an individual
makes to the gene pool of the next generation,
relative to the contributions of other individuals
– Selection favors certain genotypes by acting on
the phenotypes of certain organisms
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Directional, Disruptive, and Stabilizing Selection
• Three modes of selection:
– Directional selection favors individuals at one
end of the phenotypic range
– Disruptive selection favors individuals at both
extremes of the phenotypic range
– Stabilizing selection favors intermediate
variants and acts against extreme phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-13
Original population
Original
Evolved
population population
Phenotypes (fur color)
(a) Directional selection
(b) Disruptive selection
Environmental change
Environmental change
(c) Stabilizing
selection
The Key Role of Natural Selection in Adaptive
Evolution
• Natural selection increases the frequencies of
alleles that enhance survival and reproduction
• Adaptive evolution occurs as the match
between an organism and its environment
increases
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Because the environment can change,
adaptive evolution is a continuous process
• Genetic drift and gene flow do not consistently
lead to adaptive evolution as they can increase
or decrease the match between an organism
and its environment
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sexual Selection
• Sexual selection is natural selection for
mating success.
• Individuals with certain characteristics are more
likely than other individuals to obtain mates.
• It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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Fig. 23-15
Sexual dimorphism and sexual selection
Types of Sexual Selection
• Intrasexual selection is competition
among individuals of one sex (often
males) for mates of the opposite sex
• Intersexual selection, often called
mate choice, occurs when individuals
of one sex (usually females) are
choosy in selecting their mates
• Male showiness due to mate choice can
increase a male’s chances of attracting a
female, while decreasing his chances of
survival
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The Preservation of Genetic Variation
• What prevents natural selection from reducing
genetic variation by culling all unfavorable
genotypes?
– Various mechanisms help to preserve
genetic variation in a population
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Diploidy
• Diploidy maintains genetic variation in the form
of hidden recessive alleles
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Balancing Selection
• Balancing selection occurs when natural
selection maintains stable frequencies of two or
more phenotypic forms in a population
– Heterozygote Advantage
– Frequency Dependent Selection
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Heterozygote Advantage
• Heterozygote advantage occurs when
heterozygotes have a higher fitness than do
both homozygotes
• Natural selection will tend to maintain two or
more alleles at that locus
• The sickle-cell allele causes mutations in
hemoglobin but also confers malaria
resistance
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Fig. 23-17
Frequencies of the
sickle-cell allele
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent Selection
• In frequency-dependent selection, the
fitness of a phenotype declines if it becomes
too common in the population
• Selection can favor whichever phenotype is
less common in a population
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Fig. 23-18
“Right-mouthed”
1.0
“Left-mouthed”
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Why Natural Selection Cannot Fashion Perfect
Organisms
1. Selection can act only on existing variations
2. Evolution is limited by historical constraints
- Descent with modification
3. Adaptations are often compromises
4. Chance, natural selection, and
the environment interact
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Fig. 23-19
Fig. 23-UN1
Original
population
Evolved
population
Directional
selection
Disruptive
selection
Stabilizing
selection
Fig. 23-UN2
Sampling sites
(1–8 represent
pairs of sites)
2
1
3
4
5
6
7
8
9
10
11
Allele
frequencies
lap94 alleles
Other lap alleles
Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation,
American Scientist 75:134–141 (1987).
Salinity increases toward the open ocean
1
3
Long Island 2
Sound
9
N
W
10
E
S
7 8
6
4 5
11
Atlantic
Ocean
Fig. 23-UN3
You should now be able to:
1. Explain why the majority of point mutations
are harmless
2. Explain how sexual recombination generates
genetic variability
3. Define the terms population, species, gene
pool, relative fitness, and neutral variation
4. List the five conditions of Hardy-Weinberg
equilibrium
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5. Apply the Hardy-Weinberg equation to a
population genetics problem
6. Explain why natural selection is the only
mechanism that consistently produces
adaptive change
7. Explain the role of population size in genetic
drift
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8. Distinguish among the following sets of terms:
directional, disruptive, and stabilizing
selection; intrasexual and intersexual
selection
9. List four reasons why natural selection cannot
produce perfect organisms
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