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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
• 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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Concept 23.1: Mutation and sexual reproduction
produce the genetic variation that makes evolution
possible
• What Darwin knew:
– Life on Earth evolved over time
– Proposed natural selection as the
mechanism
– Knew the importance of heritable
differences in individuals
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• What Darwin didn’t know:
– How organisms pass heritable traits to the
offspring (Darwin never learned about genes)
• Gregor Mendel gave the answer:
– Proposed the particulate model of
inheritance, in which organisms pass discrete
heritable units (now called genes to their
offspring)
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• We now understand that two processes, mutation
and sexual reproduction, produce the variation
in gene pools that contributes to differences
among individuals
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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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Fig. 23-2
(a)
(b)
The difference in these two caterpillars is due to diet. (A) was fed on oak flowers
and thus resembles the flowers. (B) Siblings of (A) were raised on oak leave
and resemble oak twigs…..is this a genetic difference???
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: usually a single gene locus
with different alleles. Ex: purple or white
flowers
• Quantitative characters vary along a
continuum within a population: usually two or
more genes acting on a single phenotypic
character. Ex: Human skin color
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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
• Ave. heterozygosity is usually greater than nucleotide
variability.
–
Ex: Fruit flies: 13,700 genes (about 1,920 are heterozygous) = 14% Ave. heterozygosity
180 million nucleotides ( 1.8 million are different) = 1% Nucleotide variability
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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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• 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
Maine
Cold (6°C)
42
40
38
36
Latitude (°N)
34
32
30
Georgia
Warm (21°C)
Mutation
• 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
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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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• The effects of point mutations can vary:
– 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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Mutation Rates
• Mutation rates are low in animals and plants
• The average is about one mutation in every
100,000 genes per generation
• Mutations rates are often lower in
prokaryotes and higher in viruses
(but they have short generation spans, so mutations can quickly
generate genetic variation in populations: Ex: AIDS virus)
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Sexual Reproduction
• Sexual reproduction can shuffle existing
alleles into new combinations
• In organisms that reproduce sexually,
recombination of alleles is more important
than mutation in producing the genetic
differences that make adaptation possible
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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
• A locus is fixed if all individuals in a population
are homozygous for the same allele
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• The frequency of an allele in a population can
be calculated
– For diploid organisms, the total number of
alleles at a locus is the total number of
individuals x 2
– The total number of dominant alleles at a locus
is 2 alleles for each homozygous dominant
individual plus 1 allele for each heterozygous
individual; the same logic applies for recessive
alleles
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• By convention, if there are 2 alleles at a locus,
p and q are used to represent their frequencies
• The frequency of all alleles in a population will
add up to 1
– For example, p + q = 1
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The Hardy-Weinberg Principle
• The Hardy-Weinberg principle describes a
population that is not evolving
• If a population does not meet the criteria of
the Hardy-Weinberg principle, it can be
concluded that the population is evolving
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Hardy-Weinberg Equilibrium
• The Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a
population remain constant from generation
to generation
• In a given population where gametes contribute
to the next generation randomly, allele
frequencies will not change
• Mendelian inheritance preserves genetic
variation in a population
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Fig. 23-6
Alleles in the population
Frequencies of alleles
p = frequency of
CR allele
= 0.8
q = frequency of
CW allele
= 0.2
Gametes produced
Each egg:
Each sperm:
80%
20%
chance chance
80%
20%
chance chance
• 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
– where p2 and q2 represent the frequencies of
the homozygous genotypes and 2pq
represents the frequency of the heterozygous
genotype
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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:
– No mutations
– Random mating
– No natural selection
– Extremely large population size
– No gene flow
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• Natural populations can evolve at some loci,
while being in Hardy-Weinberg equilibrium at
other loci
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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
– Mate selection is random with respect to
whether or not an individual is a carrier for the
PKU allele
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– Natural selection can only act on rare
homozygous individuals who do not follow
dietary restrictions
– The population is large
– Migration has no effect as many other
populations have similar allele frequencies
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• The occurrence of PKU is 1 per 10,000 births
– q2 = 0.0001
– q = 0.01
• 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
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Natural Selection
• Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
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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
Animation: Causes of Evolutionary Change
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-8-1
CR CR
CR CR
CR CW
CR CR
CW CW
CR CW
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Fig. 23-8-2
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CW CW
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
Fig. 23-8-3
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CR
CR CW
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CR
CR CR
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
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
• The resulting gene pool may no longer be
reflective of the original population’s gene
pool
• If the population remains small, it may be
further affected by genetic drift
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Fig. 23-9
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect can
increase understanding of how human activity
affects other species
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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
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Gene Flow
• Gene flow consists of the movement of alleles
among populations
• Alleles can be transferred through the
movement of fertile individuals or gametes (for
example, pollen)
• Gene flow tends to reduce differences
between populations over time
• Gene flow is more likely than mutation to
alter allele frequencies directly
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Fig. 23-11
• Gene flow can decrease the fitness of a
population
• In bent grass, alleles for copper tolerance are
beneficial in populations near copper mines,
but harmful to populations in other soils
• Windblown pollen moves these alleles between
populations
• The movement of unfavorable alleles into a
population results in a decrease in fit between
organism and environment
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• Gene flow can increase the fitness of a
population
• Insecticides have been used to target
mosquitoes that carry West Nile virus and
malaria
• Alleles have evolved in some populations that
confer insecticide resistance to these
mosquitoes
• The flow of insecticide resistance alleles into a
population can cause an increase in fitness
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Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
• Only natural selection consistently results
in adaptive evolution
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A Closer Look at Natural Selection
• Natural selection brings about adaptive
evolution 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
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• 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
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Fig. 23-13
Original population
Original
Evolved
population population
(a) Directional selection
Phenotypes (fur color)
(b) Disruptive selection
(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
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Fig. 23-14
Examples of Adaptations
(a) Color-changing ability in cuttlefish
Movable bones
(b) Movable jaw
bones in
snakes
• 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
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Sexual Selection
• Sexual selection is natural selection for
mating success
• It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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Fig. 23-15
• 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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• How do female preferences evolve?
• The good genes hypothesis suggests that if a
trait is related to male health, both the male
trait and female preference for that trait should
be selected for
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The Preservation of Genetic Variation
• 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
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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
Neutral Variation
• Neutral variation is genetic variation that
appears to confer no selective advantage or
disadvantage
• For example,
– Variation in noncoding regions of DNA
– Variation in proteins that have little effect on
protein function or reproductive fitness
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Why Natural Selection Cannot Fashion Perfect
Organisms
1. Selection can act only on existing variations
2. Evolution is limited by historical constraints
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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