Download Chapter 23 - The Evolution of Populations

Figure 23.1
CAMPBELL
BIOLOGY
Figure 23.1a
The Smallest Unit of Evolution
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
!  A common misconception is that organisms evolve
during their lifetimes
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!  Natural selection acts on individuals, but only
populations evolve
The Evolution
of Populations
!  Consider, for example, a population of medium
ground finches on Daphne Major Island
!  During a drought, large-beaked birds were more
likely to crack large seeds and survive
!  The finch population evolved by natural selection
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
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Concept 23.1: Genetic variation makes
evolution possible
Genetic Variation
Average beak depth (mm)
Figure 23.2
10
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!  Microevolution is a change in allele frequencies
in a population over generations
!  Variation in heritable traits is a prerequisite for
evolution
!  Genetic variation among individuals is caused by
differences in genes or other DNA segments
!  Three mechanisms cause allele frequency change
!  Mendel’s work on pea plants provided evidence of
discrete heritable units (genes)
!  Phenotype is the product of inherited genotype
and environmental influences
!  Natural selection
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!  Natural selection can only act on variation with a
genetic component
!  Genetic drift
!  Gene flow
0
1976
1978
(similar to the (after
prior 3 years) drought)
!  Only natural selection causes adaptive evolution
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Figure 23.3
Animation: Genetic Variation from Sexual
Recombination
!  Some phenotypic differences are determined by a
single gene and can be classified on an either-or
basis
!  Genetic variation can be measured as gene
variability or nucleotide variability
!  For gene variability, average heterozygosity
measures the average percent of loci that are
heterozygous in a population
!  Other phenotypic differences are determined by
the influence of two or more genes and vary along
a continuum within a population
!  Nucleotide variability is measured by comparing
the DNA sequences of pairs of individuals
!  Nucleotide variation rarely results in phenotypic
variation
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Figure 23.4
Insertion sites
1
!  Some phenotypic variation does not result from
genetic differences among individuals, but rather
from environmental influences
(a)
(b)
!  Only genetically determined variation can have
evolutionary consequences
Substitution resulting
in translation of
different amino acid
1,500
Figure 23.5a
1,000
500
Intron
Exon
(a)
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Figure 23.5
Base-pair
substitutions
Deletion
2,000
2,500
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Figure 23.5b
Sources of Genetic Variation
(b)
Formation of New Alleles
!  New genes and alleles can arise by mutation or
gene duplication
!  A mutation is a random change in nucleotide
sequence of DNA
!  The effects of point mutations can vary
!  Sexual reproduction can result in genetic variation
by recombining existing alleles
!  Only mutations in cells that produce gametes can
be passed to offspring
!  A point mutation is a change in one base in a gene
!  Mutations that result in a change in protein
production are often harmful
!  Harmful mutations can be hidden from selection in
recessive alleles
!  Mutations that result in a change in protein
production can sometimes be beneficial
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!  The effects of point mutations can vary
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Altering Gene Number or Position
Rapid Reproduction
Sexual Reproduction
!  Mutation rates are low in animals and plants
!  Mutations accumulate quickly in prokaryotes and
viruses because they have short generation times
!  An ancestral odor-detecting gene has been
duplicated many times: humans have 350 copies
of the gene, mice have 1,000
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Concept 23.2: The Hardy-Weinberg equation
can be used to test whether a population is
evolving
Gene Pools and Allele Frequencies
!  In organisms that reproduce sexually,
recombination of alleles is more important than
mutation in producing the genetic differences that
make adaptation possible
!  Mutation rates are often lower in prokaryotes and
higher in viruses
!  Duplicated genes can take on new functions by
further mutation
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!  Sexual reproduction can shuffle existing alleles
into new combinations
!  The average is about one mutation in every
100,000 genes per generation
!  Duplication of small pieces of DNA increases
genome size and is usually less harmful
!  Mutations to genes can be neutral because of
redundancy in the genetic code
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!  A locus is fixed if all individuals in a population are
homozygous for the same allele
Porcupine herd
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Fortymile
herd range
CANADA
Fortymile herd
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MAP
AREA
Porcupine
herd range
!  A gene pool consists of all the alleles for all loci in
a population
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Figure 23.6a
ALASKA
!  A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
Porcupine herd
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Figure 23.6b
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Figure 23.UN01
!  If there are two or more alleles for a locus, diploid
individuals may be either homozygous or
heterozygous
!  The frequency of an allele in a population can be
calculated
CR CR
!  For diploid organisms, the total number of alleles at
a locus is the total number of individuals times 2
CW CW
!  The total number of dominant alleles at a locus is
two alleles for each homozygous dominant
individual plus one allele for each heterozygous
individual; the same logic applies for recessive
alleles
CR CW
Fortymile herd
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Figure 23.6
!  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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!  Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
!  Point mutations in noncoding regions generally
result in neutral variation, conferring no selective
advantage or disadvantage
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The Hardy-Weinberg Equation
!  By convention, if there are two 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
!  For example, consider a population of wildflowers
that is incompletely dominant for color
!  320 red flowers (CRCR)
!  The Hardy-Weinberg equation describes the
genetic makeup we expect for a population that is
not evolving at a particular locus
!  p = freq CR = 800 / (800 + 200) = 0.8
!  q = freq CW = 200 / (800 + 200) = 0.2
!  160 pink flowers (CRCW)
!  20 white flowers
!  To calculate the frequency of each allele
!  If the observed genetic makeup of the population
differs from expectations under Hardy-Weinberg, it
suggests that the population may be evolving
!  The sum of alleles is always 1
(CWCW)
!  0.8 + 0.2 = 1
!  Calculate the number of copies of each allele
!  CR = (320 × 2) + 160 = 800
!  CW = (20 × 2) + 160 = 200
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Figure 23.7
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Frequencies of alleles
Hardy-Weinberg Equilibrium
!  In a population where gametes contribute to the
next generation randomly and Mendelian
inheritance occurs, allele and genotype
frequencies remain constant from generation to
generation
p = frequency of CR allele
= 0.8
q = frequency of CW allele
= 0.2
!  Hardy-Weinberg equilibrium describes the
constant frequency of alleles in such a gene pool
Alleles in the population
!  The frequency of genotypes can be calculated
!  CRCR = p2 = (0.8)2 = 0.64
!  Consider, for example, the same population of 500
wildflowers and 1,000 alleles where
!  CWCW = q2 = (0.2)2 = 0.04
!  p = freq CR = 0.8
!  Such a population is in Hardy-Weinberg
equilibrium
!  CRCW = 2pq = 2(0.8)(0.2) = 0.32
!  The frequency of genotypes can be confirmed
using a Punnett square
!  q = freq CW = 0.2
Gametes produced
Each sperm:
Each egg:
80%
20%
chance chance
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Figure 23.8
80%
20%
chance chance
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80% CR (p = 0.8)
Figure 23.8a
20% CW (q = 0.2)
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Figure 23.8b
20% CW (q = 0.2)
80% CR (p = 0.8)
64% CRCR, 32% CRCW, and 4% CWCW
Sperm
CR p = 0.8
CW q = 0.2
CR
Sperm
CR p = 0.8
CW q = 0.2
p = 0.8
0.64 (p2)
C RC R
Eggs
CW
0.16 (pq)
C RC W
0.16 (qp)
C RC W
q = 0.2
0.04 (q2)
CWCW
Eggs
R
+ 16% C R W
= 80% CR = 0.8 = p
(from C C plants)
W
4% CW
+ 16% C
= 20% CW = 0.2 = q
(from CWCW plants)
(from CRCW plants)
W
4%
+ 16% C
= 20% CW = 0.2 = q
(from CWCW plants)
(from CRCW plants)
0.64 (p2)
C RC R
q = 0.2
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
0.16 (pq)
C RC W
0.16 (qp)
C RC W
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R
+ 16% C
= 80% CR = 0.8 = p
(from CRCW plants)
CW
CW
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
64% CR
(from CRCR plants)
p = 0.8
Gametes of this generation:
64% CR
(from CRCR plants)
Gametes of this generation:
CR
64% CRCR, 32% CRCW, and 4% CWCW
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0.04 (q2)
CWCW
!  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
64% CRCR, 32% CRCW, and 4% CWCW plants
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Conditions for Hardy-Weinberg Equilibrium
Applying the Hardy-Weinberg Equation
!  The Hardy-Weinberg theorem describes a
hypothetical population that is not evolving
!  The five conditions for nonevolving populations are
rarely met in nature
!  Natural populations can evolve at some loci, while
being in Hardy-Weinberg equilibrium at other loci
1.  No mutations
!  In real populations, allele and genotype
frequencies do change over time
!  We can assume the locus that causes
phenylketonuria (PKU) is in Hardy-Weinberg
equilibrium given that
1.  The PKU gene mutation rate is low
2.  Random mating
2.  Mate selection is random with respect to whether
or not an individual is a carrier for the PKU allele
3.  No natural selection
4.  Extremely large population size
5.  No gene flow
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Concept 23.3: Natural selection, genetic drift,
and gene flow can alter allele frequencies in a
population
!  The occurrence of PKU is 1 per 10,000 births
3.  Natural selection can only act on rare
homozygous individuals who do not follow dietary
restrictions
5.  Migration has no effect as many other populations
have similar allele frequencies
!  Differential success in reproduction results in
certain alleles being passed to the next generation
in greater proportions
!  Three major factors alter allele frequencies and
bring about most evolutionary change
!  q2 = 0.0001
!  q = 0.01
!  For example, an allele that confers resistance to
DDT in fruit flies increased in frequency after DDT
was used widely in agriculture
!  Natural selection
!  The frequency of normal alleles is
4.  The population is large
Natural Selection
!  Genetic drift
!  p = 1 - q = 1 - 0.01 = 0.99
!  Gene flow
!  The frequency of carriers is
!  2pq = 2 × 0.99 × 0.01 = 0.0198
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!  or approximately 2% of the U.S. population
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Figure 23.9–1
Figure 23.9–2
Genetic Drift
!  Natural selection can cause adaptive evolution,
an improvement in the match between organisms
and their environment
!  The smaller a sample, the greater the chance of
random deviation from a predicted result
C RC R
!  Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to the
next
C RC W
C RC R
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C RC R
C RC R
C RC R
C RC R
CWCW
C RC W
CWCW
CWCW
C RC W
C RC R
C RC W
C RC R
C RC W
C RC W
!  Genetic drift tends to reduce genetic variation
through losses of alleles
C RC W
CWCW
C RC W
C RC R
C RC R
CWCW
5 plants
leave
offspring
C RC R
C RC W
C RC R
C RC R
C RC W
C RC W
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
C RC W
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
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Generation 2
p = 0.5
q = 0.5
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BioFlix: Mechanisms of Evolution
Animation: Causes of Evolutionary Change
The Founder Effect
Figure 23.9–3
5 plants
leave
offspring
C RC R
CWCW
C RC W
C RC R
C RC R
C RC R
C RC R
CWCW
2 plants
leave
offspring
C RC W
C RC R
C RC W
C RC R
C RC R
C RC R
C RC R
C RC W
C RC R
C RC R
C RC W
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
!  Allele frequencies in the small founder population
can be different from those in the larger parent
population
C RC R
CWCW
CWCW
C RC W
C RC R
C RC W
C RC R
!  The founder effect occurs when a few individuals
become isolated from a larger population
C RC R
C RC W
Generation 2
p = 0.5
q = 0.5
C RC R
C RC R
Generation 3
p = 1.0
q = 0.0
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Figure 23.10–1
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Figure 23.10–2
Figure 23.10–3
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
Original
population
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Original
population
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Bottlenecking
event
Original
population
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Bottlenecking
event
Surviving
population
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Figure 23.11
Pre-bottleneck
(Illinois, 1820)
Case Study: Impact of Genetic Drift on the
Greater Prairie Chicken
!  Understanding the bottleneck effect can increase
understanding of how human activity affects other
species
!  Loss of prairie habitat caused a severe reduction
in the population of greater prairie chickens in
Illinois
Greater prairie
chicken
(a)
!  The surviving birds had low levels of genetic
variation, and only 50% of their eggs hatched
Number
of alleles
per locus
Percentage
of eggs
hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Illinois
1930–1960s
1993
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Figure 23.11b
Pre-bottleneck Post-bottleneck
(Illinois, 1820) (Illinois, 1993)
Range
of greater
prairie
chicken
Population
size
Location
Figure 23.11a
Post-bottleneck
(Illinois, 1993)
Greater prairie
chicken
(a)
Range
of greater
prairie
chicken
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(b)
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Figure 23.11c
Effects of Genetic Drift: A Summary
Population
size
Number
of alleles
per locus
Percentage
of eggs
hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Location
Illinois
1930–1960s
1993
!  Researchers used DNA from museum specimens
to compare genetic variation in the population
before and after the bottleneck
!  The results showed a loss of alleles at several loci
!  Researchers introduced greater prairie chickens
from populations in other states and were
successful in introducing new alleles and
increasing the egg hatch rate to 90%
Greater prairie
chicken
1.  Genetic drift is significant in small populations
2.  Genetic drift can cause 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
(b)
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Figure 23.12
Figure 23.12a
Gene Flow
Central
population
NORTH SEA
N
!  Consider, for example, the great tit (Parus major) on the
Dutch island of Vlieland
!  Alleles can be transferred through the movement
of fertile individuals or gametes (for example,
pollen)
Parus major
Survival rate (%)
!  Immigration from the mainland introduces alleles that
decrease fitness on the island
!  Natural selection removes alleles that decrease fitness
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!  Birds born in the central region with high immigration
have a lower fitness; birds born in the east with low
immigration have a higher fitness
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10
0
10
74
50 Population in which
the surviving females
eventually bred
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Central
Eastern
30
20
0
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2 km
Population in which
the surviving females
eventually bred
40
Central
Eastern
30
50
!  Mating causes gene flow between the central and
eastern populations
!  Gene flow tends to reduce variation among
populations over time
Eastern
population
Vlieland,
the Netherlands
Survival rate (%)
!  Gene flow can decrease the fitness of a population
!  Gene flow consists of the movement of alleles
among populations
Females born in
Females born in
central population eastern population
Females born in
Females born in
central population eastern population
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Figure 23.12b
!  Gene flow can increase the fitness of a population
!  Consider, for example, the spread of alleles for
resistance to insecticides
Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
!  Gene flow is an important agent of evolutionary
change in modern human populations
!  Evolution by natural selection involves both
chance and “sorting”
!  New genetic variations arise by chance
!  Insecticides have been used to target mosquitoes
that carry West Nile virus and malaria
!  Beneficial alleles are “sorted” and favored by
natural selection
!  Alleles have evolved in some populations that
confer insecticide resistance to these mosquitoes
Parus major
!  Only natural selection consistently increases the
frequencies of alleles that provide reproductive
advantage
!  The flow of insecticide resistance alleles into a
population can cause an increase in fitness
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Natural Selection: A Closer Look
Relative Fitness
!  Natural selection brings about adaptive evolution
by acting on an organism’s phenotype
Directional, Disruptive, and Stabilizing Selection
!  The phrases “struggle for existence” and “survival
of the fittest” are misleading as they imply direct
competition among individuals
!  Relative fitness is the contribution an individual
makes to the gene pool of the next generation,
relative to the contributions of other individuals
!  Reproductive success is generally more subtle
and depends on many factors
!  Selection favors certain genotypes by acting on
the phenotypes of individuals
!  There are three modes of selection
!  Directional selection favors individuals at one
extreme 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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Figure 23.14
Frequency of
individuals
Figure 23.13
Original
Evolved
population population
The Key Role of Natural Selection in Adaptive
Evolution
Original
population
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Figure 23.14a
Bones shown in
green are movable.
!  Striking adaptations have arisen by natural
selection
!  For example, certain octopuses can change color
rapidly for camouflage
Phenotypes (fur color)
Ligament
!  For example, the jaws of snakes allow them to
swallow prey larger than their heads
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection
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Figure 23.15
Sexual Selection
!  Natural selection increases the frequencies of
alleles that enhance survival and reproduction
!  Adaptive evolution occurs as the match between a
species and its environment increases
!  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
!  Because the environment can change, adaptive
evolution is a continuous process
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!  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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Figure 23.16
!  Intrasexual selection is direct 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
Experiment
Recording of LC
male’s call
Recording of SC
male’s call
Offspring of
SC father
Female gray
tree frog
LC male gray
SC male
tree frog
gray tree
SC sperm × Eggs × LC sperm
frog
Offspring of
LC father
Survival and growth of these half-sibling
offspring compared
Results
Offspring
Performance
1996
1995
Larval survival
LC better
NSD
Larval growth
NSD
LC better
Time to
metamorphosis
LC better (shorter)
LC better (shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
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Recording of LC
male’s call
Female gray
tree frog
SC male
LC male gray
gray tree
tree frog
SC
sperm
× Eggs × LC sperm
frog
!  The “good genes” hypothesis suggests that if a
trait is related to male genetic quality or health,
both the male trait and female preference for that
trait should increase in frequency
!  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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Figure 23.16a
Experiment
Recording of SC
male’s call
!  How do female preferences evolve?
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Offspring of
SC father
Offspring of
LC father
Survival and growth of these half-sibling
offspring compared
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Figure 23.16b
Balancing Selection
Results
Offspring
Performance
1995
1996
Larval survival
LC better
NSD
Larval growth
NSD
LC better
Time to
metamorphosis
LC better (shorter)
LC better (shorter)
Heterozygote Advantage
!  Diploidy maintains genetic variation in the form of
recessive alleles hidden from selection in
heterozygotes
!  Heterozygote advantage occurs when
heterozygotes have a higher fitness than do both
homozygotes
!  A mutation in an allele that codes for part of the
hemoglobin protein causes sickle-cell disease, but
also confers malaria resistance
!  Balancing selection occurs when natural
selection maintains stable frequencies of two or
more phenotypic forms in a population
!  Natural selection will tend to maintain two or more
alleles at that locus
!  In regions where the malaria parasite is common,
selection favors individuals heterozygous for the
sickle-cell allele
!  Heterozygote advantage can result from stabilizing
or directional selection
!  Balancing selection includes
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
!  Heterozygote advantage
!  Frequency-dependent selection
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Figure 23.17a
Figure 23.17b
Sickle-cell allele
on chromosome
Figure 23.17d
MAKE CONNECTIONS: The Sickle-Cell Allele
Evolution in Populations
Events at the Molecular Level
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Figure 23.17c
MAKE CONNECTIONS: The Sickle-Cell Allele
MAKE CONNECTIONS: The Sickle-Cell Allele
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MAKE CONNECTIONS: The Sickle-Cell Allele
Effects on Individual
Organisms
Template strand
Consequences for Cells
Fiber
An adenine
replaces a Sickle-cell
hemoglobin
Wild-type thymine.
allele
Low-oxygen
conditions
Normal hemoglobin
(does not aggregate
into fibers)
Sickled red
blood cell
Normal red
blood cell
Key
Frequencies of
the sickle-cell allele
3.0–6.0%
6.0–9.0%
Distribution of malaria
9.0–12.0%
12.0–15.0%
caused by Plasmodium falciparum
>15.0%
(a parasitic unicellular eukaryote)
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Infected mosquitos spread malaria when they bite people.
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Figure 23.18
Figure 23.UN02
Frequency-Dependent Selection
Why Natural Selection Cannot Fashion Perfect
Organisms
“Left-mouthed”
P. microlepis
!  Selection favors whichever phenotype is less
common in a population
!  For example, frequency-dependent selection
results in approximately equal numbers of “rightmouthed” and “left-mouthed” scale-eating fish
1.  Selection can act only on existing variations
1.0
Frequency of
“left-mouthed” individuals
!  In frequency-dependent selection, the fitness of
a phenotype declines if it becomes too common in
the population
2.  Evolution is limited by historical constraints
“Right-mouthed”
P. microlepis
3.  Adaptations are often compromises
4.  Chance, natural selection, and the environment
interact
0.5
0
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1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
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Figure 23.UN03a
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Figure 23.UN03b
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Figure 23.UN04
Figure 23.UN05
Sampling sites
(1–8 represent
pairs of sites)
Original
population
1
2
3
5
4
7
6
8
9
10
11
Allele
frequencies
Evolved
population
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
Directional selection
Disruptive selection
Stabilizing selection
1
Long Island 2 3
Sound
N
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5 6
7 8
9
10
11
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4
Atlantic
Ocean
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Figure 23.UN06
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