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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
•
Two processes, mutation and sexual reproduction, produce the variation in
gene pools that contributes to differences among individuals
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-2
Nonheritable variation. These caterpillars of the moth Nemoria arizonaria owe their
different appearances to chemicals in their diets, not to their genotypes. Caterpillars
raised on a diet of oak flowers resembled the flowers (a), whereas their siblings
raised on oak leaves resembled oak twigs (b).
(a)
(b)
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 (purple pea flowers
or white pea flowers)
•
Quantitative characters vary along a continuum within a population (human
skin color)
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-3
Most species exhibit geographic variation, differences between gene pools of
separate populations or population subgroups. The numbers represent fused
chromosomes from ancestral species.
1
2.4
8.11
9.12
3.14
5.18
10.16 13.17
6
7.15
19
XX
1
2.19
3.8
4.16 5.14
9.10 11.12 13.17 15.18
6.7
XX
Fig. 23-4
1.0
0.8
•Some examples of
geographic variation
occur as a cline,
which is a graded
change in a trait
along a geographic
axis
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 – Point Mutations, Mutations that alter
gene number or sequence, Mutation Rates
•
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
•
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 in a gene might not affect protein production because of
redundancy in the genetic code
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mutation – Point Mutations, Mutations that alter
gene number or sequence, Mutation Rates
•
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
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
•
(Remember – In meiosis genetic variation is a result of crossing over,
independent assortment of alleles, and random fertilization)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
•
A population is a localized group of individuals capable of interbreeding and
producing fertile offspring
Gene Pools and Allele Frequencies
•
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
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•
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
In Fig. 23.5 there are two caribou populations that are not totally isolated; they
sometimes share the same area. Nonetheless, members of either population
are more likely to breed with members of their own populations than with
members of the other populations.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-5
Porcupine herd
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
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
•
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
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Hardy-Weinberg Principle
•
p+q=1
–
p = frequency of the dominant allele
–
q = frequency of the recessive allele
p2 + 2pq + q2 = 1
p2 = frequency of the homozygous dominant
genotype
2pq = frequency of the heterozygous genotype
q2 = frequency of the recessive genotype
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-6
If all of these alleles could be placed in a large bin, 80% would be CR and 20%
would be Cw.
Assuming random mating, each time two gametes come together, there is an 80%
chance the egg carries a CR allele and a 20% chance it carries a Cw allele. (Same
%’s for sperm)
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
Fig. 23-7-4
20% CW (q = 0.2)
80% CR ( p = 0.8)
Sperm
(80%)
CW
(20%)
64% ( p2)
CR CR
16% ( pq)
CR CW
CR
16% (qp)
CR CW
4% (q2)
CW CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this 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
•
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
Natural populations can evolve at some loci, while being in Hardy-Weinberg
equilibrium at other loci
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 HardyWeinberg 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
–
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Applying the Hardy-Weinberg Principle
•
•
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
–
Natural selection
• Differential success (some traits are selected for and others
are selected against) in reproduction results in certain alleles
being passed to the next generation in greater proportions
–
Genetic drift
–
Gene flow
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-8-1
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
•
(Highlighted boxes represent, by chance
alone, flowers that produce fertile offspring.)
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Bottleneck Effect
•
•
•
Original
population
Bottlenecking
event
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
Surviving
population
Fig. 23-10
Pre-bottleneck Post-bottleneck
(Illinois, 1820) (Illinois, 1993)
Loss of prairie
habitat caused a
severe reduction
in the population
of greater prairie
chickens in Illinois
Range
of greater
prairie
chicken
(a)
Location
Population
size
Percentage
Number
of alleles of eggs
per locus hatched
Illinois
The surviving
birds had low
levels of genetic
variation, and only
50% of their eggs
hatched
1,000–25,000
5.2
93
<50
3.7
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Minnesota, 1998
(no bottleneck)
4,000
5.3
85
1930–1960s
1993
(b)
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
population in other
states and were
successful in
introducing new
alleles and increasing
the egg hatch rate to
90%
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 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
•
Gene flow can decrease the fitness of a population
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-11
This image
illustrates
how gene
flow can
homogenize
the gene
pools of
populations,
thereby
reducing
geographic
variation in
appearance.
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
70
60
MINE
SOIL
NONMINE
SOIL
NONMINE
SOIL
50
Prevailing wind direction
40
30
20
10
0
20
0
20
0
20
40
60
80
Distance from mine edge (meters)
100
120
140
160
Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
•
Natural selection brings about adaptive evolution by acting on an organism’s
phenotype
•
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
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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. 23-13a
Original population
Phenotypes (fur color)
– Directional
selection
favors
individuals at
one end of the
phenotypic
range
Original population
Evolved population
(a) Directional selection
Fig. 23-13b
Original population
Disruptive
selection favors
individuals at
both extremes of
the phenotypic
range
Phenotypes (fur color)
Evolved population
(b) Disruptive selection
Fig. 23-13c
Original population
Phenotypes (fur color)
– Stabilizing
selection
favors
intermediate
variants and
acts against
extreme
phenotypes
Evolved population
(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
•
As a result, what constitutes a “good match” between an organism and its
environment can be a moving target, making adaptive evolution a continuous,
dynamic process.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-14a
•
Natural selection increases the frequencies of alleles that enhance survival and
reproduction
(a) Color-changing ability in cuttlefish –
allows fish to hide from predators and surprise
its prey.
Fig. 23-14b
Movable bones
•
Adaptive
evolution occurs
as the match
between an
organism and its
environment
increases
(b) Movable jaw
bones in
snakes – allows
them to swallow
food much larger
than their head.
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•
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
•
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-16
EXPERIMENT
Female gray
tree frogs prefer
to mate with
males that give
long mating
calls.
Question:
Is the genetic
makeup of long
calling (LC)
males superior
to that of short
calling (SC)
males?
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of Offspring of
SC father
LC father
Fitness of these half-sibling offspring compared
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
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.
Conclusion:
Because offspring
fathered by an LC
male had higher
fitness than their
half siblings
fathered by an SC
male, the duration
of a male’s mating
call is indicative of
the male’s overall
genetic quality.
This result supports
the hypothesis that
female mate choice
can be based on a
trait that indicates
whether the male
has “good genes.”
The Preservation of Genetic Variation – various
mechanisms help to preserve genetic variation in a
population
•
Diploidy maintains genetic variation in the form of hidden recessive alleles
•
Balancing selection occurs when natural selection maintains stable
frequencies of two or more phenotypic forms in a population (includes
heterozygote advantage and frequency-dependent selection)
•
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
•
In frequency-dependent selection, the fitness of a phenotype declines if it
becomes too common in the population
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-17
•The sickle-cell allele causes mutations in
hemoglobin but also confers malaria
resistance
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%
Fig. 23-18
•Selection can favor whichever
phenotype is less common in a
population
“Right-mouthed”
1.0
“Left-mouthed”
Green dot
represents
adults that
reproduced had
the opposite
phenotype of that
which was most
common
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Why Natural Selection Cannot Fashion Perfect
Organisms (Make sure you read!)
1.
Selection can act only on existing variations - these variations may not be
the ideal traits
2.
Evolution is limited by historical constraints – Each species has a legacy of
descent with modification. Evolution does not scrap the ancestral anatomy
and build each structures from scratch. In birds and bats, an existing pair of
limbs took on new functions as these organisms evolved from walking
ancestors.
3.
Adaptations are often compromises – Each organism must do many different
things. Some adaptations work for one aspect of the organisms exsistence
but not for others. (Fig. 23.19)
4.
Chance, natural selection, and the environment interact – founding
populations may not transport individuals that are best suited to new
environment and natural disasters change the environment in unpredictable
ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-19
This poor little frog’s love calls also attract this bat!
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
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
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings