Chapter 23
The Evolution of Populations
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
Fig. 23-1
Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals
Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
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
Fig. 23-2
(a)
(b)
Fig. 23-2a
(a)
Fig. 23-2b
(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
Quantitative characters vary along a continuum within a population
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
Variation Between Populations
Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups
Fig. 23-3
13.17
19
XX
10.16
9.12
8.11
1
2.4
3.14
5.18
6
7.15
9.10
1
2.19
11.12
13.17
15.18
3.8
4.16
5.14
6.7
XX
Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis
Fig. 23-4
1.0
0.8
0.6
0.4
0.2
0
46
44
42
40
38
36
34
32
30
Georgia
Warm (21°C)
Latitude (°N)
Maine
Cold (6°C)
Ldh-B b allele frequency
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
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 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
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
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
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
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
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
Fig. 23-5
Porcupine herd
Porcupine
herd range
Beaufort Sea
NORTHWEST
TERRITORIES
MAP
AREA
ALASKA
CANADA
Fortymile
herd range
Fortymile herd
ALASKA
YUKON
Fig. 23-5a
Porcupine
herd range
Beaufort Sea
NORTHWEST
TERRITORIES
MAP
AREA
ALASKA
CANADA
Fortymile
herd range
ALASKA
YUKON
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
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
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
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
Fig. 23-6
Frequencies of alleles
Alleles in the population
Gametes produced
Each egg:
Each sperm:
80%
chance
80%
chance
20%
chance
20%
chance
q = frequency of
p = frequency of
CR allele = 0.8
CW allele = 0.2
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
Fig. 23-7-1
Sperm
CR
(80%)
CW
(20%)
80% CR ( p = 0.8)
CW
(20%)
20% CW (q = 0.2)
16% ( pq)
CRCW
4% (q2)
CW CW
CR
(80%)
64% ( p2)
CRCR
16% (qp)
CRCW
Eggs
Fig. 23-7-2
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR   +    16% CR    =   80% CR = 0.8 = p
4% CW     +    16% CW   =   20% CW = 0.2 = q
Fig. 23-7-3
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR   +    16% CR    =   80% CR = 0.8 = p
4% CW     +    16% CW   =   20% CW = 0.2 = q
64% CRCR, 32% CRCW, and 4% CWCW plants
Genotypes in the next generation:
Fig. 23-7-4
Gametes of this generation:
64% CR CR, 32% CR CW, and 4% CW CW
64% CR    +     16% CR    =   80% CR  = 0.8 = p
4% CW      +    16% CW    =  20% CW = 0.2 = q
64% CR CR, 32% CR CW, and 4% CW CW plants
Genotypes in the next generation:
Sperm
CR
(80%)
CW
(20%)
80% CR ( p = 0.8)
CW
(20%)
20% CW (q = 0.2)
16% ( pq)
CR CW
4% (q2)
CW CW
CR
(80%)
64% ( p2)
CR CR
16% (qp)
CR CW
Eggs
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
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

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
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
Three major factors alter allele frequencies and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow
Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
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
Fig. 23-8-1
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
Fig. 23-8-2
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Generation 2
p = 0.5
q = 0.5
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
CR CW
CR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
Fig. 23-8-3
Generation 1
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Generation 2
CR CW
CR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
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
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
Fig. 23-9
Original
population
Bottlenecking
event
Surviving
population
Understanding the bottleneck effect can increase understanding of how human activity affects other species
Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois
The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched
Fig. 23-10
Number
of alleles
per locus
Range
of greater
prairie
chicken
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Minnesota, 1998
    (no bottleneck)
Nebraska, 1998
    (no bottleneck)
Kansas, 1998
    (no bottleneck)
Illinois
1930–1960s
1993
Location
Population
size
Percentage
of eggs
hatched
1,000–25,000
<50
750,000
75,000–
200,000
4,000
5.2
3.7
93
<50
5.8
5.8
5.3
85
96
99
(a)
(b)
Fig. 23-10a
Range
of greater
prairie
chicken
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
(a)
Fig. 23-10b
Number
of alleles
per locus
Minnesota, 1998
   (no bottleneck)
Nebraska, 1998
   (no bottleneck)
Kansas, 1998
   (no bottleneck)
Illinois
1930–1960s
1993
Location
Population
size
Percentage
of eggs
hatched
1,000–25,000
<50
750,000
75,000–
200,000
4,000
5.2
3.7
93
<50
5.8
5.8
5.3
85
96
99
(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
Genetic drift is significant in small populations
Genetic drift causes allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed
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
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
Fig. 23-12
NON-
MINE
SOIL
MINE
SOIL
NON-
MINE
SOIL
Prevailing wind direction
Index of copper tolerance
Distance from mine edge (meters)
70
60
50
40
30
20
10
0
20
0
20
0
20
40
60
80
100
120
140
160
Fig. 23-12a
NON-
MINE
SOIL
MINE
SOIL
NON-
MINE
SOIL
Prevailing wind direction
Index of copper tolerance
Distance from mine edge (meters)
70
60
50
40
30
20
10
0
20
0
20
0
20
40
60
80
100
120
140
160
Fig. 23-12b
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
Only natural selection consistently results in adaptive evolution
Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
A Closer Look at Natural Selection
Natural selection brings about adaptive evolution by acting on an organism’s phenotype
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
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
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
Fig. 23-13
Original population
(c) Stabilizing selection
(b) Disruptive selection
(a) Directional selection
Phenotypes (fur color)
Frequency of individuals
Original
population
Evolved
population
Fig. 23-13a
Original population
(a) Directional selection
Phenotypes (fur color)
Frequency of individuals
Original population
Evolved population
Fig. 23-13b
Original population
(b) Disruptive selection
Phenotypes (fur color)
Frequency of individuals
Evolved population
Fig. 23-13c
Original population
(c) Stabilizing selection
Phenotypes (fur color)
Frequency of individuals
Evolved population
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
Fig. 23-14
(a) Color-changing ability in cuttlefish
(b) Movable jaw
bones in
snakes
Movable bones
Fig. 23-14a
(a) Color-changing ability in cuttlefish
Fig. 23-14b
(b) Movable jaw
bones in
snakes
Movable bones
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
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
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
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
Fig. 23-16
SC male gray
tree frog
Female gray
tree frog
LC male gray
tree frog
EXPERIMENT
SC sperm  Eggs  LC sperm
Offspring of
LC father
Offspring of
SC father
Fitness of these half-sibling offspring compared
RESULTS
1995
Fitness Measure
1996
Larval growth
Larval survival
Time to metamorphosis
LC better
NSD
LC better
(shorter)
LC better
(shorter)
NSD
LC better
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
Fig. 23-16a
SC male gray
tree frog
Female gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of
LC father
Offspring of
SC father
Fitness of these half-sibling offspring compared
EXPERIMENT
Fig. 23-16b
RESULTS
1995
Fitness Measure
1996
Larval growth
Larval survival
Time to metamorphosis
LC better
NSD
LC better
(shorter)
LC better
(shorter)
NSD
LC better
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population
Diploidy
Diploidy maintains genetic variation in the form of hidden recessive alleles
Balancing Selection
Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
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
Heterozygote Advantage
Fig. 23-17
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
Frequencies of the
sickle-cell allele
2.5–5.0%
7.5–10.0%
5.0–7.5%
>12.5%
10.0–12.5%
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
Frequency-Dependent Selection
Fig. 23-18
“Right-mouthed”
1981
“Left-mouthed”
Frequency of
“left-mouthed” individuals
Sample year
1.0
0.5
0
’82
’83
’84
’85
’86
’87
’88
’89
’90
Fig. 23-18a
“Right-mouthed”
“Left-mouthed”
Fig. 23-18b
1981
Frequency of
“left-mouthed” individuals
Sample year
1.0
0.5
0
’82
’83
’84
’85
’86
’87
’88
’89
’90
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
Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact
Fig. 23-19
Fig. 23-UN1
Stabilizing
selection
Original
population
Evolved
population
Directional
selection
Disruptive
selection
Fig. 23-UN2
Sampling sites
(1–8 represent
pairs of sites)
Salinity increases toward the open ocean
N
Long Island
Sound
Allele
frequencies
Atlantic
Ocean
Other lap alleles
lap94 alleles
Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation,
American Scientist 75:134–141 (1987).
E
S
W
1
2
3
4
5
9
10
6
7
8
11
1
11
10
2
3
4
5
6
7
8
9
Fig. 23-UN3
You should now be able to:
Explain why the majority of point mutations are harmless
Explain how sexual recombination generates genetic variability
Define the terms population, species, gene pool, relative fitness, and neutral variation
List the five conditions of Hardy-Weinberg equilibrium
Apply the Hardy-Weinberg equation to a population genetics problem
Explain why natural selection is the only mechanism that consistently produces adaptive change
Explain the role of population size in genetic drift
Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection
List four reasons why natural selection cannot produce perfect organisms