1. The Evolution of Populations
Chapter 23
The Evolution of Populations

2. 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

3. Fig. 23-1
Figure 23.1 Is this finch evolving by natural selection?

4. Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

5. 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

6. Fig. 23-2
(a)
(b)
Figure 23.2 Nonheritable variation

7. Fig. 23-2a
(a)
Figure 23.2 Nonheritable variation

8. Fig. 23-2b
(b)
Figure 23.2 Nonheritable variation

9. 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

10. 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

11. Variation Between Populations
Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups

12. Fig. 23-3 1
2.4
3.14
5.18 6
7.15
8.11
9.12
10.16
13.17 19 XX 1
2.19
3.8
4.16
5.14
6.7
Figure 23.3 Geographic variation in isolated mouse populations on Madeira
9.10
11.12
13.17
15.18 XX

13. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

14. Ldh-B b allele frequency
Fig. 23-4
1.0
0.8
0.6
Ldh-B b allele frequency
0.4
0.2
Figure 23.4 A cline determined by temperature 46 44 42 40 38 36 34 32 30
Latitude (°N)
Maine
Cold (6°C)
Georgia
Warm (21°C)

15. Animation: Genetic Variation from Sexual Recombination
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

16. Point Mutations
A point mutation is a change in one base in a gene

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. Porcupine herd Porcupine herd range Fortymile herd range
Fig. 23-5
Porcupine herd
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Figure 23.5 One species, two populations
Fortymile
herd range
ALASKA
YUKON
Fortymile herd

25. Porcupine herd range Fortymile herd range Beaufort Sea NORTHWEST
Fig. 23-5a
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Figure 23.5 One species, two populations
Fortymile
herd range
ALASKA
YUKON

26. 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

27. The frequency of all alleles in a population will add up to 1
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

28. 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

29. 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

30. Alleles in the population Frequencies of alleles
Fig. 23-6
Alleles in the population
Frequencies of alleles
Gametes produced
p = frequency of
Each egg:
Each sperm:
CR allele = 0.8
q = frequency of
80%
chance
20%
chance
80%
chance
20%
chance
CW allele = 0.2
Figure 23.6 Selecting alleles at random from a gene pool

31. 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

32. 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) Eggs CW (20%) CR CW
Fig
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
Figure 23.7 The Hardy-Weinberg principle
Eggs
64% ( p2)
CRCR
16% ( pq)
CRCW
4% (q2)
CW CW
16% (qp)
CRCW CW
(20%)

33. Gametes of this generation:
Fig
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR   +    16% CR    =   80% CR = 0.8 = p
Figure 23.7 The Hardy-Weinberg principle
4% CW     +    16% CW   =   20% CW = 0.2 = q

34. Gametes of this generation:
Fig
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR   +    16% CR    =   80% CR = 0.8 = p
4% CW     +    16% CW   =   20% CW = 0.2 = q
Genotypes in the next generation:
Figure 23.7 The Hardy-Weinberg principle
64% CRCR, 32% CRCW, and 4% CWCW plants

35. Figure 23.7 The Hardy-Weinberg principle
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
Eggs
64% ( p2)
CR CR
16% ( pq)
CR CW
4% (q2)
CW CW CW
(20%)
16% (qp)
CR CW
64% CR CR, 32% CR CW, and 4% CW CW
Figure 23.7 The Hardy-Weinberg principle
Gametes of this generation:
64% CR    +     16% CR    =   80% CR  = 0.8 = p
4% CW      +    16% CW    =  20% CW = 0.2 = q
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants

36. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population
In real populations, allele and genotype frequencies do change over time

37. 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

38. Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

39. 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

40. 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

41. The occurrence of PKU is 1 per 10,000 births
q2 =
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 =
or approximately 2% of the U.S. population

42. 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

43. Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

44. Animation: Causes of Evolutionary Change
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

45. Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3
Fig
CR CR
CR CR
CR CW
CW CW
CR CR
CR CW
CR CR
CR CW
Figure 23.8 Genetic drift
CR CR
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3

46. Generation 1 Generation 2 p (frequency of CR) = 0.7 p = 0.5
Fig
CR CR
CR CR
CW CW
CR CR
CR CW
CR CW
CW CW
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CR CW
CW CW
CR CR
Figure 23.8 Genetic drift
CR CR
CR CW
CR CW
CR CW
Generation 1
Generation 2
p (frequency of CR) = 0.7
p = 0.5
q (frequency of CW ) = 0.3
q = 0.5

47. Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7
Fig
CR CR
CR CR
CW CW
CR CR
CR CR
CR CW
CR CW
CR CR
CR CR
CW CW
CR CR
CR CR
CW CW
CR CR
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CW CW
CR CR
Figure 23.8 Genetic drift
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
Generation 1
Generation 2
Generation 3
p (frequency of CR) = 0.7
p = 0.5
p = 1.0
q (frequency of CW ) = 0.3
q = 0.5
q = 0.0

48. 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

49. 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

50. Original population Bottlenecking event Surviving population
Fig. 23-9
Figure 23.9 The bottleneck effect
Original
population
Bottlenecking
event
Surviving
population

51. Understanding the bottleneck effect can increase understanding of how human activity affects other species

52. 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

53. Fig
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Number
of alleles
per locus
Percentage
of eggs
hatched
Population
size
Location
Illinois
5.2
3.7 93
<50
1930–1960s
1,000–25,000
1993
<50
Figure Bottleneck effect and reduction of genetic variation
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
(b)

54. Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Range
Fig a
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Figure Bottleneck effect and reduction of genetic variation
Range
of greater
prairie
chicken
(a)

55. Number of alleles per locus Percentage of eggs hatched Population size
Fig b
Number
of alleles
per locus
Percentage
of eggs
hatched
Population
size
Location
Illinois
5.2
3.7 93
<50
1930–1960s
1,000–25,000
1993
<50
Kansas, 1998
   (no bottleneck)
750,000
5.8 99
Figure Bottleneck effect and reduction of genetic variation
Nebraska, 1998
   (no bottleneck)
75,000–
200,000
5.8 96
Minnesota, 1998
   (no bottleneck)
4,000
5.3 85
(b)

56. 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%

57. 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

58. 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

59. Fig
Figure Gene flow and human evolution

60. 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

61. Figure 23.12 Gene flow and selection70
NON-
MINE
SOIL
MINE
SOIL
NON-
MINE
SOIL 60 50
Prevailing wind direction
Index of copper tolerance 40 30
Figure Gene flow and selection 20 10 20 20 20 40 60 80
100
120
140
160
Distance from mine edge (meters)

62. Prevailing wind direction
Fig a 70
NON-
MINE
SOIL
MINE
SOIL
NON-
MINE
SOIL 60 50
Prevailing wind direction 40
Index of copper tolerance 30 20 10
Figure Gene flow and selection 20 20 20 40 60 80
100
120
140
160
Distance from mine edge (meters)

63. Fig b
Figure Gene flow and selection

64. 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

65. Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Only natural selection consistently results in adaptive evolution

66. A Closer Look at Natural Selection
Natural selection brings about adaptive evolution by acting on an organism’s phenotype

67. 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

68. 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

69. 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

70. Frequency of individuals
Fig
Original population
Frequency of individuals
Phenotypes (fur color)
Original
population
Evolved
population
Figure Modes of selection
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection

71. Frequency of individuals
Fig a
Original population
Frequency of individuals
Phenotypes (fur color)
Figure Modes of selection
Original population
Evolved population
(a) Directional selection

72. Frequency of individuals
Fig b
Original population
Frequency of individuals
Phenotypes (fur color)
Figure Modes of selection
Evolved population
(b) Disruptive selection

73. Frequency of individuals
Fig c
Original population
Frequency of individuals
Phenotypes (fur color)
Figure Modes of selection
Evolved population
(c) Stabilizing selection

74. 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

75. Fig
(a) Color-changing ability in cuttlefish
Movable bones
Figure Examples of adaptations
(b) Movable jaw
bones in
snakes

76. (a) Color-changing ability in cuttlefish
Fig a
Figure Examples of adaptations
(a) Color-changing ability in cuttlefish

77. Movable bones (b) Movable jaw bones in snakes Fig. 23-14b
Figure Examples of adaptations
(b) Movable jaw
bones in
snakes

78. 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

79. 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

80. Fig
Figure Sexual dimorphism and sexual selection

81. 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

82. 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

83. Larval growth NSD LC better Larval survival LC better NSD
Fig
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of
SC father
Offspring of
LC father
Fitness of these half-sibling offspring compared
RESULTS
Figure Do females select mates based on traits indicative of “good genes”?
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.

84. SC sperm  Eggs  LC sperm
Fig a
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Figure Do females select mates based on traits indicative of “good genes”?
Offspring of
SC father
Offspring of
LC father
Fitness of these half-sibling offspring compared

85. Larval growth LC better Larval survival LC better NSD
Fig b
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
Figure Do females select mates based on traits indicative of “good genes”?
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.

86. The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population

87. Diploidy
Diploidy maintains genetic variation in the form of hidden recessive alleles

88. Balancing Selection
Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

89. 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

90. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Fig
Frequencies of the
sickle-cell allele
0–2.5%
Figure Mapping malaria and the sickle-cell allele
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
7.5–10.0%
10.0–12.5%
>12.5%

91. 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

92. “left-mouthed” individuals
Fig
“Right-mouthed”
1.0
“Left-mouthed”
“left-mouthed” individuals
Frequency of
0.5
Figure Frequency-dependent selection in scale-eating fish (Perissodus microlepis)
1981
’82
’83
’84
’85
’86
’87
’88
’89
’90
Sample year

93. “Right-mouthed” “Left-mouthed” Fig. 23-18a
Figure Frequency-dependent selection in scale-eating fish (Perissodus microlepis)
“Left-mouthed”

94. “left-mouthed” individuals
Fig b
1.0
“left-mouthed” individuals
Frequency of
0.5
Figure Frequency-dependent selection in scale-eating fish (Perissodus microlepis)
1981
’82
’83
’84
’85
’86
’87
’88
’89
’90
Sample year

95. 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

96. 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

97. Fig
Figure Evolutionary compromise

98. Original population Evolved population Directional selection
Fig. 23-UN1
Original
population
Evolved
population
Directional
selection
Disruptive
selection
Stabilizing
selection

99. Salinity increases toward the open ocean
Fig. 23-UN2
Sampling sites
(1–8 represent
pairs of sites) 1 2 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 7 8 5 6 4 3
Long Island
Sound 2 1 9 N 10
Atlantic
Ocean W E 11 S

100. Fig. 23-UN3

101. 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

102. 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

103. 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