1. Why did this happen?

2. The Evolution of Populations
Chapter 23
The Evolution of Populations

3. Did this bird evolve in its lifetime?
Figure 23.1 Is this finch evolving?

4. Average beak depth (mm)
Evidence of selection by food source. Microevolution is a change in allele frequencies in a population over generations 10 9
Average beak depth (mm) 8
Figure 23.2 Evidence of selection by food source.
1976
(similar to the
prior 3 years)
1978
(after
drought)

5. Three mechanisms cause allele frequency change:
Natural selection
Genetic drift
Gene flow
Only natural selection causes adaptive evolution
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6. Concept 23.1: BIG PICTURE: Genetic variation makes evolution possible
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7. Nonheritable variation of Nemoria moths: different phenotypes due to dietary chemicals Natural selection can only act on variation with a genetic component
(a)
(b)
Figure 23.3 Nonheritable variation.

8. 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
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9. Variation Between Populations
Most species exhibit geographic variation, differences between gene pools of separate populations
For example, Madeira is home to several isolated populations of mice
Chromosomal variation among populations is due to drift, not natural selection
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10. Figure 23.4 1
2.4
3.14
5.18 6
7.15
8.11
9.12
10.16
13.17 19 XX
Figure 23.4 Geographic variation in isolated mouse populations on Madeira. 1
2.19
3.8
4.16
5.14
6.7
9.10
11.12
13.17
15.18 XX

11. HOW DID THIS COME TO BE? 1.0 0.8 0.6 Ldh-Bb allele frequency 0.4 0.2
mummichog fish vary in a cold-adaptive allele along a temperature gradient
1.0
0.8
0.6
Ldh-Bb allele frequency
0.4
0.2
Figure 23.5 A cline determined by temperature. 46 44 42 40 38 36 34 32 30
Latitude (ºN)
Maine
Cold (6°C)
Georgia
Warm (21ºC)
HOW DID THIS COME TO BE?

12. Sources of Genetic Variation
New genes and alleles can arise by mutation or gene duplication
The average is about one mutation in every 100,000 genes per generation
An ancestral odor-detecting gene has been duplicated many times: humans have 1,000 copies of the gene, mice have 1,300
What is the consequence?
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13. Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Three major factors alter allele frequencies and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow
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14. Genetic Drift
Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next due to chance events
The smaller a sample, the greater the chance of deviation from a predicted result
genetic drift tends to reduce genetic variation through losses of alleles
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15. Animation: Causes of Evolutionary Change
Right-click slide / select “Play”
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16. Example of Genetic drift
Figure
Example of Genetic drift
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCW
Figure 23.9 Genetic drift.
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

17. Example of Genetic drift
Figure
Example of Genetic drift 5
plants
leave
off-
spring
CRCR
CRCR
CWCW
CRCR
CRCW
CRCW
CWCW
CRCR
CRCR
CWCW
CRCW
CRCW
CRCR
CRCW
CWCW
CRCR
Figure 23.9 Genetic drift.
CRCR
CRCW
CRCW
CRCW
Generation 1
Generation 2
p (frequency of CR) = 0.7
p = 0.5
q (frequency of CW) = 0.3
q = 0.5

18. Example of Genetic drift5
plants
leave
off-
spring 2
plants
leave
off-
spring
CRCR
CRCR
CWCW
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CWCW
CRCR
CRCR
CWCW
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CWCW
CRCR
CRCR
Figure 23.9 Genetic drift.
CRCR
CRCW
CRCW
CRCW
CRCR
CRCR
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

19. The Founder Effect
The founder effect occurs when a few individuals become isolated from a larger population
 Allele frequencies in the small founder population can be different from those in the larger parent population
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20. Original population Bottleneck effect
Figure The bottleneck effect.
Original
population

21. Original population Bottlenecking event
Figure
Figure The bottleneck effect.
Original
population
Bottlenecking
event

22. Original population Bottlenecking event Surviving population
The bottleneck effect is a sudden reduction in population size due to a change in the environment If the population remains small, it may be further affected by genetic drift
Figure The bottleneck effect.
Original
population
Bottlenecking
event
Surviving
population

23. How is the bottleneck effect seen in humans affecting wild organism populations?
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24. 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
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25. Gene Flow
Gene flow consists of the movement of alleles amongst populations
Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
Gene flow tends to reduce variation among populations over time
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26. Gene flow can decrease the fitness of a population
Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland
Mating causes gene flow between the central and eastern populations
Immigration from the mainland introduces alleles that decrease fitness
Natural selection selects for alleles that increase fitness
Birds in the central region with high immigration have a lower fitness; birds in the east with low immigration have a higher fitness
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27. Gene flow and local adaptation
Population in which the
surviving females
eventually bred 60
Central
population
Central 50
NORTH SEA
Eastern
population
Eastern
Vlieland,
the Netherlands 40
2 km
Survival rate (%) 30 20 10
Figure Gene flow and local adaptation.
Females born
in central
population
Females born
in eastern
population
Parus major

28. Gene flow can also increase the fitness of a population
Consider, for example, the spread of alleles for resistance to insecticides
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
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29. Bioflix: Selection within hypothetical beetle population

30. Come up with a new example of:
Natural selection
Gene flow
Genetic drift

31. Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Evolution by natural selection involves both chance and “sorting”
New genetic variations (mutations) arise by chance
Beneficial alleles are “sorted” and favored by natural selection
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32. Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Typically individuals are NOT in direct competition
Selection favors certain genotypes by acting on the phenotypes of certain organisms
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33. Directional, Disruptive, and Stabilizing Selection
Three modes of selection:
Directional selection favors individuals at one end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes
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34. Frequency of individuals
Figure 23.13
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

35. Striking adaptations have arisen by natural selection
Bones shown in
green are movable.
Ligament
Striking adaptations have arisen by natural selection
Figure Movable jaw bones in snakes.

36. Evolution over time, process and outcomes
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
Because the environment can change, adaptive evolution is a continuous process
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37. Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment
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38. Why are males prettier than females?
Figure Sexual dimorphism and sexual selection.

39. Why are males prettier than females?
Figure Sexual dimorphism and sexual selection.

40. Why are males prettier than females?
Figure Sexual dimorphism and sexual selection.

41. Why are males prettier than females?
Figure Sexual dimorphism and sexual selection.

42. Why are males prettier than females?
Figure Sexual dimorphism and sexual selection.

43. Why are males prettier than females? outcome is sexual dimorphism
Figure Sexual dimorphism and sexual selection.

44. Intersexual selection, often called mate choice
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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45. vs. intrasexual selection
Competition within gender for mates

46. What is so special about him?
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47. Do all animal species care about looks?

48. Offspring Performance 1995 1996
Figure 23.16
EXPERIMENT
Recording of SC
male’s call
Recording of LC
male’s call
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
Survival and growth of these half-sibling offspring compared
Figure Inquiry: Do females select mates based on traits indicative of “good genes”?
RESULTS
Offspring Performance
1995
1996
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.

49. Diploidy
What’s the benefit?
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50. Diploidy
Diploidy maintains genetic variation in the form of hidden recessive alleles
by carrying recessive alleles that are hidden from the effects of selection
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51. Balancing Selection
Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
Balancing selection includes
Heterozygote advantage
Frequency-dependent selection
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52. Heterozygote Advantage
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
Natural selection will tend to maintain two or more alleles at that locus
The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance
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53. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Figure 23.17
Key
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%

54. 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
For example, frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish
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55. “left-mouthed” individuals
Figure 23.18
“Left-mouthed”
P. microlepis
1.0
“Right-mouthed”
P. microlepis
“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

56. Why Natural Selection Cannot Fashion Perfect Organisms
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57. 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
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58. Evolutionary compromise…for the frog
Figure Evolutionary compromise.

59. Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population
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60. 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
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61. The Hardy-Weinberg Principle
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation, ∴ not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving
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62. Extremely large population size No gene flow
The five conditions for nonevolving populations are rarely met in nature, and include:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow
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63. Beaufort Sea Porcupine herd range Porcupine herd Fortymile herd range
Figure 23.6
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Porcupine herd
Fortymile
herd range
Figure 23.6 One species, two populations.
ALASKA
YUKON
Fortymile herd

64. 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
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65. Calculate the number of copies of each allele:
For example, consider a population of wildflowers that is incompletely dominant for color:
320 red flowers (CRCR)
160 pink flowers (CRCW)
20 white flowers (CWCW)
Calculate the number of copies of each allele:
CR  (320  2)  160  800
CW  (20  2)  160  200
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66. To calculate the frequency of each allele:
p  freq CR  800 / (800  200)  0.8
q  freq CW  200 / (800  200)  0.2
The sum of alleles is always 1
0.8  0.2  1
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67. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
Consider, for example, the same population of 500 wildflowers and 1,000 alleles where
p  freq CR  0.8
q  freq CW  0.2
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68. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
p2  2pq  q2  1
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
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69. The frequency of genotypes can be calculated
CRCR  p2  (0.8)2  0.64
CRCW  2pq  2(0.8)(0.2)  0.32
CWCW  q2  (0.2)2  0.04
The frequency of genotypes can be confirmed using a Punnett square
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70. Figure 23.8
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
64% (p2)
CRCR
16% (pq)
CRCW
Eggs CW
16% (qp)
CRCW
4% (q2)
CWCW
(20%)
64% CRCR, 32% CRCW, and 4% CWCW
Figure 23.8 The Hardy-Weinberg principle.
Gametes of this generation:
64% CR
(from CRCR plants)
16% CR
(from CRCW plants) + =
80% CR = 0.8 = p
4% CW
(from CWCW plants)
16% CW
(from CRCW plants) + =
20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants

71. 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
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72. 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  0.99  0.01 
or approximately 2% of the U.S. population
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73. Try this:
A population of mice displays the recessive trait for loving the banjo. A survey says it is about 20% of them.
What % dislike the banjo?
What is the frequency of the dominant and recessive allele?
What % is heterozygous for this trait?

74. What is the problem with dog breeding?

75. What is the problem with dog breeding?
Limit genetic variability
Preserve harmful mutations
Fitness decreased

76. Review quiz
What are the three mechanisms for altering allele frequency?
What is the type of selection reflecting the fact that really small wolves and really big wolves don’t survive as well as mediums?
Draw the curve.
In population of fruit flies, 70% of the gametes contain A1 alleles. If the pop in in H-W equilibrium, what proportion carry both A1 and A2? 0.09