1. Evolutionary Change is Random
AP BIOLOGY
BIG IDEA #1 – Part A – Section 3
Evolutionary Change is Random

2. 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

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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

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

11. 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

12. 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)

13. 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)

14. 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)

15. 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%

16. 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

17. 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

18. Fig
Figure Gene flow and human evolution

19. Windblown pollen moves these alleles between populations
Examples of Gene flow that 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

20. 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)

21. 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)

22. Fig b
Figure Gene flow and selection

23. Examples of Gene flow that 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

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

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

26. 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

27. 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

28. 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

29. 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

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

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

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

33. 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

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

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

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

37. 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

38. 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

39. Fig
Figure Sexual dimorphism and sexual selection

40. 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

41. 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

42. 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.

43. 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

44. 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.

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

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

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

48. 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

49. 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%

50. 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

51. “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

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

53. “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

54. Variation in noncoding regions of DNA
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

55. 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

56. Fig
Figure Evolutionary compromise

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

58. 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

59. Fig. 23-UN3

60. 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

61. 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

62. 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