1. Objective 7:
TSWBAT discuss the factors that alter allele frequencies in a population.

2. Concept 21.3: 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 2

3. Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture 3

4. Genetic Drift
The smaller a sample, the more likely it is that chance alone will cause 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, especially in small populations 4

5. Animation: Causes of Evolutionary Changes
Right click slide / Select play

6. Video: Mechanisms of Evolution

7. p (frequency of CR)  0.7 q (frequency of CW)  0.3
Figure
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCW
Figure Genetic drift (step 1)
CRCR
CRCW
Generation 1
p (frequency of CR)  0.7
q (frequency of CW)  0.3 7

8. p (frequency of CR)  0.7 q (frequency of CW)  0.3 p  0.5 q  0.5
Figure
CRCR
CRCR
CWCW
CRCR
CRCW
5 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CWCW
CRCW
CRCW
CRCR
CRCW
CWCW
CRCR
Figure Genetic drift (step 2)
CRCR
CRCW
CRCW
CRCW
Generation 1
Generation 2
p (frequency of CR)  0.7
q (frequency of CW)  0.3
p  0.5
q  0.5 8

9. Generation 1 Generation 2 Generation 3 p (frequency of CR)  0.7
Figure
CRCR
CRCR
CWCW
CRCR
CRCR
CRCW
5 plants
leave
offspring
CRCW
2 plants
leave
offspring
CRCR
CRCR
CWCW
CRCR
CRCR
CWCW
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CWCW
CRCR
CRCR
Figure Genetic drift (step 3)
CRCR
CRCW
CRCW
CRCW
CRCR
CRCR
Generation 1
Generation 2
Generation 3
p (frequency of CR)  0.7
q (frequency of CW)  0.3
p  0.5
q  0.5
p  1.0
q  0.0 9

10. 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 due to chance 10

11. The Bottleneck Effect
The bottleneck effect can result from a drastic reduction in population size due to a sudden environmental change
By chance, 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 11

12. Original population Bottlenecking event Surviving population
Figure 21.10
Original
population
Bottlenecking
event
Surviving
population
(a) By chance, blue marbles are overrepresented in
the surviving population.
Figure The bottleneck effect
(b) Florida panther (Puma concolor coryi) 12

13. (a) By chance, blue marbles are overrepresented in the
Figure 21.10a-1
Figure 21.10a-1 The bottleneck effect (part 1, step 1)
Original
population
(a) By chance, blue marbles are overrepresented in the
surviving population. 13

14. Original population Bottlenecking event
Figure 21.10a-2
Figure 21.10a-2 The bottleneck effect (part 1, step 2)
Original
population
Bottlenecking
event
(a) By chance, blue marbles are overrepresented in the
surviving population. 14

15. Original population Bottlenecking event Surviving population
Figure 21.10a-3
Figure 21.10a-3 The bottleneck effect (part 1, step 3)
Original
population
Bottlenecking
event
Surviving
population
(a) By chance, blue marbles are overrepresented in the
surviving population. 15

16. (b) Florida panther (Puma concolor coryi)
Figure 21.10b
Figure 21.10b The bottleneck effect (part 2: photo)
(b) Florida panther (Puma concolor coryi) 16

17. Understanding the bottleneck effect can increase understanding of how human activity affects other species 17

18. 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 18

19. Greater prairie chicken
Figure 21.11a
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken
Figure 21.11a Genetic drift and loss of genetic variation (part 1: map)
(a) 19

20. Number of alleles per locus Percentage of eggs hatched Population size
Figure 21.11b
Number
of alleles
per locus
Percentage
of eggs
hatched
Population
size
Location
Illinois
1930–1960s
1993
1,000–25,000
50
5.2
3.7 93
50
Kansas, 1998
(no bottleneck)
750,000
5.8 99
Figure 21.11b Genetic drift and loss of genetic variation (part 2: table)
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8 96
(b) 20

21. Greater prairie chicken
Figure 21.11c
Greater prairie chicken
Figure 21.11c Genetic drift and loss of genetic variation (part 3: photo) 21

22. The results showed a loss of alleles at several loci
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 populations in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90% 22

23. Effects of Genetic Drift: A Summary
Genetic drift is significant in small populations
Genetic drift can cause 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 23

24. 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 genetic variation among populations over time 24

25. Gene flow can decrease the fitness of a population
Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland
Immigration of birds from the mainland introduces alleles that decrease fitness in island populations
Natural selection reduces the frequency of these alleles in the eastern population where immigration from the mainland is low
In the central population, high immigration from the mainland overwhelms the effects of selection 25

26. Females born in central population Females born in eastern population
Figure 21.12
Central
population
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
2 km
Population in which the
surviving females
eventually bred 60
Parus major 50
Central
Eastern 40
Survival rate (%) 30
Figure Gene flow and local adaptation 20 10
Females born in
central population
Females born in
eastern population 26

27. Females born in central population Females born in eastern population
Figure 21.12a
Population in which the
surviving females
eventually bred 60
Central 50
Eastern 40
Survival rate (%) 30 20
Figure 21.12a Gene flow and local adaptation (part 1: graph) 10
Females born in
central population
Females born in
eastern population 27

28. Figure 21.12b
Figure 21.12b Gene flow and local adaptation (part 2: photo)
Parus major 28

29. Gene flow can 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 other diseases
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 29

30. Gene flow is an important agent of evolutionary change in modern human populations30

31. Concept 21.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Evolution by natural selection involves both chance and “sorting”
New genetic variations arise by chance
Beneficial alleles are “sorted” and favored by natural selection
Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness 31

32. Natural Selection: A Closer Look
Natural selection brings about adaptive evolution by acting on an organism’s phenotype 32

33. 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 33

34. Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Selection indirectly favors certain genotypes by acting directly on phenotypes 34

35. Directional, Disruptive, and Stabilizing Selection
There are three modes of natural 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 35

36. Frequency of individuals
Figure 21.13
Original
population
Frequency of
individuals
Original
population
Evolved
population
Phenotypes (fur color)
Figure Modes of selection
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection 36

37. The Key Role of Natural Selection in Adaptive Evolution
Striking adaptations have arisen by natural selection
For example, certain octopuses can change color rapidly for camouflage
For example, the jaws of snakes allow them to swallow prey larger than their heads 37

38. Bones shown in green are movable. Ligament Figure 21.14
Figure Movable jaw bones in snakes 38

39. 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, dynamic process 39

40. 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 40

41. 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 41

42. Figure 21.15
Figure Sexual dimorphism and sexual selection 42

43. 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 43

44. How do female preferences evolve?
The “good genes” hypothesis suggests that if a trait is related to male genetic quality or health, both the male trait and female preference for that trait should increase in frequency 44

45. The Preservation of Genetic Variation
Neutral variation is genetic variation that does not confer a selective advantage or disadvantage
Various mechanisms help to preserve genetic variation in a population 45

46. Diploidy
Diploidy maintains genetic variation in the form of hidden recessive alleles
Heterozygotes can carry recessive alleles that are hidden from the effects of selection 46

47. 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 47

48. 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
For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance 48

49. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Figure 21.17
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% 49

50. Selection can favor whichever phenotype is less common in a population
Frequency-dependent selection occurs when 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 50

51. “left-mouthed” individuals
Figure 21.18
“Left-mouthed”
P. microlepis
1.0
“Right-mouthed”
P. microlepis
“left-mouthed” individuals
Frequency of
0.5
Figure Frequency-dependent selection
1981
’83
’85
’87
’89
Sample year 51

52. 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 52

53. Figure 21.19
Figure Evolutionary compromise 53

54. Original population Evolved population Directional selection
Figure 21.UN04
Original
population
Evolved
population
Figure 21.UN04 Summary of key concepts: modes of selection
Directional
selection
Disruptive
selection
Stabilizing
selection 54