1. The Evolution of Populations21
The Evolution of Populations 1

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

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. 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
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

6. The Founder Effect (Gen. Drift)
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 6

7. Example of The Founder Effect
The Pennsylvania Amish: In the 1700s, a small group of Europeans settled in Eastern Pennsylvania. Among this small group was an individual who carried an allele for Ellis-van Creveld syndrome, which is a very rare form of dwarfism, causing short stature, extra fingers (known as polydactyly), abnormal teeth and nails, and heart defects. The allele for Ellis- van Creveld syndrome is found at a frequency of 7% in the Pennsylvania Amish in comparison to only 0.1% in the general population. 7

8. The Bottleneck Effect (Gen. Drift)
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 8

9. Example of The Bottleneck Effect
Cheetahs have been through 2 bottlenecks in recent history:
Ice age (ended about 12,000 years ago)
Overhunting
Living cheetahs are as identically similar to each other as identical twins 9

10. 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 10

11. Gene Flow
Gene flow consists of the movement of alleles among populations (migration)
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 11

12. 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 12

13. 13 Central population NORTH SEA Eastern population Vlieland,
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 20 10
Females born in
central population
Females born in
eastern population 13

14. 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

15. It is often hard to predict the effects of gene flow
Gene flow is an important agent of evolutionary change in modern human populations 15

16. 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 16

17. Natural Selection: A Closer Look
Natural selection brings about adaptive evolution by acting on an organism’s phenotype 17

18. 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 indirectly favors certain genotypes by acting directly on phenotypes 18

19. Relative Fitness of Mules is ZERO
Sterile 19

20. 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 20

21. 21 Original population Frequency of individuals Original population
Figure 21.13
Original
population
Frequency of
individuals
Original
population
Evolved
population
Phenotypes (fur color)
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection 21

22. The Key Role of Natural Selection in Adaptive Evolution
Striking adaptations have arisen by natural selection
Examples:
certain octopuses can change color rapidly for camouflage
the jaws of snakes allow them to swallow prey larger than their heads 22

23. 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

24. 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 24

25. 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 25

26. 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 26

27. 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 27

28. 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 28

29. 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 29

30. 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

31. 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
Example: the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance 31

32. 32 Key Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5%
Figure 21.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
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% 32

33. 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
Example, frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale- eating fish 33

34. “left-mouthed” individuals
Figure 21.18
“Left-mouthed”
P. microlepis
1.0
“Right-mouthed”
P. microlepis
“left-mouthed” individuals
Frequency of
0.5
1981
’83
’85
’87
’89
Sample year 34

35. 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 35