1. The Evolution of Populations21
The Evolution of Populations

2. Overview: The Smallest Unit of Evolution
Natural selection acts on individuals, but only populations evolve 2

3. Figure 21.1
Figure 21.1 Is this finch evolving? 3

4. 1976 (similar to the prior 3 years) 1978 (after drought)
Figure 21.2 10 9
Average beak depth (mm) 8
Figure 21.2 Evidence of selection by food source
1976
(similar to the
prior 3 years)
1978
(after
drought) 4

5. Only natural selection causes adaptive evolution
Microevolution …a change in allele frequencies in a population over generations
3 mechanisms
Natural selection
Genetic drift
Gene flow
Only natural selection causes adaptive evolution 5

6. Figure 21.3
Figure 21.3 Phenotypic variation in horses 6

7. Genetic Variation
Genetic variation can be measured at whole gene level as gene variability
Gene variability can be quantified as the average % of loci that are heterozygous
Natural selection only acts on phenotypic variations with genetic component 7

8. (a) Caterpillars raised on a diet of oak flowers
Figure 21.5
Figure 21.5 Nonheritable variation
(a) Caterpillars raised on a diet of
oak flowers
(b) Caterpillars raised on a diet of
oak leaves 8

9. Altering Gene Number or Position
Duplicated genes can take on new functions by further mutation
An ancestral odor-detecting gene has been duplicated many times: Humans have 350 functional copies of the gene; mice have 1,000 9

10. Concept 21.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
population ya localized group of individuals capable of interbreeding and producing fertile offspring
A gene pool yall the alleles for all loci in a population
An allele for a particular locus is fixed if all individuals in a population are homozygous for the same allele 10

11. Porcupine herd Porcupine herd range Fortymile herd range
Figure 21.6
Porcupine herd
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Fortymile
herd range
Figure 21.6 One species, two populations
YUKON
ALASKA
Fortymile herd 11

12. The Hardy-Weinberg; Calculating Allele Frequencies
For diploid organisms, the total number of alleles at a locus is the number of individuals times 2 12

13. Conditions for Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes a population that is not evolving - allele frequencies will not change
In real populations, allele and genotype frequencies do change over time 13

14. The five conditions for nonevolving populations are rarely met in nature
No mutations
Random mating
No natural selection
large population size
No gene flow 14

15. Natural populations can evolve at some loci while being in Hardy-Weinberg equilibrium at other loci15

16. The Hardy-Weinberg equation
p2  2pq  q2  1
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype 16

17. Factors that change allele frequencies
Natural selection
Genetic drift
Gene flow 17

18. Genetic Drift
Genetic drift random changes in allele frequencies especially in small populations
Animation: Causes of Evolutionary Changes
Animation: Mechanisms of Evolution 18

19. 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 19

20. 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 20

21. 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 21

22. The Founder Effect
founder effect - when a few individuals become isolated from a larger population
Allele frequencies in founder population can be different from original population 22

23. The Bottleneck Effect
bottleneck effect results from a drastic reduction in population size 23

24. 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) 24

25. (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. 25

26. 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. 26

27. 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. 27

28. 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 28

29. 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) 29

30. 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) 30

31. Researchers using DNA from museum specimens showed a loss of alleles at several loci after the bottleneck
Introduction of greater prairie chickens from populations in other states introduced new alleles and increasing hatch rate to 90% 31

32. Gene Flow Gene flow - movement of alleles among populations
reduces genetic variation between populations 32

33. Concept 21.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Only natural selection consistently results in adaptive evolution, (increase in frequency of alleles that improve fitness) 33

34. Directional, Disruptive, and Stabilizing Selection
3 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 34

35. 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 35

36. Directional Selection
Fig. 18-6a1, p.288

37. Directional Selection
Fig. 18-6a2, p.288

38. Directional Selection
Fig. 18-6b1, p.288

39. Directional Selection
Fig. 18-6b2, p.288

40. Directional Selection
Fig. 18-7a, p.289

41. Directional Selection
Fig. 18-7b-e, p.289

42. Bones shown in green are movable. Ligament Figure 21.14
Figure Movable jaw bones in snakes 42

43. Figure 21.14a
Figure 21.14a Movable jaw bones in snakes (photo) 43

44. Sexual Selection
Sexual selection is natural selection for mating success
can result in sexual dimorphism, marked differences between the sexes 44

45. Sexual Selection
Fig , p.292

46. Sexual Selection

47. Figure 21.15
Figure Sexual dimorphism and sexual selection 47

48. The Preservation of Genetic Variation
Neutral variation -genetic variation giving no selective advantage or disadvantage 48

49. Balancing Selection
Balancing selection - natural selection maintains stable frequencies of two or more phenotypes in a population
Balancing selection includes
Heterozygote advantage
Frequency-dependent selection 49

50. example, sickle-cell allele
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
example, sickle-cell allele 50

51. 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% 51

52. Frequency-dependent selection occurs when the advantage of a phenotype declines if it becomes too common
example, frequency-dependent selection selects for equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish 52

53. “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 53

54. Figure 21.19
Figure Evolutionary compromise 54

55. Adaptation to What?
Fig a, p.297

56. Adaptation to What?
Fig b, p.297

57. Fig , p.299

58. Physician during the Plague