1. The Evolution of Populations21
The Evolution of Populations

2. The Smallest Unit of Evolution- Microevolution
One common misconception is that organisms evolve during their lifetimes
Natural selection acts on individuals, but only populations evolve
Consider, for example, a population of medium ground finches on Daphne Major Island
During a drought, large-beaked birds were more likely to crack large seeds and survive.
The finch population evolved by natural selection. 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. The Smallest Unit of Evolution- Microevolution
Microevolution is a change in allele frequencies in a population over generations
Three mechanisms cause allele frequency change
Natural selection
Genetic drift (alteration of allele frequencies)
Gene flow (transfer of alleles between populations)
Only natural selection causes adaptive evolution 5

6. Concept 21.1: Genetic variation makes evolution possible
Variation in heritable traits is a prerequisite for evolution
Genetic variation among individuals is caused by differences in genes or other DNA sequences
Genetic variation observed in phenotype variation
Mendel’s work on pea plants provided evidence of discrete heritable units (genes)
Single gene differences
Two or more gene differences for a trait 6

7. Figure 21.3
Figure 21.3 Phenotypic variation in horses 7

8. Genetic variability
Genetic variation can be measured at the whole gene level as gene variability
Gene variability can be quantified as the average percent of loci that are heterozygous Ex. Drosophila is heterozygous for 14% of loci.
Genetic variation can be measured at the molecular level of DNA as nucleotide variability
rarely results in phenotypic variation ‘cos in noncoding (introns) regions
Variations in coding regions (exons) rarely change the amino acid sequence of the encoded protein 8

9. DNA sequence of Adh gene in various Drosophila
Base-pair
substitutions
Insertion sites 1
500
1,000
Exon
Intron
Substitution resulting
in translation of
different amino acid
26 base pair
Deletion
Figure 21.4 Extensive genetic variation at the molecular level
1,500
2,000
2,500 9

10. Phenotype is the product of inherited genotype and environmental influences
Natural selection can only act on phenotypic variation that has a genetic component 10

11. Effect of environment on caterpillars of a moth Nemoria
Figure 21.5 Nonheritable variation
(a) Caterpillars raised on a diet of
oak flowers
(b) Caterpillars raised on a diet of
oak leaves 11

12. Sources of Genetic Variation - Mutation
The effects of point mutations can vary
Mutations in noncoding regions of DNA are often harmless
Redundancy of genetic code does not change protein sequence
Mutations that alter the phenotype are often harmful
Mutations that result in a change in protein production can sometimes be beneficial 12

13. Sources of Genetic Variation - Altering Gene Number or Position
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Duplication of small pieces of DNA increases genome size and is usually less harmful (meiosis errors)
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 13

14. Sources of Genetic Variation - Rapid Reproduction
Mutation rates are low in animals and plants
The average is about one mutation in every 100,000 genes per generation
Mutation rates are often lower in prokaryotes and higher in viruses
Short generation times allow mutations to accumulate rapidly in prokaryotes and viruses 14

15. Sources of Genetic Variation - Sexual Reproduction
In organisms that reproduce sexually, most genetic variation results from recombination of alleles
Sexual reproduction can shuffle existing alleles into new combinations through three mechanisms: crossing over, independent assortment, and fertilization 15

16. How can we tell if a population is evolving?
The Hardy-Weinberg equation can be used.
When the gene pool is not evolving – they are in Hardy- Weinberg equilibrium.
Frequencies of alleles and genotypes in a population will remain constant from generation to generation i.e. combination of crosses of all individuals within a population 16

17. Some definitions:-
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
An allele for a particular locus is fixed if all individuals in a population are homozygous for the same allele 17

18. One species two populations:
Porcupine herd
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
One species two populations:
Fortymile
herd range
Figure 21.6 One species, two populations
YUKON
ALASKA
Fortymile herd 18

19. The frequency of an allele in a population can be calculated
For diploid organisms, the total number of alleles at a locus is the total number of individuals times 2
The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles 19

20. 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 20

21. How to calculate allele frequencies:
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 21

22. How to calculate allele frequencies:
To calculate the frequency of each allele
p  freq CR  800 / (800  200)  0.8 (80%)
q  1  p  0.2 (20%)
The sum of alleles is always 1
0.8  0.2  1 22

23. The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving 23

24. Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a population 24

25. 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 25

26. 80% chance 20% chance 80% chance 20% chance
Figure 21.7
Frequencies of alleles
p  frequency of CR allele
 0.8
q  frequency of CW allele
 0.2
Alleles in the population
Figure 21.7 Selecting alleles at random from a gene pool
Gametes produced
Each egg:
Each sperm:
80%
chance
20%
chance
80%
chance
20%
chance 26

27. 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 27

28. CR CW CR CW   Figure 21.8 80% CR (p  0.8) 20% CW (q  0.2) Sperm
CRCR
0.16 (pq)
CRCW
Eggs CW
0.16 (qp)
CRCW
0.04 (q2)
CWCW
q  0.2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
Figure 21.8 The Hardy-Weinberg principle
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
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants 28

29. 0.64 (p2) CRCR 0.16 (pq) CRCW 0.16 (qp) CRCW 0.04 (q2) CWCW
Figure 21.8a
80% CR (p  0.8)
20% CW (q  0.2)
Sperm CR
p  0.8 CW
q  0.2 CR
p  0.8
0.64 (p2)
CRCR
0.16 (pq)
CRCW
Figure 21.8a The Hardy-Weinberg principle (part 1: Punnett square)
Eggs CW
0.16 (qp)
CRCW
0.04 (q2)
CWCW
q  0.2 29

30. Gametes of this generation:
Figure 21.8b
64% CRCR, 32% CRCW, and 4% CWCW
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
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
Figure 21.8b The Hardy-Weinberg principle (part 2: equations)
64% CRCR, 32% CRCW, and 4% CWCW plants 30

31. 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 31

32. Figure 21.UN02
Figure 21.UN02 In-text figure, Hardy-Weinberg equation, p. 405 32

33. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population that is not evolving
In real populations, allele and genotype frequencies do change over time 33

34. Extremely large population size No gene flow
The five conditions for nonevolving populations are rarely met in nature
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow 34

35. Natural populations can evolve at some loci while being in Hardy-Weinberg equilibrium at other loci
Some populations evolve slowly enough that evolution cannot be detected 35

36. Applying the Hardy-Weinberg Principle
We can assume the locus that causes phenylketonuria (PKU) a recessive trait, is in Hardy-Weinberg equilibrium in USA, 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 36

37. 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 37

38. 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 38

39. 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 39

40. 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 40

41. 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 41

42. 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 42

43. 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 43

44. Genetic Drift - 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
Amish and polydactyly
Huntington’s disease in Dutch settlers of South Africa 44

45. Genetic Drift - 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
Northern elephant seals due to excessive hunting
Florida panther
Humans 70,000 years ago – Toba erruption 45

46. 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) 46

47. Greater prairie chicken
Figure 21.11
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken
(a)
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
Figure Genetic drift and loss of genetic variation
Kansas, 1998
(no bottleneck)
750,000
5.8 99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8 96
(b) 47

48. 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 48

49. 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 49

50. Results of gene flow:
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 50

51. 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 51

52. Results of gene flow:
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 52

53. 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 53

54. Natural Selection: A Closer Look54

55. Natural Selection: A Closer Look
Natural selection brings about adaptive evolution by acting on an organism’s phenotype
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 55

56. 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 56

57. 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 57

58. 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 58

59. 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 59

60. Bones shown in green are movable. Ligament Figure 21.14
Figure Movable jaw bones in snakes 60

61. 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 61

62. Figure 21.15
Figure Sexual dimorphism and sexual selection 62

63. 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 63

64. 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 64

65. 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% 65

66. 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 66

67. “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 67

68. 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 68