Gen Bio II BIO102

1. The EvolutionThe Evolution
of Populationsof Populations
BIOLBIOL 102:102:
General Biology IIGeneral Biology II
ChapterChapter 2323
RobRob SwatskiSwatski
Associate ProfessorAssociate Professor of Biologyof Biology
HACCHACC--YorkYork

2. Overview ofOverview of
NaturalNatural
SelectionSelection
Natural selection
acts on individuals,
but only
populations evolve
Evolution occurs
through genetic
variations in
populations
Ex: Medium
ground finch &
beak size during
drought2 Medium ground finchMedium ground finch

3. MicroevolutionMicroevolution
Microevolution:
changes in a
population’s allele
frequencies over
generations
Three mechanisms
cause allele frequency
change:
1) Natural selection,
2) Genetic drift, 3)
Gene flow
3
1976
(similar to the
prior 3 years)
1978
(after
drought)
Averagebeakdepth(mm)
10
9
8
0
3

4. 4

5. 5

6. 6

7. AllelesAlleles
7

8. Two main sources ofTwo main sources of
gene pool variation:gene pool variation:
Mutation
Sexual
Reproduction
8

9. 9

10. 10
Harold and Maude (1971)Harold and Maude (1971)

11. GeneticGenetic
VariationVariation
Variation in
individual genotype
leads to variation in
individual phenotype
Natural selection can
only act on variation
with a genetic
component
Not all phenotypic
variation is heritable
11

12. Moth caterpillars raised on oak flower diet resemble oak flowers
NonheritableNonheritable VariationVariation
12

13. Moth caterpillar siblings raised on oak leaves resemble oak twigs13

14. Population variation is thePopulation variation is the
result of:result of:
Discrete
characters
Are classified
as “either/or”
Quantitative
characters
Vary along a
continuum within
a population
Phenotype is
often influenced
by 2 or more
genes 14

15. DiscreteDiscrete
CharactersCharacters
15

16. 16

17. Quantitative CharactersQuantitative Characters
17

18. GenotypesGenotypes
Homozygous
Individual having
2 of the same
alleles for a given
locus
Heterozygous
Individual having
2 different alleles
for a given locus
18

19. AverageAverage
HeterozygosityHeterozygosity
A measure of gene
variability
Measures the average
% of loci that are
heterozygous in a
population
19

20. NucleotideNucleotide
VariabilityVariability
Measured by comparing
the differences between
DNA sequences of pairs
of individuals
20

21. 21

22. DrosophilaDrosophila melanogastermelanogaster
Average
heterozygosity
13,700 genes
in genome
= 14% (1,920 loci)
Nucleotide
variability
180 million
nucleotides
in genome
= 1% (1.8 million)
22

23. GeographicGeographic
VariationVariation
Differences between gene
pools of separate
populations or population
subgroups
23

24. 13.17 19 XX10.169.128.11
1 2.4 3.14 5.18 6 7.15
9.10
1 2.19
11.12 13.17 15.18
3.8 4.16 5.14 6.7
XX
Geographic Variation in Isolated MouseGeographic Variation in Isolated Mouse
Populations on MadeiraPopulations on Madeira
Isolated populations have differences in
fused chromosomes 24
karyotypes

25. ClineCline
A graded change in a
trait along a
geographic axis
Ex: lactate
dehydrogenase
frequency is higher in
cold water (allows
faster swimming in
fish)
25

26. 1.0
0.8
0.6
0.4
0.2
0
46 44 42 40 38 36 34 32 30
Georgia
Warm (21°C)
Latitude (Latitude (°°N)N)
Maine
Cold (6°C)
Mummichog
26

27. MutationMutation
A change in the
nucleotide
sequence of DNA
Causes new genes
& alleles to arise
Only mutations in
gamete-producing
cells can be passed
to offspring
27

28. Point Mutation:Point Mutation: a change in 1 base in a gene
28

29. Effects ofEffects of
PointPoint
MutationsMutations
Mutations in noncoding
regions of DNA are often
harmless due to
redundancy
Mutations resulting in a
change in protein
production are often
harmful
Mutations may also be
beneficial & increase an
organism’s fit into its
environment
29

30. Types of MutationsTypes of Mutations
Deletions
More
harmful
Disruptions
More
harmful
Rearrangements
More
harmful
Duplication
Less
harmful
Genes can
take on new
functions
30

31. 31

32. MutationMutation
RatesRates
Low in animals &
plants: avg 1 mutation
in every 100,000 genes
per generation
Often higher in
prokaryotes & viruses
Prokaryotes & viruses
have short generation
times so mutations can
quickly produce
genetic variation
32

33. SexualSexual
ReproductionReproduction
Shuffles existing
alleles into new
combinations
Recombination
More important
than mutation in
producing genetic
differences …Why?
33

34. FlowerFlower
SymmetrySymmetry
inin
AntirrhinumAntirrhinum
SpeciesSpecies
34

35. Recombination During MeiosisRecombination During Meiosis
35

36. Gene PoolGene Pool: all the alleles for all loci in a population
36

37. Porcupine herd
PorcupinePorcupine
herdherd
rangerange
Beaufort Sea
MAP
AREA
FortymileFortymile
herdherd
rangerange
Fortymile herd
overlapoverlap
37

38. Calculate the Frequency of an Allele in aCalculate the Frequency of an Allele in a
Population:Population:
Total # of alleles at a locus = total # of individuals x 2
38

39. Total # of Dominant orTotal # of Dominant or
Recessive Alleles at a LocusRecessive Alleles at a Locus
2 alleles for each
homozygous
dominant or recessive
individual plus…
1 allele for each
heterozygous
individual
39

40. If there are 2 alleles at a locus, p & q are used to represent
their frequencies
The frequency of all alleles in a population will add up to 1
p + q = 1
p q
40

41. HardyHardy--
WeinbergWeinberg
PrinciplePrinciple
Describes a hypothetical
population that is not
evolving
In real populations,
allele & genotype
frequencies change
over time
If a population does not
meet H-W criteria, then
the population is
evolving
41

42. HardyHardy--
WeinbergWeinberg
EquilibriumEquilibrium
Allele & genotype
frequencies in a population
remain constant from
generation to generation
In a population where
gametes randomly
contribute to the next
generation, allele
frequencies will not change
Mendelian inheritance
preserves genetic variation in
a population
42

43. CRCR
CWCW
CRCW
43

44. Frequencies
of alleles
Alleles in theAlleles in the
populationpopulation
Gametes produced
Each
egg:
Each
sperm:
80%
chance
80%
chance
20%
chance
20%
chance
q = frequency of
p = frequency of
CR allele = 0.8
CW allele = 0.2
equilibrium
random
Selecting Alleles at Random from aSelecting Alleles at Random from a
Gene PoolGene Pool
44

45. p2 & q2 are the frequencies of the homozygous genotypes
2pq is the frequency of the heterozygous genotype
If p & q represent the relative frequencies of the
only two possible alleles in a population at a
particular locus, then:
45

46. SpermSperm
CR
(80%)
80% CR (p = 0.8)
CW
(20%)
20% CW (q = 0.2)
16% (pq)
CRCW
4% (q2)
CW CW
64% (p2)
CRCR
16% (qp)
CRCW
Parent
Generation:
F1:
46

47. Gametes of this generation:
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Genotypes in the next generation:
With random mating, these gametes will
result in the same mix of genotypes
64% CRCR, 32% CRCW, and 4% CWCWF1:
64% CRCR, 32% CRCW, and 4% CWCW plantsF2:
47

48. No mutations
Random
mating
No natural
selection
Extremely
large
population size
No gene flow
The 5 H-W conditions for nonevolving populations
are rarely met in nature:
48

49. 3 Major Factors of3 Major Factors of
Evolutionary ChangeEvolutionary Change
Natural
Selection
Genetic
Drift
Gene Flow
49

50. NaturalNatural
SelectionSelection
Differential
reproductive
success
Results in certain
alleles being
passed to the next
generation in
greater
proportions than
other alleles
50

51. GeneticGenetic
DriftDrift
Allele frequencies
can fluctuate
unpredictably from
one generation to
the next
Reduces genetic
variation through
loss of alleles
The smaller a
sample, the greater
the chance of
deviation from a
predicted result 51

52. 52

53. 53

54. Generation 1Generation 1
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Generation 2Generation 2
CR CWCR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
p = 0.5
q = 0.5
Generation 3Generation 3
p = 1.0
q = 0.0
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR CR CR
CR CR
CR CR CR CR
54

55. FounderFounder
EffectEffect
Occurs when a few
individuals become
isolated from a larger
population
Allele frequencies in the
small founder population
may differ from those in
the larger parent
population
Ex: Amish
55

56. PolydactylyPolydactyly 56

57. BottleneckBottleneck
EffectEffect
Occurs when population
size is reduced due to a
sudden change in the
environment
The resulting gene pool
may no longer reflect the
original population’s
gene pool
If the population remains
small, it may be further
affected by genetic drift
57

58. Original
population
Bottlenecking
event
Surviving
population
58

59. Genetic Drift &Genetic Drift &
the Greaterthe Greater
Prairie ChickenPrairie Chicken
Habitat loss caused a
severe reduction in the
population of greater
prairie chickens in Illinois
The surviving birds had
low levels of genetic
variation
Only 50% of their eggs
hatched
59

60. Range
of greater
prairie
chicken
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
60

61. Greater PrairieGreater Prairie
ChickenChicken
Research, cont.Research, cont.
DNA from museum
specimens used to
compare genetic
variation before & after
bottleneck
Results showed a loss of
alleles at several loci
Introduced prairie
chickens from other
states to increase gene
pool diversity
Successfully introduced
new alleles & increased
egg hatch rate to 90%
61

62. NumberNumber
of allelesof alleles
per locusper locus
Minnesota, 1998Minnesota, 1998
(no bottleneck)
Nebraska, 1998Nebraska, 1998
(no bottleneck)
Kansas, 1998Kansas, 1998
(no bottleneck)
IllinoisIllinois
1930–1960s
1993
LocationLocation
PopulationPopulation
sizesize
%%
of eggsof eggs
hatchedhatched
1,000–25,000
<50
750,000
75,000–
200,000
4,000
5.2
3.7
93
<50
5.8
5.8
5.3 85
96
99
62

63. Effects ofEffects of
GeneticGenetic
DriftDrift
Significant in
small populations
Causes allele
frequencies to
change at random
Can lead to a loss
of genetic
variation within
populations
May cause
harmful alleles to
become fixed
63

64. Gene FlowGene Flow
Movement of alleles
among populations
Transferred through
movement of fertile
individuals or gametes
Usually reduces
differences between
populations over time
More likely than
mutation to directly
alter allele
frequencies 64

65. 65

66. 66

67. Gene Flow &Gene Flow &
DecreasingDecreasing
FitnessFitness
Ex: Bent grass
Alleles for copper tolerance
are beneficial in populations
near copper mines, but
harmful to those in other
soils
Windblown pollen moves
alleles between populations
Movement of unfavorable
alleles into a population
decreases the fitness
between organism &
environment 67

68. NON-
MINE
SOIL
MINE
SOIL
NON-
MINE
SOIL
Prevailing wind direction
Indexofcoppertolerance
Distance from mine edge (meters)
70
60
50
40
30
20
10
0
20 0 20 0 20 40 60 80 100 120 140 160
68

69. Population in which the
surviving females
eventually bred
Central
Eastern
Survivalrate(%)Survivalrate(%)
Females born
in central
population
Females born
in eastern
population
Parus major
60
50
40
30
20
10
0
Central
population
NORTH SEA Eastern
population
Vlieland,
the Netherlands
2 km
69

70. Gene Flow &Gene Flow &
IncreasingIncreasing
FitnessFitness
Ex: Insecticide
resistance in mosquitoes
Insecticides have been
used to kill mosquitoes
that carry West Nile
virus & malaria
Alleles have evolved in
some mosquito
populations that confer
insecticide resistance
The flow of these
resistance alleles into a
population can increase
its fitness
70

71. 71

72. 72

73. 73

74. Why are the phrases
“survival of the fittest”
and
“struggle for existence”
misleading?
74

75. RelativeRelative
FitnessFitness
Reproductive success is
generally more subtle &
depends on many
factors
The contribution an
individual makes to the
gene pool of the next
generation…
…relative to the
contributions of other
individuals
75

76. 76

77. 3 Types of Selection3 Types of Selection
Directional Disruptive Stabilizing
77

78. Directional SelectionDirectional Selection
Favors individuals at
one extreme of the phenotypic range
Original population
Phenotypes (fur color)
Evolved population
78

79. 79

80. Disruptive SelectionDisruptive Selection
Favors individuals at
both extremes of the phenotypic range
Original population
Phenotypes (fur color)
Evolved population
80

81. 81

82. Stabilizing SelectionStabilizing Selection
Favors intermediate variants
& acts against extreme phenotypes
Original population
Phenotypes (fur color)
Evolved population
82

83. 83

84. NaturalNatural
Selection &Selection &
AdaptiveAdaptive
EvolutionEvolution
Natural selection increases
the frequencies of alleles
that enhance survival &
reproduction
Adaptive evolution occurs
as the match between an
organism & its environment
increases
Because the environment
can change, adaptive
evolution is a continuous
dynamic process 84

85. 85

86. Bones shown in
green are movable.
Ligament
Movable jaw
bones in snakes 86

87. SexualSexual
SelectionSelection
Natural selection for
mating success
May result in sexual
dimorphism
Can lead to significant
differences between
secondary sexual traits
87

88. 88

89. 89

90. 90

91. Types of SexualTypes of Sexual
SelectionSelection
Intrasexual
selection
Intersexual
selection
(Mate choice)
91

92. 92
Competition between individuals of one
sex (often males) for mates of the opposite sex
IntrasexualIntrasexual SelectionSelection

93. 93

94. 94
Occurs when individuals of one sex (usually
females) are more choosy in selecting their mates
Intersexual Selection (Mate Choice)Intersexual Selection (Mate Choice)
SootySooty
GrouseGrouse
mating ritualmating ritual

95. Male showiness can increase
his chances of attracting a female, but
also decrease his overall chances of survival 95

96. Good GenesGood Genes
HypothesisHypothesis
One explanation for the
evolution of female
preference
If a trait is related to
male health, selection
should favor both the
male trait & the female
preference for that trait
Ex: Gray tree frog mating
call
96

97. Significance ofSignificance of
Call Duration onCall Duration on
Mate ChoiceMate Choice
Long-Calling (LC) &
Short-Calling (SC)
Does call duration
indicate the male’s
overall genetic quality?
Do females choose
mates based upon this
trait?
97

98. SC male
Female gray tree frog
LC male
SC sperm  Eggs  LC sperm
Offspring of
LC father
Offspring of
SC father
Fitness of these half-sibling offspring compared
EXPERIMENTEXPERIMENT
98

99. RESULTSRESULTS
Time to metamorphosis
Larval survival
Larval growth
NSD = no significant difference; LC better = offspring of LC males superior to
offspring of SC males.
Offspring Performance 1995 1996
LC better NSD
NSD
LC better
(shorter)
LC better
(shorter)
LC better
99

100. The Preservation of Genetic VariationThe Preservation of Genetic Variation
Diploidy
Balancing
selection
Heterozygote
advantage
Frequency-
dependent
selection
Neutral
variation
100

101. DiploidyDiploidy Maintains genetic variation in the form of
hidden recessive alleles
101

102. Biston betularia morpha typica Biston betularia morpha carbonaria
BalancingBalancing
SelectionSelection
Natural selection maintains stable
frequencies of 2 or more phenotypic
forms in a population
102

103. HeterozygoteHeterozygote
AdvantageAdvantage
Heterozygotes have a
higher fitness than both
homozygotes
Natural selection will tend
to maintain 2 or more
alleles at that locus
The sickle-cell allele causes
mutations in hemoglobin,
but also provides malaria
resistance
103

104. Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
104

105. FrequencyFrequency--
DependentDependent
SelectionSelection
The fitness of a phenotype
decreases if it becomes
too common in the
population
Selection can favor the
least common phenotype
in a population
Ex: scale-eating fish
(Perissodus)
105
“Left-mouthed”
P. microlepis
“Right-mouthed”
P. microlepis
1.0
0.5
0
1981
Sample year
’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Frequencyof
“left-mouthed”individuals

106. NeutralNeutral
VariationVariation
Genetic variation that
appears to provide no
selective advantage or
disadvantage
Ex: Variations in noncoding
regions of DNA
Ex: Variations in proteins
that have little effect on
function or reproductive
fitness
106

107. Selection can act
only on existing
variations
Evolution is limited
by historical
constraints
Adaptations are
often compromises
Chance, natural
selection, & the
environment
interact
Why Natural Selection Cannot FashionWhy Natural Selection Cannot Fashion
“Perfect” Organisms“Perfect” Organisms
107

108. 108

109. 109
CreditsCredits
by Rob Swatski, 2013
http://robswatskibiology.wetpaint.com
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