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Title: Biology 2900 Principles of Evolution and Systematics


1
Biology 2900Principles of Evolutionand
Systematics
  • Dr. David Innes
  • Dr. Ted Miller
  • Jennifer Gosse
  • Valerie Power

2
Announcements
  • Lab 2 (Group 1) handout ? print from course web
    page
  • Do the population genetics review
    before Lab.
  • Readings for Lab. 2 (Futuyma)
  • HWE Ch 9
    (pp. 190 - 197)
  • Genetic Drift Ch 10 (pp.
    226 231)
  • Selection Ch 12
    (pp. 273 282)
  • Gene Flow Ch 12 (pp.
    278 280)
  • http//www.mun.ca/biology/dinnes/B2900/B2900.html

3
Evolution in the News
Science 26 January 2007Vol. 315. no. 5811, p.
457Whale Worm Sperm Factories Five years
ago, researchers were thrilled by a decomposing
whale carcass they found on the floor of
California's Monterey Canyon, 2900 meters
underwater. The carcass was home to a thriving
community of bacteria-filled tubeworms, called
Osedax, embedded in its decaying bones. The
visible worms were all females, each attended by
up to 100 microscopic males that live in the
female's gelatinous tube (Science, 30 July 2004,
p. 668). Now, biologists have taken a closer look
at these dwarf males and found that they have a
distinctly odd but highly targeted development
They fail to mature, except with respect to their
ability to produce sperm. The dwarf males
have no mouth, no anus, and no gut at all.
There's no circulatory system, nor any of the
internal stores of bacteria that females depend
on for nourishment.
Female
100 Males
4
Biology 2900Principles of Evolution and
Systematics
  • Topics
  • - the fact of evolution
  • - natural selection
  • - population genetics
  • - natural selection and adaptation
  • - speciation, systematics and
  • phylogeny
  • - the history of life

5
Hardy-Weinberg TheoremChapter 9
  • Null model
  • Allele and genotype frequencies will not change
    across generations (equilibrium)
  • Assuming - random mating
  • - large population size
  • - no selection
  • - no migration
  • - no mutation

6
Relax Assumptions
  • Processes that can change allele and/or genotype
    frequencies
  • - Mutation
  • - Migration
  • - Non-random mating
  • - Finite population size
  • - Selection ? differential survival,
  • fecundity etc. among genotypes

7
Hardy-Weinberg
p2 2pq q2 AA Aa aa
  • Relax Assumptions
  • ? - Mutation
  • ? - Migration
  • ? - Non-random mating
  • ? - Finite population size
  • - Selection - differential survival,
  • fecundity etc. among genotypes

8
Selection
  • - Random drift-------gt stochastic
  • - Selection------------gt deterministic
  • Fitness differences
  • differences in the potential to donate genes to
    future generations among phenotypes

  • (genotypes)
  • Fitness values relative

9
Selection
  • Differential fitness
  • differences among phenotypes (genotypes) in
    survival, fertility, fecundity, mating success,
    etc.

  • Example differential survival
  • survival rate ( U )
  • relative fitness (w)

10
Selection
  • Genotype A1A1 A1A2
    A2A2
  • Survival (U) 0.8 0.6
    0.2
  • Fitness(w) w11 w12
    w22
  • 1.00 gt 0.75
    gt 0.25

Directional selection favouring the A1 allele
11
Computer SimulationExample of Directional
Selection
  • Genotype A1A1 A1A2
    A2A2 Fitness(w) w11
    w12 w22
  • 1.00 gt 0.75
    gt 0.25
  • Box 12A Population Mean fitness
  • w p2 w11 2pq w12 q2 w22

12
w111.0 w12 .75 w22 .25
Freq(A1) allele
Directional Selection
13
Initial p 0.40
A1A1 A1A2
A2A2
14
? p rate of change of allele freq.
Maximum rate
15
w p2 w11 2pq w12 q2 w22
16
Strength of Selection
Directional selection
17
Directional Selection
Outcome fixation of one allele (loss of other
allele) Rate dependent on strength of
selection Pattern of change in allele frequency
a function of dominance relationship
18
Selection
  • Selection (fitness of
    phenotype)
  • Favoured allele
  • 1) Dominant w11 w12
    gt w22
  • 2) Recessive w11
    w12 lt w22

19
Fig. 12.6
Fitness Dominant Intermediate
Recessive
A1A1 1.0 1.0 1.0
A1A2 1.0 0.9
0.8 A2A2 0.8 0.8 0.8
Increase of an advantageous allele (directional
selection) Depends on - initial allele
frequency - selection coefficient -
degree of dominance
20
Examples of Selection
  • Single gene polymorphisms
  • Colour Polymorphisms
  • British School of Ecological Genetics
  • (Snails, Butterflies)

21
Cepaea nemoralis
Snail
Butterflies
Peppered moth Biston betularia
22
Peppered Moth
Cryptic coloration
23
Decline in melanic form as air pollution declines
Fig. 12.25
24
http//www.biologycorner.com/worksheets/pepperedmo
th.html
25
Mytilus edulis
Cepaea nemoralis
26
Examples of Selection
  • Single gene polymorphisms
  • 1966 Lewontin and Hubby
  • Protein electrophoresis
  • Many polymorphic enzyme loci
  • Variation neutral or maintained by selection ?

27
Protein Electrophoresis
Pgm
Origin
28
Examples of Selection
  • 1. Laboratory natural selection experiments

29
Directional selection
AdhF allele
30
Examples of Selection
  • 2. Geographic clines in allele frequency
  • - gradient due to migration history
    (neutral) ?
  • - selection due to environmental gradient ?

31
Geographic clines
  • Migration history
  • mixing of alleles
  • (neutral)

32
Six enzyme loci
insecticide
none
33
Geographic clines
  • Mosquito enzyme genes
  • cline for AceR allele correlated with
  • pesticide usage
  • Selection ?
  • Five control genes no cline
  • What type of experiment would be useful ?

34
Selection for Pesticide Resistance
  • Chemical Year Deployed Resistance observed
  • DDT 1939 1948
  • 2,4-D 1945 1954
  • Dalapon 1953 1962
  • Atrazine 1958 1968
  • Picloram 1963
    1988
  • Trifluralin 1963
    1988
  • Triallate 1964
    1987
  • Diclofop 1980
    1987

35
Number of insecticide resistance pest species
Fig. 12.9
Total
36
Fig. 12.8
Rat poison
37
Selection for Antibiotic Resistance
  • Antibiotic Year Deployed
    Resistance observed
  • Penicillin 1943
    1946
  • Streptomycin 1943
    1959
  • Tetracycline 1948
    1953


methicillin-resistant Staphylococcus aureus, or
MRSA
38
Genetic Variation
  • Loss of genetic variation
  • - random genetic drift
  • - inbreeding
  • - migration
  • - directional selection
  • How can genetic variation be maintained ?

39
Maintenance of Genetic Variation
  • Balance of gain and loss of alleles
  • - balance of forward and reverse mutation
  • - selection - mutation balance
  • - selection - migration balance
  • - heterozygote advantage
  • - frequency-dependent selection

40
Mutation Balance
  • two-way (reversible)
  • v equilibrium
    q 0
  • A a
  • u q
  • p

u u v
V
v u v
V

41
Mutation Balance
Equilibrium Freq. (A)
v u v
V
V
(equilibrium) p

0.00001
u v
42
Selection - Mutation Balance
  • Most mutations deleterious
  • Selection acts to remove deleterious alleles
  • New mutations created continuously
  • Balance - rate mutations added
  • - rate selection removes
  • q equilibrium frequency of deleterious

  • allele

v
43
Selection - Mutation Balance
  • A1 dominant, A2 recessive deleterious mutation
  • w11 w12 1 w22 1 - s m
    mutation

  • rate
  • q Ö

s selection coefficient (lethal s 1)
m
v
s
44
Selection - Mutation Balance
m
  • q Ö
  • s low and high then q high
  • s high and low then q low
  • if s 1 then q Ö m
  • (lethal)

v
s
v
m
v
m
45
Selection Mutation Balance
? 1.0 x 10-6
v
m
v
q Ö
s
Strong (lethal)
(selection)
weak
46
Selection - Mutation Balance
  • Human genetic diseases
  • Cystic fibrosis (recessive allele c)
  • f(cc) 1/2500 0.0004 q2 s
    1(lethal)
  • q
    .02
  • q Ö

m
m 0.0004
v
s
47
Selection - Mutation Balance
  • Mutation - selection balance ?

m 0.0004 unusually high Assumptions
incorrect ??? - selection scheme (Fitness
of CC lt Cc?) - not in equilibrium ( f(c)
allele decreasing?) - genetic drift
increased f(c) allele?
48
Selection Migration Balance
Spatial varying fitness among genotypes -
different environments favour different alleles
in different populations - frequency of
the favoured allele will increase to fixation
(loss of unfavoured alleles) - gene flow
can introduce alleles removed by selection -
polymorphism (genetic variation) maintained by a
balance between selection (removing) and gene
flow (reintroducing)
49
Selection Migration Balance
Fig. 12.10
g gene flow
A2
width
Spatial varying fitness
Cline in allele frequency
50
Migration - Selection Balance
Fig. 12.11
High salinity
Low salinity
Selection against ap94 allele maintained by gene
flow
51
Maintenance of Genetic Variation
  • Balance of gain and loss of alleles
  • Ö - balance of forward and reverse mutation
  • Ö - selection - mutation balance
  • Ö - selection - migration balance
  • - heterozygote advantage
  • - frequency-dependent selection

52
Genetic Variation
  • Loss of genetic variation
  • - random genetic drift
  • - inbreeding
  • - migration
  • - directional selection
  • How can genetic variation be maintained ?

53
Heterozygote Advantage
  • Directional selection - one allele or other fixed
  • Selection favours heterozygotes (heterosis

  • overdominance)
  • A1A2 maintains both alleles
  • A1A2 X A1A2 1A1A1 2 A1A2
    1A2A2

54
Selection FavouringHeterozygotes
  • Genotype A1A1 A1A2
    A2A2
  • p2 2pq
    q2
  • Fitness(w) w11 w12
    w22
  • 1 - s 1
    1 - t
  • w12 gt w11, w22 if s gt 0 and
    t gt 0

55
Selection FavouringHeterozygotes
  • Equilibrium
  • q
    p
  • if t 0 q 1.0 (A2
    dominant)
  • if s 0 q 0.0 (A1
    dominant)

t
s
v
v
s t
s t
v
v
56
Selection FavouringHeterozygotes
  • Example
  • t 0.80
  • s 0.90
  • Fitness

w11 w12 w22 1 - .90
1 1 - .80 0.10
1 0.20
57
v
w11 w12 w22 1 - .90
1 1 - .80
q 0.33
v
p .66
58
stable equilibrium

-
p
59
PopulationMean Fitness ( w )
  • w p2 w11 2pq w12 q2
    w22

A1A1 1.0
Directional Selection
60
Heterozygote advantage
p .66
61
, 0, -
gt 0
p
62
Selection FavouringHeterozygotes
  • Sickle-cell anemia
  • hemoglobin gene w
  • AA normal 0.9
  • AS some sickle 1.0
  • SS sickle 0.2
  • favoured in the
  • presence of malaria

63
Sickle-cell allele frequency
Malaria Zone
64
Selection FavouringHeterozygotes
  • Further information
  • http//en.wikipedia.org/wiki/OverdominanceHeteroz
    ygote_advantage_and_sickle-cell_anemia

65
Frequency-DependentSelection
  • Fitness of a genotype constant (w11 w12 w22)
  • Genotype may have different fitness depending on
    the environment (food, pop density etc.)
  • Genotype x Environment Interaction
  • including the frequency of other genotypes

66
Frequency-DependentSelection
  • Polymorphism 2 or more types

Predator search image selects most
common (protects rare form)
67
Rare morph protected
in population
68
Frequency-DependentSelection
  • Cepaea snail shell-colour polymorphism and
    predation by thrushes
  • - frequency in
    population
  • - frequency taken

Lab. 1
Variation maintained by crypsis
frequency-dependent selection
69
Aquatic bug and fish predator
Increased mortality when common
Decreased mortality when rare
70
Maintenance of Genetic Variation
  • Balance of gain and loss of alleles
  • balance of forward and reverse mutation
  • selection - mutation balance
  • selection - migration balance
  • heterozygote advantage
  • frequency-dependent selection

Ö
Ö
Ö
Balancing Selection
Ö
Ö
71
Population Genetics Summary
  • Synthesis of Mendelian genetics and Darwinian
    evolution
  • Hardy-Weinberg Null model
  • Genetic variation is present in natural
    populations
  • Maintenance of genetic variation a dynamic
    process

72
Population Genetics Question
  • Is natural selection the only mechanism of
    evolution ?
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