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

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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
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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

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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
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Initial p 0.40
A1A1 A1A2
A2A2
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? p rate of change of allele freq.
Maximum rate
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w p2 w11 2pq w12 q2 w22
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Strength of Selection
Directional selection
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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

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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)

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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
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http//www.biologycorner.com/worksheets/pepperedmo
th.html
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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

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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 ?

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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

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Number of insecticide resistance pest species
Fig. 12.9
Total
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Fig. 12.8
Rat poison
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Selection for Antibiotic Resistance

• Antibiotic Year Deployed
  Resistance observed
• Penicillin 1943
  1946
• Streptomycin 1943
  1959
• Tetracycline 1948
  1953
• 
  

methicillin-resistant Staphylococcus aureus, or
MRSA
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Genetic Variation

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

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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

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Mutation Balance

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

u u v
V
v u v
V

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

0.00001
u v
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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
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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
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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
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Selection Mutation Balance
? 1.0 x 10-6
v
m
v
q Ö
s
Strong (lethal)
(selection)
weak
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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
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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?
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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)
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Selection Migration Balance
Fig. 12.10
g gene flow
A2
width
Spatial varying fitness
Cline in allele frequency
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Migration - Selection Balance
Fig. 12.11
High salinity
Low salinity
Selection against ap94 allele maintained by gene
flow
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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

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Genetic Variation

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

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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

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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
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Selection FavouringHeterozygotes

• Example
• t 0.80
• s 0.90
• Fitness

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

-
p
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PopulationMean Fitness ( w )

• w p2 w11 2pq w12 q2
  w22

A1A1 1.0
Directional Selection
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Heterozygote advantage
p .66
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, 0, -
gt 0
p
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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
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Selection FavouringHeterozygotes

• Further information
• http//en.wikipedia.org/wiki/OverdominanceHeteroz
  ygote_advantage_and_sickle-cell_anemia

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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

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Frequency-DependentSelection

• Polymorphism 2 or more types

Predator search image selects most
common (protects rare form)
67
Rare morph protected
in population
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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
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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
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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 ?