Download Natural Selection

10/05/16
Natural Selection
Martin Lascoux
Dpt of Ecology and Genetics
Interacting forces
Evolution through Natural selection
1. The principle of variation
Among individuals within any
population there is variation in
morphology, physiology and
behavior.
2. The principle of heredity
This variation is geneticallycontrolled. Offspring resemble
their parents more than they
resemble unrelated individuals
3. The principle of selection
Some forms are more successful at
surviving and reproducing than
other forms in a given
environment.
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The evidence
Evidence of natural selection I
Evidence of natural selection II
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Evidence of natural selection II
•  Beak size is heritable
•  Beak size is correlated with climatic
fluctuations.
Evidence of natural selection II
Calmodulin
expression
correlates
with beak
morphology
Abzhanov et al 2006
Evidence of natural selection III
Widespread parallel evolution in
sticklebacks by repeated fixation
of Ectodysplasin alleles.
Colosimo et al. , Science 307:1928-1933
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Three-spines sticklebacks
(Gasterosteus aculeatus): an interesting
evolutionary system
• Sticklebacks have undergone
one of the most recent
evolutionary radiations on earth,
generating a large number of
distinct populations with
dramatic changes in
morphological, physiological, and
behavioral characteristics.
Lateral plates
• Easy to manipulate in the lab,
large clutches, availability of
genomic data.
Evolution of lateral armor plates:QTL mapping
Gac4174: QTL explains 77% of the variation in plate morph/number
Genetic, physical and linkage disequilibrium
map of the plate morph interval
1cm
Largest differences in allele frequency
between complete and low morphs.
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Parallel evolution
most low-plate populations have a shared history at the Eda locus
(B) but not at other nuclear genes (C)
Low-plate alleles are present in detectable
frequencies in completely-plated sticklebacks
Transgenics
Low-plate
Transgenic
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The mechanics of natural
selection
A simple case: haploid model
•  To keep things simple assume two bacterial
genotypes A and B that reproduce asexually.
•  Genotype A grows faster than genotype B.
At=(1+a)tA0 and Bt=(1+b)tB0
•  What will happen in the long run if a=0.5 and
b=0.4?
Selection takes place when a≠b
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Selection takes place when a≠b:
B’s are replaced by A’s
Selection takes place when a≠b:
B’s are replaced by A’s
•  The quantity that matters here is:
w=
1+ b
1+ a
the relative fitness
€
Example: experimental
evolution of E. coli
Mutations with higher fitness
Travisiano & Lenski 1994
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Two major points!
•  It’s relative fitness that matters, not
absolute fitness.
•  Relative fitness depends on the
environment: an environmental change
can change the relative fitness
Fitness and environment
Experimental evolution
In Pseudomonas.
Different morphs evolve
to occupy the different
ecological niches in the
Petri dish.
Travisiano & Rainey 1998
Complex
environment
Homogeneous
environment
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Relative fitness of the different
morphs
Selection in diploids:
viability selection
Removing assumptions from the
basic model
• 
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• 
• 
Infinite population size
No mutation
No migration
No selection
Random mating
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Viability selection: basic model
•  Genetic system
• 
• 
• 
Single, biallelic, autosomal locus
Diploidy
Random mating among individuals
•  Selection
• 
• 
• 
Selection identical in both sexes
Selection occurs through differences in viability
Constant fitness values
•  Other factors
• 
• 
• 
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• 
Nonoverlapping generations
No inbreeding
Infinite population size
No gene flow
No mutation
Relative Fitnesses
Genotypes AA
Aa
aa
Number
of zygotes
100
200
100
Number of
adults
Survival
Relative
fitnesses
80
160
50
0.80
1
0.80
1
0.5
0.625
Viability selection
Genotype
Frequency
Fitness
AA
p2
WAA
Aa
2pq
WAa
aa
q2
Waa
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Change in genotype
frequency
AA
Aa
aa
W
2 pq Aa
W
W AA
p
W
2
Waa
q
W
2
Change in allele frequency
WAA=1, WAa=1, Waa=0.5
p0=0.1
1
0.9
A
0.8
0.7
0.6
p(A)
W
p' = p
W
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
35
40
45
Generations
€
WAA=1, WAa=1, Waa=0.5
0.12
Change in allele frequency
0.1
0.08
0.06
0.04
0.02
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Allele frequency (A)
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WAA=1, WAa=0.5, Waa=0.5 p0=0.1
1
0.9
0.8
0.7
p(A)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
35
40
45
Generations
WAA=1, WAa=0.5, Waa=0.5
0.12
Change in allele frequency
0.1
0.08
0.06
Series1
0.04
0.02
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Allele frequency (A)
Purging recessive
deleterious genes
Full recessive
Partly recessive
WAA=0.1, WAa=0.8, Waa=1
WAA=0.1, WAa=1, Waa=1
1
1
0.9
0.9
0.8
0.8
0.7
0.7
0.6
p(A)
p(A)
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
0
0
5
10
15
20
25
Generations
30
35
40
45
0
5
10
15
20
25
30
35
40
45
Generations
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WAA=0.5, WAa=1, Waa=0.5 p0=0.1 and p0=0.9
1
0.9
0.8
0.7
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
35
40
45
Generations
WAA=0.5, WAa=1, Waa=0.5
0.08
Change in allele frequency
0.06
0.04
0.02
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-0.02
-0.04
-0.06
-0.08
Allele frequency (A)
WAA=0.1, WAa=1, Waa=0.1
1
0.9
0.8
0.7
0.6
p(A)
p(A)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
35
40
45
Generations
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WAA=0.1, WAa=1, Waa=0.1
0.3
Change in allele frequency
0.2
0.1
0
Series1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-0.1
-0.2
-0.3
Allele frequency (A)
WAA=0.1, WAa=1, Waa=0.5
1
0.9
0.8
0.7
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
35
40
45
Generations
WAA=0.1, WAa=1, Waa=0.5
0.1
Equilibrium value
0.05
Change in allele frequency
p(A)
0.6
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-0.05
-0.1
Series1
-0.15
-0.2
-0.25
-0.3
Allele frequency (A)
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WAA=1, WAa=0.5, Waa=1 p0=0.1 and p0=0.9
1
0.9
0.8
0.7
p(A)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
35
40
45
Generations
Another parameterization
Fitness
AA
Aa
aa
WAA
WAa
Waa
1
1-hs
1-s
s: selection coefficient
h: dominance coefficient (h=0: a fully
recessive, h=0.5, additivity of allelic effects)
In the case of overdominance
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Viability selection in
diploids
Contrary to the haploid case
there exists a case, and only
one, where, in the absence of
mutation, selection can
maintain genetic variation: this
is called overdominance
Overdominance: an example
β-globin locus (HbS/HbA)
Malaria
•  Became common
with the spread
of agriculture (ca
10,000 ya)
•  Many genes
involved in
resistance to
malaria (complex
trait)
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Table 1: Percentage Frequencies of the Sickle Cell Trait in the New World
Malarious Regions
23.3
20.3
11.5
14.0
Honduras
Surinam
Jamaica
St.Lucia
Non-Malarious Regions
Curacao
St.Vincent
Dominique
Barbados
7.2
8.7
9.5
7.0
Frequency-dependent
selection
Gametophytic SI system
(Solanaceae)
Sporophytic SI system
(Brassicaceae)
Gametophytic SI with 3 alleles
Progeny
Female
parent
Pollen
Freq
S1S2
S1S3
S2S3
S1S2
S3
P12
-
1/2P12
1/2P12
S1S3
S2
P13
1/2P13
S2S3
S1
P23
1/2P23
1/2P23
1/2(1-P12)
1/2(1-P13)
1/2P13
1/2(1-P23)
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Gametophytic SI with 3 alleles
0.6
0.5
Allele frequency
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
6
7
8
9
Generation
Mutation-selection balance
•  Recessive mutant
•  Partially recessive mutant
(more mutations are eliminated)
qˆ =
qˆ ≈
µ
s
µ
hs
Conclusion
Selection is a force that is more
difficult to model than, say, genetic
drift.
There are many examples in the
wild and the efficacy of directional
selection has been shown
repeatedly by breeding
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