Download Variation and its response to selection

Biol 418:
Evolutionary Ecology
Variation
Overview
Variation
• Link between natural selection & evolution
– The breeder’s equation
– The paradox of variation
Variation and its response to selection
• Fisher’s fundamental theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
1
Biol 418:
Evolutionary Ecology
Variation
Linking variation, natural selection & evolution
•Natural selection requires variation in a trait that shows fitness differences. It is a
process (change in phenotypic distribution within a generation)
•Evolution is the outcome (change in phenotypic distribution across generations)
•Inheritance is the mechanism that transmits the effects of selection across
generations, causing evolutionary change
Variation is the raw material of evolution
no natural selection without phenotypic variation
no evolution without genetic variation
Mean Zbefore
Variation
• Link between natural
selection & evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
variation among
individuals in some
attribute or trait:
variation
S = mean Zafter – mean Zbefore
the trait frequency
Mean Zafter
distribution will differ
among age classes
Mean Zkids
the trait distribution of
2
R=h S
all offspring will be
different from that of all
parents
Jennifer Gow
2
Biol 418:
Evolutionary Ecology
Variation
Linking variation, natural selection & evolution
Biol 418:
Evolutionary Ecology
Variation
The breeder’s equation:
the response to selection
= transmission of withingeneration changes to the
next generation
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
R=h2S
The paradox of variation
Directional selection
heritability
=proportion of varn that is genetic
= 0 if no genetic variability
selection differential
= 0 if no phenotypic variation
i.e. selection + genetic variance should lead
to evolution of a phenotypic trait
Trait mean changes towards an
extreme and variance decreases.
Brodie et al. (1995) TREE 10:
313-318.
Variation
• Link between natural selection &
evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Stabilising selection
Trait mean does not changes but the
variance decreases.
Traits closely related to fitness should be less variable
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Biol 418:
Evolutionary Ecology
Variation
The paradox of variation
Biol 418:
Evolutionary Ecology
Variation
The paradox of variation
Fisher’s fundamental theorem:
Frequency
‘The rate of increase in fitness of any organism
Brodie et al. (1995) TREE 10:
313-318.
S
Bill size
S
Bill size
Jennifer Gow
The stronger selection, the
greater the reduction in
variance
Frequency
Variation
• Link between natural selection &
evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
at any time is equal to its genetic variance in
fitness at that time’ (Fisher)
Fisher (1958) The Genetical
Theory of Natural Selection.
2nd edn.
Variation
• Link between natural
selection & evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
In population at equilibrium rate of fitness increase = 0
=> genetic variance in fitness = 0
=> genetic variance in traits related to fitness = 0
Jennifer Gow
4
Biol 418:
Evolutionary Ecology
Variation
Types of variation: polymorphisms
Biol 418:
Evolutionary Ecology
Variation
Types of variation: polymorphisms
Genetic polymorphism
Black-bellied seedcracker,
Pyrenestes ostrinus
Skúlason & Smith (1995)
TREE 10:366-370.
Smith (1993) Nature 363:
618-620.
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Jennifer Gow
5
Biol 418:
Evolutionary Ecology
Variation
Types of variation: polymorphisms
Biol 418:
Evolutionary Ecology
Variation
Types of variation: polymorphisms
Non-genetic polymorphism
Atypical morph:
bent
Skúlason & Smith (1995)
TREE 10:366-370.
Lively (1986) Ecology
67:858-864.
Lively (1986) Evolution
40:232-242.
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Jennifer Gow
Typical morph:
conic
Acorn barnacle,
Chthamalus anisopoma
Acanthina angelica
• Environmental-inducement of bent by
presence of predators
• Bent confers protection from predator
• Bent is less fecund and grows slower
• Bent morph is only advantageous in
presence of predators but predation is
patchy
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Biol 418:
Evolutionary Ecology
Variation
Types of variation: continuous variation
Biol 418:
Evolutionary Ecology
Variation
Types of variation: continuous variation
The genetic basis of continuous variation
Frequency
X = mean
σ
•Most continuous traits have at least a partly genetic basis
σ = standard deviation
Coefficient of variation, CV =
σ
•Number of genes affecting traits can be large
X
•Traits are ‘polygenic’ cf ‘monogenic’ or ‘oligogenic’
(i.e. the standard deviation scaled by the mean)
e.g. Body size: outcome of complex developmental, biochemical and
X
Quantitative trait
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
•CV allows comparison of variability of traits of very different sizes
•Organisms showing determinate growth show restricted CV:
•Birds have a CV of 2-4% for most traits
•Microtus voles have a CV of about 10 % for body weight
•CVs may be larger for organisms with indeterminate growth (e.g. fish)
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
physiological processes: expect that many genes are involved.
•How can we analyse such traits from a genetic perspective?
•Could try and identify all genes that underlie variation
•One way is called ‘Quantitative Trait Locus’ or QTL analysis
Jennifer Gow
7
Biol 418:
Evolutionary Ecology
Variation
Types of variation: continuous variation
QTL analysis
Biol 418:
Evolutionary Ecology
Variation
Types of variation: continuous variation
QTL analysis
X
1. Choose species &
identify genetic markers
e.g. RAPDs, msats etc
2. Cross contrasting
populations
3. Genotype progeny
Schön et al (2004) Genetics
167: 485-498.
QTL for size
Linkage group 1
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
marker 1
Linkage group 2
Linkage group 1
marker 8
marker 1
marker 2
marker 3
Linkage group 2
marker 8
marker 2
marker 3
marker 13
marker 13
marker 7
4. Construct linkage map
5. Raise progeny &
score phenotype
marker 7
6. Compare phenotype
& genotype
Sample size affects QTL detection power, the proportion
of genotypic variance explained by QTL, and bias
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Difficult to identify all genes when many genes of small effect contribute to a trait
8
Biol 418:
Evolutionary Ecology
Variation
Types of variation: continuous variation
Quantitative geneticss
Biol 418:
Evolutionary Ecology
Variation
Difficult and expensive to explore variation molecularly
How can we analyse genetics?
Quantitative genetics: partition of variance (Fisher again!)
Types of variation: continuous variation
Quantitative genetics
h2 is estimated from the degree of similarity between relatives
e.g. parent-offspring regression
VP = VA + VG + VE
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Phenotypic variance
Environmental and
among individuals
developmental variance
VP = Σ(xi – x)2
n-1
Additive genetic variance
Non-additive genetic
(portion of variation due to simple variance (dominance and
genetic effects)
epistasis).
(narrow sense) heritability h2 = VA
VP
72
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
median of offspring height
Falconer & MacKay (1996)
Introduction to Quantitative
Genetics.
70
The slope = h2
68
y = 0.69x + 21.5
66
64
64
66
68
70
72
mean of parents height
74
Jennifer Gow
9
Jennifer Gow
Biol 418:
Evolutionary Ecology
Variation
Types of variation: continuous variation
Quantitative genetics
72
Meadow vole
Microtus pennsylvanicus
70
r = 0.20
68
Song sparrow
Melospiza melodia
66
Smith & Dhondt (1980)
Evolution 34: 1155-1158.
64
64
66
68
70
Weight of mother
72
74
Maternal effects:
effect of the maternal
parent's genotype on the
phenotype of her offspring
that are unrelated to the
offspring's own genotype
72
70
r = 0.07
68
66
64
64
66
68
70
Weight of father
72
74
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Mean family beak depth
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Quantitative genetics
Weight of offspring at weaning
Hansen & Boonstra (2000)
OIKOS 89: 81-94.
Types of variation: continuous variation
Weight of offspring at weaning
Biol 418:
Evolutionary Ecology
Variation
True mid-parent beak depth
Fostered young do not
resemble foster parents but do
resemble true parents, showing
that there is strong heritability.
True foster-parent beak depth
Jennifer Gow
10
Biol 418:
Evolutionary Ecology
Variation
The paradox of variation
Biol 418:
Evolutionary Ecology
Variation
Genetic variation in nature
Mousseau & Roff (1987)
Heredity 59: 181-191.
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Traits closely related to fitness should be less variable
(narrow sense) heritability h2 = VA
VP
low?
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Most continuous traits have non-zero heritability .
But 50% of morphological traits h2 <0.44 and 50% of life history traits
h2 <0.25.
i.e. h2 is often low.
11
Biol 418:
Evolutionary Ecology
Variation
Genetic variation in nature
Biol 418:
Evolutionary Ecology
Variation
Genetic variation in nature
CVA for life history traits NOT less than for morphology
Drosophila pseudoobscura
Houle (1992) Genetics 130:
195-204.
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
h 2 = VA = VA
(VA + VR)
VP
VR = VG + VE
Low heritability but high additive genetic variance with high residual
variance (from other genetic and environmental sources)
Hubby & Lewontin (1966)
Genetics 54: 577-594 & 595609.
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
electrophoresis!
12
Biol 418:
Evolutionary Ecology
Variation
Maynard Smith (1998)
Evolutionary Genetics. 2nd
Edn.
Variation
• Link between natural selection
& evolution
– The breeder’s equation
– The paradox of variation
• Fisher’s
fundamental
theorem
• Types of variation
– Polymorphism
– Continuous
• Genetic variation in nature
Jennifer Gow
Genetic variation in nature
Average proportion of loci polymorphic per population
Insects
Drosophila
wasps
Others
Marine invertebrates
Marine snails
Land snails
Fish
Amphibians
Reptiles
Birds
Rodents
Large mammals
Plants
Biol 418:
Evolutionary Ecology
Variation
Overview
Variation
• Link with natural selection & evolution
0.53
0.24
0.53
0.59
0.18
0.44
0.31
0.34
0.23
0.14
0.20
0.23
0.46
– The breeder’s equation
– The paradox of variation
• Fisher’s fundamental theorem
• Types of variation
– Polymorphism, Continuous
• Amount of variation in nature
• Maintenance of variation
• Lack of response of variation to selection
Jennifer Gow
13