Download lecture 8

7/25/11
Natural selection
Molecular Evolution
Natural selection: differential reproduction due to
differences in mortality, fertility, fecundity, offspring
viability, etc.
Selection
Fitness: (w) measure of ability to survive and reproduce,
Can be presented as relative or absolute
Dr. Erica Bree Rosenblum
Fitness surfaces
Fitness surfaces
Junco work (Brodie lab)
Frequency dependent vs non-frequency dependent selection
Does the fitness of a phenotype depend on its frequency
relative to other phenotypes in the population?
If it does then the selection surface is like a water bed –
other individuals affect the fitness optima.
Types of selection
Types of NON-frequency-dependent selection
Disruptive
(or Diversifying)
frequency
frequency
Directional
Stabilizing
(+ Positive or - Purifying ) (or Normalizing)
trait
trait
Mean shifts
Variance decreases
Variance decreases
Variance increases
Note these graphs are illustrating phenotypic expectations
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Types of selection
Types of selection
Adult Human Lactose Tolerance
Stabilizing
(or Purifying)
Human Birth Weight
frequency
Directional
(+ or - )
trait
Mean shifts
Variance decreases
If cattle domestication ~10,000 years ago
If initial mutation frequency was 1/100,000
Only need selection coefficient of 0.04
Variance decreases
Photo: Peter Price
Types of selection
Darwin’s Finches
Types of selection
Disruptive
(or Diversifying)
Sexual selection in Side-blotched lizards
Variance increases
Disruptive
(or Diversifying)
Variance increases
Photo: B. Sinervo
Frequency-dependent selection
Frequency dependent selection
Balancing selection
Balancing selection can be due to any number of
mechanisms that maintains genetic (phenotypic)
variation within a population
For example:
frequency dependent selection
heterozygote advantage
environmental fluctuations
disruptive selection
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loss
Fate of a mutation
under selection
Natural selection
polymorphism
Natural selection: differential reproduction due to
differences in mortality, fertility, fecundity, offspring
viability, etc.
Fitness: (w) measure of ability to survive and reproduce
?
Fitness can also be measured at the genic level
For example in a diploid:
wAA
fixation
wAa
w aa
Relative fitness of ancestral homozygous genotype = 1
If new allele is:
<1
selective disadvantage
>1
selective advantange
=1
selective neutrality
Natural selection
Modes of allelic interaction
Natural selection: differential reproduction due to
differences in mortality, fertility, fecundity, offspring
viability, etc.
Even in a simple 2 allele system there are different
possible allelic interactions:
Fitness: (w) measure of ability to survive and reproduce
Codominant: homozygotes have different fitnesses,
heterozygote fitness is mean of that of homozygotes
Fitness can also be measured at the genic level
For example in a diploid:
wAA
wAa
Complete Recessive or Complete Dominant:
homozygotes have different fitnesses, heterozygote fitness is
same as one of the homozygotes
w aa
Although it is convenient to refer to “A” and “a” alleles,
this implies that “a” is recessive so prefer A 1 and A 2
Overdominant or Underdominant: heterozygote
Modes of allelic interaction
Modes of allelic interaction
Even in a simple 2 allele system there are different
possible allelic interactions:
A 1 A 1 A 1 A 2 A2 A2
Codominant: Example: human blood type (A and B both
Codominant:
1
1+s
1+2s
Complete Recessive:
1
1
1+s
Complete Dominant:
1
1+s
1+s
Overdominant:
1
1+s
1+t
(s>0, t<0)
Underdominant:
1
1+s
1+t
(s<0, t>0)
fitness is higher (overdominant) or lower (underdominant)
than either homozygote
dominant over O but neither dominant over each other)
http://learn.genetics.utah.edu
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Modes of allelic interaction
Modes of allelic interaction
Complete Recessive or Complete Dominant:
Overdominant or Underdominant:
Example: human bitter tasting
Example: heterosis in crops
papillae
Yield related traits like
seed number per plant,
fruit number, total yield,
and biomass exhibit
heterosis in Solanum;
hybrid tomatoes
dominate the market.
taste buds
gustatory
cells
Generally used as an example of
complete dominance: heterozygotes
can taste PTC (thought to have
evolved to discern poisonous plants)
nerve
taste receptor
shapes have
genetic basis
http://learn.genetics.utah.edu
Modes of allelic interaction
Modes of allelic interaction
When A2 is at
intermediate or
high frequency,
homozygotes will
be constantly
generated (and thus
visible to selection)
Efficiency can be described as the rate of
change in allele frequencies due to selection
Efficiency depends on mode of allelic
interaction and strength of selection
Inefficiency is often due to codominant or
recessive alleles residing mostly in
heterozygotes at low frequencies
A1A 2
A2 A2
Modes of allelic interaction
Inefficiency is often due to codominant or
recessive alleles residing mostly in
heterozygotes at low frequencies
A1A
1
A1A 2
A2 A2
A1A
When A2 is at low
frequency,
homozygotes will
rarely be generated
(and thus A2 is not
visible to selection)
1
A1A 2
A2 A2
Fate of a mutation under selection
Codominant
Dominant
Allele freq
1
Allele freq
A1A
Inefficiency is often due to codominant or
recessive alleles residing mostly in
heterozygotes at low frequencies
Recessive
Fate of a new
advantageous allele
Generation
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Fate of a mutation under selection
Underdominant
Allele freq
Overdominant
Modes of allelic interaction
Complete Recessive:
directional selection, inefficient
Complete Dominant:
directional selection, efficient
Codominant:
directional selection, inefficient
Overdominant:
balancing selection, stable
Underdominant:
unstable*
Generation
Modes of allelic interaction
and initial efficiency of selection
directional selection, inefficient
A2 more advantageous than A1 but initially hidden in heterozygotes
Complete Dominant:
directional selection, efficient
A2 more advantageous than A1 and not hidden in heterozygotes
Codominant:
directional selection, inefficient
A2 more advantageous than A1 and “partially” hidden in heterozygotes
Overdominant:
balancing selection, stable
Heterozygotes more fit than either homozygote, both alleles maintained
Underdominant:
Neutral and selected mutations behave differently
Selected mutation
t
1/k
Neutral mutation
time
t
1/k
unstable*
Heterozygotes less fit than homozygotes, frequency determines outcome
Allele freq
Complete Recessive:
Fate of a mutation
Allele freq
Fate of a new advantageous allele with varying initial frequencies
*important demonstration that an allele with a selective advantage
doesn’t always increase in a population
time
Neutralist/Selectionist Controversy
Now that we know roughly how neutral and non-neutral
mutations behave…
Which represents the bread and butter of Evolution?
Do we actually live in a neutralist or selectionist world?
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