Download Natural Selection and Change in Allele Frequency

Polygenic traits are Many Gene Traits
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Polygenic traits are Many Gene Traits
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Positive Allele +
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Trait Value
Negative Allele –
Decreases
Trait Value
Natural Selection on Polygenic traits
Individuals
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Natural Selection 3
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by death of small
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by death of small
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After Natural Selection
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Number of individuals
Before Natural Selection
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Frequency
Distribution of a Phenotype in Population BEFORE
1
NATURAL SELECTION
Variance
Average
0
Small
Intermediate
Phenotype: Body Size
Large
Distribution of a Phenotype in Population BEFORE
NATURAL SELECTION
1
Frequency
Peak is
Average Value
Spread around the
Peak is
Variance
Variance
Average
0
Small
Intermediate
Phenotype: Body Size
Large
Distribution of a Phenotype in 2 Populations:
Black and Red
1
Frequency
Peaks are equal:
Average = Average
Variance
Spread around Peaks
Are unequal:
Variance > Variance
Variance
Average
Average
0
Small
Intermediate
Phenotype: Body Size
Large
Three kinds of Natural Selection
Pink individuals die; Blue individuals live.
Number of individuals
Disruptive Selection Directional Selection Stabilizing Selection
Die
Die
Die
Die
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0 25 50 75 100 125
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Body size (g)
Body size (g)
Body size (g)
Selection for larger individuals
Selection for mid-size individuals
Peak shifts
Peak gets narrower
Selection for small and large individuals
Number of individuals
Before
Natural
Selection
Two peaks form
After
Natural
Selection
0 25 50 75 100 125
0 25 50 75 100 125
0 25 50 75 100 125
Body size (g)
Body size (g)
Body size (g)
An Example of Stabilizing Natural
Selection
• Natural Selection on Larval Body Size in
the goldenrod gall fly.
• The Ecology involves 5 species:
1. The golden rod gall fly.
2. The golden rod plant.
3. A parasitoid wasp predator.
4. Two avian or bird predators: chicadees
and woodpeckers.
All Pictures from Dr. Warren Abrahamson or Paul Heinrich
Goldenrod, the ‘host’ plant,
Solidago altissima
(hexaploid).
130 species of goldenrod
in N. America.
Solidago altissima is a clonal
plant. That is, a cluster of stems
that all belong to the same individual.
Each stem is called a ramet and
they are connected underground by
a ‘root’ called a rhizome. All of the
ramets connected to the same
rhizome underground and
are genetically identical.
Ramets = Stems
Solidago altissima is a clonal
plant. That is, a cluster of stems
that all belong to the same individual.
Each stem is called a ramet and
they are connected underground by
a ‘root’ called a rhizome. All of the
ramets connected to the same
rhizome underground and
are genetically identical.
Connected
underground
Eurosta solidaginis, goldenrod gall fly
Female
Male
This is the species that is experiencing evolution by
Natural Selection.
The kind of Natural Selection is Stablizing Selection.
Eurosta solidaginis
female ovipositing
into a goldenrod
ramet. When the
fly larva hatches, the
maggot’s saliva
causes the
plant to make a
gall. The fly larva
will feed on the
plant tissue inside
the protective gall.
Gall, three weeks after egg.
Gall, six weeks after egg.
Galls or swellings on the
ramets of S. altissima.
These galls are made by the
plant, but are induced by chemicals
in the saliva of the goldenrod gall fly maggot.
In Winter: the
above-ground
Plant dies,
But the maggot
Lives on
inside the gall!
The adult fly has no chewing mouthparts,
so it cannot ‘chew its way out’ of the
gall. The fly larva or maggot chews an
exit hole in the late fall, when the above-ground plant is dead.
When the maggot becomes an adult, it escapes out of the gall
through the exit hole.
The gall is an imperfect defense for the fly larva.
Why does the fly maggot live inside the gall?
The gall is a defense against predators
BUT
The gall is an imperfect defense for the fly larva.
Eurytoma gigantea, is a
parasitoid wasp that preys only
on gall fly larvae.
This is a female wasp
on a green gall
getting ready to oviposit her
egg into the gall. Her egg will
hatch and her larvae will eat
the gall fly maggot.
Agent of Natural Selection
Because her ovipositor is short,
the female wasp attacks the
smaller galls. Maggots in
the smaller galls are eaten
by wasp larvae.
ovipositor
Agent of Natural Selection
Fly larvae in the
bigger galls are protected
from wasp attack by the
thick walls of the gall.
Maggot survival strategy: make
a BIG gall to escape wasps.
MORE Agents
of Natural Selection
In the winter, downy
woodpeckers and the
black capped chickadees
eat the gall fly larvae.
As many as 60% of all
galls are attacked by
birds during the winter.
In order to get to the ‘chewy center’ of the gall where the maggot lives,
a bird must peck through the protective outer wall of the gall.
The birds seek the
largest food reward for
their pecking effort
and they attack
the larger galls.
Fly larvae in smaller
galls are protected
from bird attack.
Maggot survival strategy:
make a SMALL gall
to escape birds.
Note: this is the opposite
of the best strategy for
escaping wasps!
In order to study Natural Selection acting on gall size, we
collect 50 to100 galls from a single field in the early spring,
after predation by wasps
and birds has occurred.
Then, we (1) measure the size of each gall;
(2) open each gall and observe its contents (if any);
And,
(3) Sort the galls into four categories:
Live maggot, bird predation, wasp predation, ‘other.’
Typical Data Set
Type of Gall
Average
Gall Size
Gall-Fly in
Galls
21.96 mm
Wasp-Eaten
Galls
17.58 mm
Bird-Damaged
22.37 mm
Galls
Average Gall
Size
20.51 mm
Type of Gall
Gall-Fly in
Galls
Wasp-Eaten
Galls
Average
Gall Size
Inference
21.96 mm
Flies survive in
Intermediate size
galls
17.58 mm
Bird-Damaged
22.37 mm
Galls
Average
Gall Size
20.51 mm
Wasps attack
smaller galls
Birds attack
larger galls
Average gall
Size: all galls
Conclusion: Stabilizing Natural Selection by the combination
of predators favors fly larvae which induce intermediate size galls.
Average
Gall Size
Interpretation
Gall-Fly in
Galls
21.96 mm
Highest Fitness
Wasp-Eaten
Galls
17.58 mm
Lower Fitness
Bird Attacked
Galls
22.37 mm
Lower Fitness
Average
Gall Size
20.51 mm
Average Fitness
Type of Gall
Conclusion: Natural selection by predation favors those fly
larvae which induce intermediate size galls.
Frequency
1
Distribution of Gall Sizes BEFORE
NATURAL SELECTION
Variance
0
20.51 mm
Large
Small
Intermediate
Fly Phenotype: Gall Size in mm
Distribution of Gall Sizes AFTER
NATURAL SELECTION by Wasps
1
Wasp predation on the smaller galls,
pushes the mean toward larger size.
Natural Selection
by wasps
favors large gall
size: Directional
Selection. Average
gall size with live
Fly is larger.
Frequency
Wasp predation
eliminates
many of these
flies
0
20.51
Large
Small
Intermediate
Fly Phenotype: Gall Size in mm
Fly Viability Fitness:
Relative Probability of Survival
Directional Selection: Trait values as one extreme
have the Highest Fitness.
Trait values at the other extreme have the Lowest Fitness
Wasp Predation Alone
High Fitness
1
Low Fitness
20.51 mm
0
Small
Intermediate
Large
Fly Phenotype: Gall Size in mm
1
Relative Fitness
Distribution =
SELECTION
Fly Phenotype: Gall Size in mm
Frequency
X multiply two curves
Phenotype Frequency
Distribution BEFORE
SELECTION
Fly Phenotype: Gall Size in mm
Frequency
Relative Fitness: w
WASP PREDATION and SELECTION
Fly Phenotype: Gall Size in mm
Phenotype Frequency
Distribution AFTER
SELECTION
Distribution of Gall Sizes AFTER
NATURAL SELECTION by Birds
1
Bird predation on the larger galls,
pushes the mean toward smaller size.
Frequency
Natural Selection
favors small gall
size: Directional
Selection.
Bird predation
eliminates
many of these
flies
20.51
0
Large
Small
Intermediate
Fly Phenotype: Gall Size in mm
Distribution of Gall Sizes AFTER
NATURAL SELECTION by Birds
1
Bird predation on the larger galls,
pushes the mean toward smaller size.
Frequency
Natural Selection
by bird predation
favors small gall
size: Directional
Selection.
20.51
0
Large
Small
Intermediate
Fly Phenotype: Gall Size in mm
Fly Viability Fitness:
Relative Probability of Survival
Directional Selection: One extreme
phenotype has the Highest Fitness.
The other extreme has the Lowest Fitness
Bird Predation Alone
High Fitness
1
0
Small
20.51 mm
Low Fitness
Intermediate
Large
Fly Phenotype: Gall Size in mm
Relative Fitness
Distribution =
SELECTION
1
Fly Phenotype: Gall Size in mm
Frequency
X multiply two curves
Phenotype Frequency
Distribution BEFORE
SELECTION
Fly Phenotype: Gall Size in mm
Frequency
Relative Fitness: w
BIRD PREDATION and SELECTION
Fly Phenotype: Gall Size in mm
Phenotype Frequency
Distribution AFTER
SELECTION
Frequency
Distribution of Gall Sizes AFTER
NATURAL SELECTION by Birds AND by Wasps
Wasp predation
has eliminated
many of these
flies
Variance
Bird predation
has eliminated
many of these flies
Variance
20.51 21.96
Large
Small
Intermediate
Fly Phenotype: Gall Size in mm
Fly Viability Fitness:
Relative Probability of Survival
Distribution of Relative Fitness
in relation to Gall Size
High Fitness
1
Low
Fitness
Variance
Low
Fitness
20.51 mm
0
Small
Intermediate
Large
Fly Phenotype: Gall Size in mm
Fly Viability Fitness:
Relative Probability of Survival
Stabilizing Selection: Intermediate
phenotypic values have the Highest Fitness,
extreme phenotypic values have the Lowest Fitness.
1
Variance
20.51 mm
0
Small
Intermediate
Large
Fly Phenotype: Gall Size in mm
Frequency
Relative Fitness: w
BIRD and WASP PREDATION and SELECTION
Fly Phenotype: Gall Size in mm
Frequency
X multiply two curves
Phenotype Frequency
Distribution BEFORE
SELECTION
Fly Phenotype: Gall Size in mm
Fly Phenotype: Gall Size in mm
Phenotype Frequency
Distribution AFTER
SELECTION
Note: Variance is
Reduced by Stabilizing
Selection
Stabilizing Selection on Birth Weight in Humans
100
70
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Birth weight in pounds
Percent infant mortality
Percent of births in population
20
Mortality at Birth
is HIGH
For Very Small
and
For Very Large
babies