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Evolution by Natural Selection as a Syllogism
1. If individuals in a population vary with respect to a particular trait that has some genetic basis
AND
2. If the variants differ with respect to their abilities to survive and reproduce in the present environment
THEN
3. There will be an increase in the frequency of individuals having those traits that increased fitness in the next generation
The Syllogism Parallels the Breeder’s Equation
R = h2S
The breeder’s equation
Parallel between the Syllogism and the Breeder’s Equation
1. If individuals in a population vary with respect to a particular trait that has some genetic basis
AND
2. If the variants differ with respect to their abilities to survive and reproduce in the present environment
THEN
3. There will be an increase in the frequency of individuals having those traits that increased fitness in the next generation
h2
S
R
Evolutionary Response to Selection on a Quantitative Trait
Offspring trait value Slope = 1.0
h2 = 1.0
Parent trait value
S
Mean before
Mean after
Population mean
Mean of offspring of selected parents
R
When h2 = 1,R = S
Evolutionary Response to Selection on a Quantitative Trait
Offspring trait value Slope = 0.5
h2 = 0.5
Parent trait value
S
Mean before
Mean after
Population mean
Mean of offspring of selected parents
When h2 < 1,R < S
R
Selection Changes the Phenotypic Distribution of Quantitative Traits
Across One Generation• The displacement of the
mean of the character each generation is the response to selection
• Given the same strength of selection, a larger heritability means a larger response.
• If heritability doesn’t change, constant selection yields constant responsez0
_
R1
1z
Evolutionary Response to Selection on a Quantitative Trait
Across Multiple Generations• The displacement of the
mean of the character each generation is the response to selection
• Given the same strength of selection, a larger heritability means a larger response.
• If heritability doesn’t change, constant selection yields constant responsez0
_
R1 R2 R3
1z 2z 3z
Selection Changes the Phenotypic Distribution of a Population
frequency
phenotype
Mean phenotypic trait value BEFORE selection
Mean phenotypic trait value of selected parents
Selection differential (S) = mean Zafter – mean Zbefore
Response (R) = mean Zoffspring – mean Zparents
Mean phenotypic trait in next generation
R= h2S
The Response to Selection also Depends on the type of Selection
Selection as a Function• The response to selection depends on h2 and
selection (R= h2S)• Selection is the relationship between an
individual’s phenotype and its fitness
Phenotype
Fitness
Directional Selection
• Directional implies a continually increasing value of fitness as a function of the trait
Phenotype
Fitn
ess
Effects of Directional Selection:
Directional Selection- Example
• Remember Darwin’s Finches?
9.2 before drought
10.1 survivors
R= h2SMean before drought= 9.2mmMean of Survivors= 10.1mmMean of next generation = 9.7mm
Year
Stabilizing Selection
• Extremes have the lowest fitness F
itne
ss
Phenotype
Stabilizing Selection- Example
• Karn and Penrose, 1951• Data on >7000 male
babies• Survival to 28 days
Optimum= 7lbs. 8oz
Disruptive Selection
• Extremes have the highest fitness F
itne
ss
Phenotype
Disruptive Selection-Example• Fire-bellied
seedcracker finch• 2 types of seeds
available: large and small
Dark bars show individuals that survived to adulthood
Selection Surfaces
• What about combinations of traits?
• Adaptive Landscapes• Can view as topographic maps• Selection moves populations
to nearest peak
Example- Garter snakes• Brodie (1999)• Individuals with certain
combination of traits (stripe + direct escape, unstriped + evasive escape) had higher survival than other combinations
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