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
Mean of
offspring of
selected
parents
Slope = 1.0
h2 = 1.0
R
Population
mean
When h2 = 1,
R=S
S
Mean
before
Mean
after
Parent trait value
Evolutionary Response to Selection
on a Quantitative Trait
Offspring
trait value
Slope = 0.5
h2 = 0.5
Mean of
offspring of
selected
parents
Population
mean
R
When h2 < 1,
R<S
S
Mean
before
Mean
after
Parent trait value
Selection Changes the Phenotypic
Distribution of Quantitative Traits
Across One Generation
R1
_
z0
• 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.
z1
• If heritability doesn’t change,
constant selection yields
constant response
Evolutionary Response to Selection
on a Quantitative Trait
Across Multiple Generations
R1
_
z0
R2
z1
R3
z2
• 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.
z3
• If heritability doesn’t change,
constant selection yields
constant response
Selection Changes the Phenotypic
Distribution of a Population
Response (R)
= mean Zoffspring – mean Zparents
Mean phenotypic trait in
next generation
R= h2S
frequency
Selection differential (S)
= mean Zafter – mean
Zbefore
Mean phenotypic trait
value BEFORE selection
phenotype
Mean phenotypic trait
value of selected parents
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
Fitness
Phenotype
• Directional implies a continually
increasing value of fitness as a
function of the trait
Effects of Directional Selection:
Fitness
Directional Selection
Phenotype
Directional Selection- Example
• Remember Darwin’s Finches?
Year
10.1
9.2
before survivors
drought
R= h2S
Mean before drought= 9.2mm
Mean of Survivors= 10.1mm
Mean of next generation = 9.7mm
• Extremes have the
lowest fitness
Fitness
Stabilizing Selection
Phenotype
Stabilizing Selection- Example
Optimum= 7lbs. 8oz
• Karn and Penrose, 1951
• Data on >7000 male
babies
• Survival to 28 days
• Extremes have the
highest fitness
Fitness
Disruptive Selection
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