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