1. Evolution by Natural Selection as a Syllogism
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
Note, the content of this lecture and any figures were taken liberally from other sources….

2. The Syllogism Parallels the Breeder’s Equation
R = h2S
The breeder’s equation

3. Parallel between the Syllogism and the Breeder’s Equation
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

4. Evolutionary Response to Selection on a Quantitative Trait
Offspring trait value
Slope = 1.0
h2 = 1.0
Mean of offspring of selected parents R
Population mean
When h2 = 1,
R = S
Parent trait value S
Mean
before
after

5. Evolutionary Response to Selection on a Quantitative Trait
Offspring trait value
Slope = 0.5
h2 = 0.5
Mean of offspring of selected parents R
Population mean
When h2 < 1,
R < S
Parent trait value S
Mean
before
after

6. Selection Changes the Phenotypic Distribution of Quantitative Traits
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 response
Across One Generation R1 z0 _

7. Evolutionary Response to Selection on a Quantitative Trait
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 response
Across Multiple Generations R1 R2 R3 z0 _

8. 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
Mean phenotypic trait value of selected parents
phenotype

9. The Response to Selection also Depends on the type of Selection

10. 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

11. Directional Selection
Directional implies a continually increasing value of fitness as a function of the trait
Fitness
Effects of Directional Selection:
Phenotype

12. Directional Selection- Example
Remember Darwin’s Finches?
Year
10.1
survivors
R= h2S
Mean before drought= 9.2mm
Mean of Survivors= 10.1mm
Mean of next generation = 9.7mm
9.2
before drought

13. Stabilizing Selection
Fitness
Phenotype
Extremes have the lowest fitness

14. Stabilizing Selection- Example
Optimum= 7lbs. 8oz
Karn and Penrose, 1951
Data on >7000 male babies
Survival to 28 days

15. Disruptive Selection Extremes have the highest fitness Fitness
Phenotype

16. Disruptive Selection-Example
Fire-bellied seedcracker finch
2 types of seeds available: large and small
Dark bars show individuals that survived to adulthood

17. Selection Surfaces What about combinations of traits?
Adaptive Landscapes
Can view as topographic maps
Selection moves populations
to nearest peak

18. Example- Garter snakes
Brodie (1999)
Individuals with certain combination of traits (stripe + direct escape, unstriped + evasive escape) had higher survival than other combinations