Evolution by Natural Selection as a Syllogism

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

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

3. h2 S R 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

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

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

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

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

8. Mean phenotypic trait in next generation Selection Changes the Phenotypic Distribution of a Population Response (R) = mean Zoffspring – mean Zparents 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 R= h2S Mean before drought= 9.2mm Mean of Survivors= 10.1mm Mean of next generation = 9.7mm 10.1 survivors 9.2 before drought

13. Fitness Phenotype Stabilizing Selection • 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 Fitness • Extremes have the highest 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