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….

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

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

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

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

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 _

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 _

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

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

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

Stabilizing Selection Fitness Phenotype Extremes have the lowest fitness

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

Disruptive Selection Extremes have the highest fitness Fitness 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