Lecture on Quantitative Genetics and Evolution of Phenotypes

Beyond Alleles

Explain the various ways selection can act in a population.
Explain the term “Selection differential (S).”
Describe how to estimate the selection differential.
Understand how the breeder’s equation R = h^2 imes S is used to describe the evolution of polygenic traits.
Chapter 6 Homework due on Oct 10.
Test 2 covering Chapters 4, 5, and 6 scheduled for Oct 21 & 22.

Mean Phenotype Distribution
- Directional Selection:
- Favors individuals on one end of the distribution of phenotypes in a population.
- Example: In a given example, smaller individuals have higher fitness than larger individuals. After selection, the distribution of phenotypes shifts to the left toward a smaller mean body size.
- Visual Representation: Illustrates a phenotype distribution before and after selection, clearly indicating a leftward shift in the mean.
- Stabilizing Selection:
- Favors individuals in the middle of the distribution of phenotypes.
- Example: Fish with intermediate body sizes exhibit the highest fitness. After selection, the variance in the population decreases relative to the preceding generation.
- Visual Representation: Shows a curve of phenotypes with the middle section highlighted, indicating a preference for average-sized individuals.
- Disruptive Selection:
- Favors individuals at the extremes (tails) of the distribution of phenotypes present in a population.
- If the selection pressure is strong enough, populations may diverge in their phenotypic characteristics.
- Visual Representation: Illustrates the selection preference at both ends of the distribution, leaving the average phenotype less fit.

What would be the result of selection favoring large-bodied individuals in a population?
- Options:
  - Stabilizing selection
  - Directional selection
  - Disruptive selection
What does stabilizing selection do to a trait in a population?
- Options:
  - It favors individuals at the ends of the distribution of the phenotype.
  - It favors individuals in the middle of the distribution of the phenotype.
  - It favors individuals at the low end of the distribution of the phenotype.
  - It favors individuals at the high end of the distribution of the phenotype.
What would disruptive selection do to a population that varied in body size?
- Options:
  - Select against individuals who were large.
  - Select against individuals who were small.
  - Randomly select against individuals, leading to a random pattern.
  - Select against individuals who were medium-sized, favoring large and small individuals.

If only the largest individuals reproduce, the selection is strong for body size.
If both large and medium individuals reproduce, the selection is less strong and the mean size of the reproducing individuals is closer to the population mean size.

Defined as the proportion of phenotypic variance attributed to the additive effects of alleles.
The greater the narrow sense heritability, the more similar the offspring are to their parents.
Example: For human height, a narrow-sense heritability of approximately 0.8 indicates that 80% of the variation in height within a population is due to genetic differences. The remaining 20% of height variation is primarily due to environmental factors such as nutrition.

High heritability of a trait (e.g., body size) leads to larger evolutionary changes (R).
The evolutionary change/response (R) can be calculated using the breeder’s equation:
R = h^2 imes S
Scenarios:
- A: Body size is not heritable (size of parents not correlated with size of offspring), leading to no change in the population mean size in the next generation.
- B: Body size is somewhat heritable but also influenced by environmental conditions.
- C: Body size is fully heritable, where offspring size completely tracks parental size, thus R = S because h^2 = 1.

The evolutionary response to selection (R) is calculated as: R = h^2 imes S
- Where:
- R = Evolutionary Response
- h^2 = Heritability (narrow sense)
- S = Selection differential
According to Darwin's principles:
- Organisms must vary in a trait that is crucial for survival and reproduction (S).
- The trait must be heritable (able to be passed to offspring), represented by h^2.

To assess the evolution of mandible size, the breeder’s equation R = h^2 S is applied.
The mean mandible size in generation 1 is denoted as x1.
Beetles with longer mandibles exhibit higher mating success compared to those with shorter mandibles.
The mean trait value of individuals that successfully reproduce is denoted as x*.
The selection differential (S) quantifies the difference between successful individuals and the initial population.
The evolutionary response (R) to selection is calculated by subtracting the mean of generation 1 from the mean of generation 2, indicating changes in the trait across generations.
Example: Males in generation 2 exhibit mandibles 0.2 mm longer than those in generation 1.

Narrow sense heritability for mandible size amongst beetles is low, recorded at 0.08.
Mandible size is significantly affected by the environmental conditions experienced by a male beetle during development.

When would a population evolve most rapidly (exhibit a large R)?
- Options:
  - When selection is strong, and the trait is not heritable.
  - When selection is weak, and the trait is very heritable.
  - When selection is strong, and the trait is very heritable.
  - When selection is weak, and the trait is weakly heritable.