Overview
This lecture introduces methods for studying selection on continuously distributed (quantitative) traits, focusing on forms of selection, the relationship between traits and fitness, and lays the groundwork for analyzing selection on multiple traits.
Requirements for Phenotypic Evolution
- There must be phenotypic variation in the trait of interest within the population.
- Additive genetic variance (heritability) is required for traits to respond to selection.
- A relationship between trait variation and organismal fitness must exist.
Visualizing and Measuring Selection
- Selection can be visualized by plotting phenotype (standardized as a z-score) against relative fitness (w).
- Relative fitness measures individual success compared to the population mean; it can represent lifetime reproductive output or survival.
- The fitness function describes the relationship between phenotype and relative fitness, defined by its strength (slope) and form (shape).
Forms of Selection
- Directional Selection: Favors one phenotypic extreme; shifts the population mean and reduces variance.
- Described by a linear regression: w = a + bz.
- Example: Positive directional selection shifts mean phenotype rightward.
- Stabilizing Selection: Favors intermediate phenotypes; maintains mean, reduces variance.
- Modeled with a quadratic regression: w = a + bz + Ī³zĀ² (Ī³ negative).
- Human birth weight is a classic example: lowest infant mortality at average birth weight.
- Disruptive Selection: Favors both phenotypic extremes; mean unchanged, variance increases.
- Fitness highest at trait extremes, lowest at intermediates.
- Example: Coho salmon exhibit alternative mating tactics (sneaky "jack" males and large "hooknose" males), both with equal fitness; intermediates perform poorly.
- Disruptive selection can promote genetic variance and drive speciation, as seen in Lord Howe Island palms.
Key Terms & Definitions
- Additive Genetic Variance ā The heritable portion of trait variance that responds to selection.
- Relative Fitness (w) ā Individual fitness compared to the population average.
- Z-score ā Standardized value for phenotype (mean = 0, SD = 1).
- Fitness Function ā The relationship between phenotype and relative fitness.
- Directional Selection ā Selection favoring one end of the phenotypic spectrum.
- Stabilizing Selection ā Selection favoring intermediate trait values.
- Disruptive Selection ā Selection favoring both extremes of a trait.
- Selection Differential (S) ā Difference between mean phenotype of selected individuals and the original population mean.
Action Items / Next Steps
- Prepare for next segment on direct, indirect, and correlated selection.
- Review the univariate breeder's equation from previous lectures.
- Familiarize yourself with examples of stabilizing and disruptive selection.