Types of Selection
This lesson covers:
- Directional, stabilising, and disruptive selection
- The effects of these types of selection on populations
How variation drives selection
Certain environmental and genetic factors introduce variation within populations.
This variation can drive evolution:
- It generates a range of phenotypes within a population, enhancing the likelihood that some individuals will have alleles for advantageous traits.
- Individuals with these beneficial traits are more likely to survive and reproduce under changing conditions, transmitting the advantageous alleles to their offspring.
- Natural selection occurs.
Types of natural selection and their effects on populations
Natural selection can be categorised based on its effects on phenotypes in populations: directional, stabilising, and disruptive selection.
How these types of selection affect phenotypes:
- Directional selection - Selects for one extreme phenotype over other phenotypes.
- Stabilising selection - Selects for the average phenotype and selects against extreme phenotypes.
- Disruptive selection - Selects for extreme phenotypes and selects against the intermediate phenotype, especially when an environmental factor takes two or more distinct forms.
| Type of selection | Effect on allele frequency | Phenotypes selected for | Effect on normal distribution curve | Example |
|---|---|---|---|---|
| Directional selection | Increases allele frequency for one extreme phenotype | One extreme phenotype | Shifts curve in the direction of the favoured extreme | Antibiotic resistance in bacteria |
| Stabilising selection | Increases allele frequency for the average phenotype, decreases allele frequency for extremes | Average phenotypes | Narrows the curve | Human birth weights |
| Disruptive selection | Increases allele frequency for multiple extreme phenotypes, decreases allele frequency for intermediates | Very different extreme phenotypes | The curve shifts into multiple peaks either side of where the average phenotype peak was | Bird beaks adapting to become larger and smaller when there are two different food sources |