Types of Selection
This lesson covers:
- Selection pressures
- Directional, stabilising, and disruptive selection
- The effects of these types of selection on populations
Selection pressures
All organisms encounter selection pressures, which are environmental factors that affect their survival and reproductive success. The organisms that are best adapted to these pressures have a higher chance of passing on their alleles, and thus their traits, to future generations.
Selection pressures include:
- Predation
- Competition for resources
- Climate change
- Disease
The over-production of offspring is important in the process of natural selection. This is because it leads to competition for limited resources, and those best adapted to their environment are more likely to survive, reproduce, and pass on their genes. This process results in selection.
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 |