7.3.4 Types of Selection

1. Directional Selection

• Occurs when environmental conditions change.

• Individuals with phenotypes better suited to the new environment are more likely to survive, reproduce, and pass on their advantageous alleles.

• Over time, the mean phenotype shifts in the direction of the advantageous traits. Example:

• Antibiotic Resistance in Bacteria:

  • A mutation allows a bacterium to survive antibiotics (e.g., penicillin).
  • This bacterium reproduces, increasing the frequency of the resistance allele in the population.

2. Stabilising Selection

• Occurs in stable environments.

• Individuals with phenotypes closest to the mean are more likely to survive and reproduce.

• Extremes of the phenotype are selected against, reducing diversity within the population. Example:

• Human Birth Weight:

  • Babies with weights around 3kg have the highest survival rates.
  • Very light or very heavy babies are less likely to survive due to complications.

3. Disruptive Selection

• Favours extreme phenotypes at both ends of the distribution, selecting against the mean.

• Leads to increased genetic diversity and may result in the formation of two distinct groups. Example:

• Body Size in Mammals:

  • Large animals survive due to fat reserves, while small animals survive due to their low food requirements.
  • Medium-sized animals are less suited to either strategy and are selected against.