4.4.4 Directional & Stabilising Selection

1. Directional Selection

• Definition: Occurs when environmental conditions change, favouring individuals with phenotypes that are suited to the new conditions.
• Effect: Over time, the mean phenotype shifts towards the favoured traits.
• Example:
  • Antibiotic resistance in bacteria:
  • A mutation in some bacteria allows them to produce the enzyme penicillinase, which breaks down penicillin.
  • These resistant bacteria survive, reproduce, and pass on the advantageous allele.
  • Over generations, the population becomes increasingly resistant to penicillin.
• Graph: The curve shifts towards one extreme phenotype.

2. Stabilising Selection

• Definition: Occurs when environmental conditions remain stable, favouring individuals with phenotypes closest to the mean.
• Effect: Reduces variation within the population and selects against extremes.
• Example:
  • Human birth weights:
  • Babies with a birth weight around 3–4 kg are more likely to survive.
  • Those with very low or very high weights are less likely to survive, and these extremes are selected against.
• Graph: The curve becomes narrower, with most individuals clustering around the mean.