Directional & Stabilising Selection (AQA A Level Biology)

• Environmental factors that affect the chance of survival of an organism are selection pressures
  • For example, there could be high competition for food between lions if there is not plentiful prey available; this environmental factor ‘selects’ for faster, more powerful lions that are better hunters

• These selection pressures can have different effects on the allele frequencies of a population through natural selection
• There are different types of selection:
  • Stabilising
  • Directional

Stabilising selection

• Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations
• This means things stay as they are unless there is a change in the environment
• A classic example of stabilising selection can be seen in human birth weights
  • Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights

Stabilising selection selects against the extreme phenotypes (high and low birth weights) and selects for the intermediate phenotypes (medium birth weights)

Directional selection

• Directional selection is natural selection that produces a gradual change in allele frequencies over several generations
• This usually happens when there is a change in environment/selection pressures or a new allele has appeared in the population that is advantageous
• For example, antibiotic-resistant bacteria strains are becoming more common due to the overuse of antibiotics
  • The presence of antibiotics is a selection pressure
  • Mutations are occurring in bacteria populations randomly
  • A mutation arises that confers antibiotic resistance - it is a beneficial allele
  • Bacteria with this mutation are more likely to survive and reproduce
  • Most bacteria without the resistance mutation die
  • Over generations, this leads to an increase in the frequency of beneficial allele that produces antibiotic resistance

Directional selection favours one extreme phenotype. This causes the mean trait value (in the population) to change over time