Directional & Stabilising Selection (AQA AS Biology): Revision Note

Directional & stabilising selection

• 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

  • Babies with very low birth weight have a higher risk of health problems and death

  • Babies with very high birth weight may have complications during birth, which also increases mortality risk

  • Babies with a medium birth weight have the highest survival rates

• Very-low and very-high birth weights are selected against, leading to the maintenance of the intermediate 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 bacterial strains are becoming more common due to the overuse of antibiotics

  • The presence of antibiotics is a selection pressure

  • Mutations are occurring in bacterial 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 the beneficial allele that produces antibiotic resistance