Types of Selection (AQA A Level Biology): Revision Note

Types of selection

• Environmental factors that affect the chance of survival of an organism are referred to as selection pressures

• These selection pressures can have different effects on the allele frequencies of a population through natural selection

• There are three types of selection:

  • stabilising selection

  • directional selection

  • disruptive selection

Stabilising selection

• Stabilising selection favours the average phenotype and removes extremes, maintaining average values, e.g. human birth weight

• This topic is covered in detail in Unit 4: Directional & Stabilising Selection

Directional selection

• Directional selection favours individuals with a phenotype at one extreme, causing a shift in a population’s traits over time, e.g. antibiotic resistance in bacteria

• This topic is also covered in detail in Unit 4: Directional & Stabilising Selection

Disruptive selection

• Disruptive selection is natural selection that maintains high frequencies of two different sets of alleles

  • In other words, individuals with intermediate phenotypes or alleles are selected against

• Disruptive selection causes polymorphism: the continued existence of two or more distinct phenotypes in a species

• This can occur in an environment that shows variation

• For example, birds that live on the Galapagos Islands use their beaks to forage for different-sized seeds

  • The size of the bird's beak is either small or large, with the intermediate medium-sized beak selected against

  • The reason for this is that the different types of seed available are more efficiently foraged by a shorter or longer beak