AQA A Level Biology

Revision Notes

7.3.4 Types of Selection

Types of Selection & Their Effects

  • Environmental factors that affect the chance of survival of an organism are referred to as selection pressures
    • For example, there could be high levels of 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 three types of selection:
    • Stabilising selection
    • Disruptive selection
    • Directional selection

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 on birth weight, downloadable AS & A Level Biology revision notes

Stabilising selection on human birth weight

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: A recent finding has shown that climate change is having an effect on fish size in certain habitats
    • The increase in temperature is selecting for a smaller body size and against a larger body size
    • Warmer seas cause fish metabolism to speed up and so increases their need for oxygen; oxygen levels are lower in warmer seas
    • Larger fish have greater metabolic needs than smaller fish, and so they feel the effect of increased temperatures more strongly
    • Organisms are sensitive to changes in temperature primarily because of the effect that temperature can have on enzyme activity
    • Fish with a smaller body size are therefore fitter and better adapted to living in seas experiencing increased temperatures
    • Fish body size is determined by both genetic and environmental factors
    • Fish of a smaller size are more likely to reproduce and pass on their alleles to offspring
    • Over generations, this leads to an increase in the frequency of alleles that produce a small body size and a decrease in the frequency of alleles that produce a larger body size

Directional selection on fish body size, downloadable AS & A Level Biology revision notes

Directional selection acting on fish body size

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 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 beaks are 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

Disruptive selection on beak size, downloadable AS & A Level Biology revision notes

Disruptive selection acting on beak size in a bird population

Exam Tip

Become familiar with the shapes of the graphs above. They can help you answer questions about the type of selection that is occurring in a population. For example, you may be given a graph like one of those above and be asked to identify the type of selection that is occurring. Alternatively, an exam question could contain a description of the phenotypes in a population, including the relative frequencies of these phenotypes and how these are changing over time. From this, you should be able to identify the type of selection occurring.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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