AQA AS Biology

Revision Notes

4.4.4 Directional & Stabilising Selection

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 three types of selection:
    • Stabilising
    • Disruptive
    • 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

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

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

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.


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