Explain how stabilising selection differs from directional selection in its effect on the distribution of phenotypes within a population.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Stabilising selection reduces variation by favouring intermediate phenotypes and selecting against extremes, so the phenotypic distribution narrows and the mean remains unchanged. Directional selection favours one extreme phenotype, so the mean of the population shifts toward that extreme; the distribution may widen or narrow but the mean moves. For example, human birth weight shows stabilising selection – babies of very low or very high weight are selected against, keeping the mean birth weight stable. In contrast, antibiotic resistance in bacteria is a case of directional selection – bacteria with a resistance mutation are favoured, shifting the mean resistance level upward.
Examiner tips
- Use the terms 'stabilising' and 'directional' explicitly
- State the effect on mean and variance separately
- Give a clear example for each type of selection
- Keep the answer concise and to the point
Common mistakes
- Confusing the effect on the mean for stabilising selection
- Using an example that actually illustrates stabilising selection for directional selection
- Failing to mention the change (or lack of change) in variance
Mark scheme (4 marks)
- Stabilising selection reduces variation by selecting against extreme phenotypes / favours intermediate phenotypes
- Directional selection favours one extreme phenotype, shifting the mean of the population towards that extreme
- Stabilising selection narrows the phenotypic distribution / decreases variance without shifting the mean, whereas directional selection moves the mean but may maintain or alter variance
- Correct use of a named example to illustrate either stabilising or directional selection (e.g. human birth weight for stabilising; antibiotic resistance in bacteria / beak size change during drought for directional)
Key terms in this question
stabilising selection · directional selection
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Evolution and speciation questions
- Explain how variation within a population enables natural selection to occur.
- Explain how geographic isolation can lead to the formation of two new species fr…
- Explain how the concept of reproductive isolation is central to the biological s…
- Explain how the accumulation of mutations over time can contribute to speciation…
- Explain how artificial selection differs from natural selection in the mechanism…
- Explain how comparative anatomy provides evidence for the common ancestry of ver…