# Explain how stabilising selection differs from directional selection in its effect on the distribution of phenotypes within a population.

> IB DP Biology Standard Level (2023 syllabus) — A4.1 Evolution and speciation · Explain · 4 marks

## Mark scheme (4 marks)

1. Stabilising selection reduces variation by selecting against extreme phenotypes / favours intermediate phenotypes
2. Directional selection favours one extreme phenotype, shifting the mean of the population towards that extreme
3. Stabilising selection narrows the phenotypic distribution / decreases variance without shifting the mean, whereas directional selection moves the mean but may maintain or alter variance
4. 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

- [stabilising selection](https://www.gradenine.co.uk/glossary/stabilising-selection)
- [directional selection](https://www.gradenine.co.uk/glossary/directional-selection)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-stabilising-selection-differs-from-78185e38) · Published by Druglandscape Ltd.