# Explain how stabilising selection maintains phenotypic stability in a population over successive generations.

> IB DP Biology Higher Level (2023 syllabus) — D4.2 Stability and change · Explain · 4 marks

## Mark scheme (4 marks)

1. Stabilising selection acts against individuals at both extremes of a phenotypic distribution / selects for intermediate phenotypes
2. Alleles associated with extreme phenotypes are removed from the gene pool at a higher rate / their frequency decreases over generations
3. The mean / modal phenotype remains relatively unchanged across generations because individuals near the mean contribute disproportionately to the next generation
4. A named, relevant example illustrates how an optimal phenotype is maintained, e.g. human birth weight — very low or very high birth weights are associated with higher infant mortality, maintaining an intermediate optimal birth weight

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-stabilising-selection-maintains-phenotypic-26eb13b9) · Published by Druglandscape Ltd.