# Explain how a bottleneck event can alter allele frequencies in a population and affect its long-term evolutionary potential.

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

## Mark scheme (4 marks)

1. A bottleneck event drastically reduces population size due to a catastrophic event, leaving only a small number of surviving individuals.
2. The surviving individuals carry only a random subset of the original alleles, so allele frequencies in the post-bottleneck population differ from those of the original population by chance (genetic drift).
3. Rare alleles are likely to be lost entirely from the population, reducing genetic diversity / heterozygosity.
4. Reduced genetic diversity limits the population's ability to adapt to future environmental changes, increasing its vulnerability to extinction or constraining the rate of evolution by natural selection.

## Key terms

- [bottleneck event](https://www.gradenine.co.uk/glossary/bottleneck-event)
- [evolutionary potential](https://www.gradenine.co.uk/glossary/evolutionary-potential)

## 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-a-bottleneck-event-can-4db5e8eb) · Published by Druglandscape Ltd.