# A species of freshwater fish lives in a river. The fish vary in body size. Larger fish require more food but are better at escaping predators, whilst smaller fish require less food but are more frequently caught by predators. Explain how natural selection could lead to a change in the average body size of this fish population over many generations, if the river experiences a long-term reduction in food availability.

> OCR A-Level Biology B — Advancing Biology (H422) — 4.4 Populations and evolution · Explain · 5 marks

> A species of freshwater fish lives in a river. The fish vary in body size. Larger fish require more food but are better at escaping predators, whilst smaller fish require less food but are more frequently caught by predators. Over several decades, the amount of food available in the river decreases significantly due to changes in the surrounding environment.

## Mark scheme (5 marks)

1. There is variation in body size within the fish population
2. With reduced food availability, smaller fish are better adapted / have a selective advantage because they require less food to survive
3. Larger fish are less likely to survive because they cannot obtain enough food / they are outcompeted for the limited food resource
4. Smaller fish are more likely to survive and reproduce, passing on the alleles / genes for smaller body size to offspring
5. Over many generations the proportion of fish with smaller body size increases / the average body size of the population decreases

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](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/a-species-of-freshwater-fish-lives-cfa3fcc0) · Published by Druglandscape Ltd.