# Explain how natural selection could lead to a change in the colour of a population of pebble crabs over many generations, after a new predatory fish begins hunting in the area.

> OCR GCSE Biology A: Gateway Science (J247) — B5.2 Natural selection and evolution · Explain · 4 marks

> Pebble crabs live on a rocky seabed. Their shell colour varies — some crabs have shells that closely match the grey rock, whilst others have brightly coloured shells. A new predatory fish arrives in the habitat and hunts the crabs by sight.

## Mark scheme (4 marks)

1. There is genetic variation / variation in shell colour within the crab population due to mutations / different alleles
2. Brightly coloured crabs are more easily seen / eaten by the predatory fish, so they are less likely to survive
3. Grey/camouflaged crabs survive and reproduce, passing the allele(s) for grey shell colour to their offspring
4. Over many generations the frequency of the allele for grey shell colour increases in the population / the population becomes predominantly grey

## Key terms

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

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](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-natural-selection-could-lead-6e7b432f) · Published by Druglandscape Ltd.