A species of Arctic fox lives in a tundra ecosystem. The foxes have white fur and feed on lemmings. Over many generations, climate change causes the tundra to become warmer and the snow cover to decrease, exposing more bare brown ground for longer periods each year. Explain how natural selection could lead to a new variety of Arctic fox with brown fur becoming more common in this population over many generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Arctic foxes currently have white fur that camouflages them against snow. They are predators that hunt lemmings, but they are also preyed upon by wolves and eagles.
Model answer (5 marks)
1. There is variation in fur colour within the fox population – some individuals are white, others brown.
2. This variation is inherited; the different fur colours are caused by genetic differences.
3. White‑furred foxes are less well camouflaged against the exposed brown ground, so they are more easily spotted and predated by wolves and eagles.
4. Brown‑furred foxes are better camouflaged, so they survive and reproduce more successfully, passing the brown‑fur allele to their offspring.
5. Over many generations the proportion of brown‑furred foxes (and the frequency of the brown‑fur allele) increases in the population.
2. This variation is inherited; the different fur colours are caused by genetic differences.
3. White‑furred foxes are less well camouflaged against the exposed brown ground, so they are more easily spotted and predated by wolves and eagles.
4. Brown‑furred foxes are better camouflaged, so they survive and reproduce more successfully, passing the brown‑fur allele to their offspring.
5. Over many generations the proportion of brown‑furred foxes (and the frequency of the brown‑fur allele) increases in the population.
Examiner tips
- Use the exact sequence of cause–effect steps; include inheritance and survival advantage.
- Show the link between allele frequency and population change.
- Keep each point short and to the point.
- Use UK spelling (e.g. ‘colour’).
Common mistakes
- Failing to mention that the variation is inherited.
- Confusing the role of mutation with the existing variation.
- Not linking increased survival to increased reproduction and allele frequency.
Mark scheme (5 marks)
- There is variation in fur colour within the fox population (e.g. some foxes have white fur and some have brown fur)
- This variation is caused by (random) mutation / is inherited / genetic
- White-furred foxes are more easily seen (less camouflaged) against bare brown ground and are therefore more likely to be predated / less likely to survive
- Brown-furred foxes are better camouflaged so are more likely to survive, reproduce and pass on the allele/gene for brown fur to their offspring
- Over many generations, the proportion of foxes with brown fur increases / brown fur allele becomes more common in the population
Key terms in this question
natural selection · population
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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