A species of Arctic fox has thick white fur, which provides camouflage in snowy environments. Scientists observed that, over many generations, foxes with thinner brown fur became increasingly rare in the population. Explain how natural selection led to white fur becoming the most common phenotype in this Arctic fox population.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Arctic foxes live in environments covered in snow for most of the year. Predators hunt the foxes, and foxes with better camouflage are more likely to survive.
Model answer (5 marks)
1. There is variation in fur colour and thickness within the population.
2. White fur gives better camouflage against the snow.
3. Consequently, white‑furred foxes are less likely to be eaten by predators.
4. They are therefore more likely to survive to breeding age and to reproduce.
5. Their offspring inherit the allele for white fur, so over many generations the allele for white fur becomes more frequent and the brown‑fur allele becomes rarer.
2. White fur gives better camouflage against the snow.
3. Consequently, white‑furred foxes are less likely to be eaten by predators.
4. They are therefore more likely to survive to breeding age and to reproduce.
5. Their offspring inherit the allele for white fur, so over many generations the allele for white fur becomes more frequent and the brown‑fur allele becomes rarer.
Examiner tips
- Use the exact wording from the mark scheme (variation, camouflage, survival, reproduction, allele frequency).
- Show the causal chain: variation → selective advantage → differential reproduction → allele frequency change.
- Keep each point brief and to the point – one sentence per mark.
Common mistakes
- Mixing up cause and effect (e.g. saying predators *cause* white fur rather than white fur *reduces* predation).
- Using vague terms like "better survival" without linking to reproduction or allele transmission.
Mark scheme (5 marks)
- There is variation in fur colour/thickness within the Arctic fox population
- White fur provides better camouflage against the snow, so white-furred foxes are less likely to be eaten by predators
- White-furred foxes are more likely to survive and reproduce / are more likely to live long enough to breed
- White-furred foxes pass the allele(s) for white fur to their offspring
- Over many generations, the frequency of the allele for white fur increases in the population / brown fur allele becomes less common
Key terms in this question
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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