A species of moth lives in a forest. The moths vary in wing pattern — some have spotted wings and some have plain wings. The moths rest on tree bark during the day. Birds are the main predators of the moths. Explain how natural selection could lead to one wing pattern becoming more common in the moth population over many generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In this forest, the tree bark has an uneven, mottled texture. Moths with spotted wings are harder for birds to see against the bark than moths with plain wings.
Model answer (5 marks)
There is variation in wing pattern within the moth population.
Spotted moths are better camouflaged against the mottled bark, so they are less visible to birds.
Plain‑winged moths are more likely to be eaten, giving them a selective disadvantage.
Spotted moths survive and reproduce more successfully, passing the allele for spotted wings to their offspring.
Over many generations the proportion of spotted moths in the population increases.
Spotted moths are better camouflaged against the mottled bark, so they are less visible to birds.
Plain‑winged moths are more likely to be eaten, giving them a selective disadvantage.
Spotted moths survive and reproduce more successfully, passing the allele for spotted wings to their offspring.
Over many generations the proportion of spotted moths in the population increases.
Examiner tips
- Use the word "variation" to show you recognise the starting point. Explain the advantage of camouflage. Show the link between survival, reproduction and gene transmission. Mention the increase in frequency over generations.
Common mistakes
- Failing to state that variation exists. Confusing camouflage with other mechanisms. Not linking survival to reproduction. Not mentioning the increase in frequency over time.
Mark scheme (5 marks)
- There is variation in wing pattern within the moth population
- Spotted moths are better camouflaged / less visible to predators on the mottled bark
- Plain-winged moths are more likely to be eaten by birds / have a selective disadvantage
- Spotted moths are more likely to survive and reproduce, passing on the allele/gene for spotted wings to offspring
- Over many generations, the proportion of spotted moths in the population increases
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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