A species of peppered moth exists in two forms: a pale speckled form and a dark form. In an unpolluted woodland, pale speckled moths are more common. After industrial pollution kills the pale lichens on tree bark, turning the bark dark, the dark form of the moth becomes more common over many generations. Explain how the dark form became more common in the population after the bark became darker.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Peppered moths rest on tree bark during the day. Birds are predators of peppered moths and hunt by sight. Before pollution, pale speckled moths were well camouflaged against lichen-covered bark. After pollution, the bark became dark and the lichens died.
Model answer (5 marks)
1. There is variation in colour within the moth population – some moths are pale speckled and others are dark.
2. The dark moths are better camouflaged against the now‑dark bark, so they are less likely to be seen by birds.
3. The pale moths are more visible on the dark bark and are more likely to be eaten, giving them a lower chance of survival.
4. Because the dark moths survive more, they are more likely to reproduce and pass on the allele for dark colouring to their offspring.
5. Over many generations the proportion of dark moths increases in the population as more dark individuals survive and reproduce.
2. The dark moths are better camouflaged against the now‑dark bark, so they are less likely to be seen by birds.
3. The pale moths are more visible on the dark bark and are more likely to be eaten, giving them a lower chance of survival.
4. Because the dark moths survive more, they are more likely to reproduce and pass on the allele for dark colouring to their offspring.
5. Over many generations the proportion of dark moths increases in the population as more dark individuals survive and reproduce.
Examiner tips
- Use the word ‘variation’ to show genetic diversity. Mention camouflage as the key selective advantage. Explain that survival leads to reproduction. Show the cumulative effect over generations.
Common mistakes
- Failing to mention that variation exists. Confusing ‘adaptation’ with ‘mutation’. Not linking survival to reproduction. Omitting the long‑term increase in frequency.
Mark scheme (5 marks)
- There is variation in colour/form within the moth population (pale and dark individuals exist)
- Dark moths are better camouflaged / better adapted on the dark bark, so they are less likely to be seen and eaten by birds
- Pale moths are easier for birds to see on dark bark and are more likely to be eaten / have a lower chance of survival
- Dark moths are more likely to survive and reproduce, passing on the allele/characteristic for dark colouring to their offspring
- Over many generations, the proportion of dark moths in the population increases (because more dark offspring survive to reproduce)
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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