Explain how a population of peppered moths could change over time so that dark-coloured moths become more common than pale-coloured moths through natural selection.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Peppered moths exist in two forms: pale-coloured and dark-coloured. Both forms rest on tree bark. In areas where tree bark becomes darkened by pollution, dark-coloured moths are harder for predators to spot.
Model answer (4 marks)
Variation exists in the population – some moths are pale and some are dark.
Dark moths are better camouflaged on the soot‑darkened bark, so they are less likely to be eaten by predators. This predation pressure favours the dark colour.
Because more dark moths survive, they are more likely to reach breeding age and reproduce, passing the allele for dark colouration to their offspring.
Over many generations the frequency of the dark allele rises, so dark moths become more common than pale moths.
Dark moths are better camouflaged on the soot‑darkened bark, so they are less likely to be eaten by predators. This predation pressure favours the dark colour.
Because more dark moths survive, they are more likely to reach breeding age and reproduce, passing the allele for dark colouration to their offspring.
Over many generations the frequency of the dark allele rises, so dark moths become more common than pale moths.
Examiner tips
- Mention variation first, then explain the selective advantage, survival/reproduction, and change in allele frequency.
- Use the exact terms: variation, selection pressure, survival, reproduction, allele frequency.
- Show the causal chain clearly: variation → selective advantage → differential survival → differential reproduction → change in population.
- Keep the answer concise – 4 marks only.
Common mistakes
- Forgetting to state that variation exists in the population.
- Confusing the colour of the moths with the colour of the tree bark.
- Not linking survival to reproduction or not mentioning allele frequency change.
Mark scheme (4 marks)
- There is variation / a mutation in the population, producing both pale-coloured and dark-coloured moths
- Dark-coloured moths are better camouflaged on darkened bark and are less likely to be eaten by predators (predation is a selection pressure)
- Dark-coloured moths are more likely to survive and reproduce, passing on the gene/allele for dark colouration to offspring
- Over many generations, the frequency of the dark colouration trait increases in the population / dark moths become more common
Key terms in this question
Related
- Revision notes: Evolution by natural selection →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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