A species of moth lives in woodland areas. The moths rest on tree bark during the day. The tree bark is covered in pale-coloured lichen. Most moths in the population have pale-speckled wing patterns, but a small number have dark wing patterns. Explain how natural selection could lead to a change in the proportion of dark-winged moths in this population if the woodland trees lose their lichen covering and the bark becomes dark in colour.

Pearson Edexcel A-Level Biology A (9BI0) — 2.6 Adaptation and natural selection · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A species of moth lives in woodland areas. The moths rest on tree bark during the day. The tree bark is covered in pale-coloured lichen. Most moths in the population have pale-speckled wing patterns, but a small number have dark wing patterns.

Model answer (5 marks)

Variation in wing colour exists in the moth population.
When the bark becomes dark, dark‑winged moths are better camouflaged and less likely to be seen by predators.
Pale‑speckled moths are more visible and are more likely to be eaten, reducing their survival.
Dark‑winged moths survive and reproduce, passing on alleles for dark wing colour to their offspring.
Over many generations the proportion of dark‑winged moths in the population increases.

Examiner tips

  • Use the word ‘variation’ to start; mention ‘camouflage’ and ‘predation’ as selective pressures.
  • Show the causal chain: change in environment → differential survival → differential reproduction → change in allele frequency.
  • Keep each point brief but include key terms: ‘better camouflaged’, ‘lower chance of survival’, ‘pass on alleles’, ‘over many generations’.
  • Use the exact wording from the mark scheme where possible.

Common mistakes

  • Failing to mention that variation already exists in the population.
  • Confusing ‘survival’ with ‘reproduction’ or omitting the inheritance step.
  • Using vague terms like ‘better suited’ without linking to camouflage and predation.

Mark scheme (5 marks)

  1. There is variation in wing colour/pattern within the moth population
  2. When bark becomes dark, dark-winged moths are better camouflaged / better adapted to the environment
  3. Pale-speckled moths are more easily seen by predators and are more likely to be eaten / have a lower chance of survival
  4. Dark-winged moths are more likely to survive and reproduce, passing on their alleles/genes for dark wing colour to offspring
  5. Over many generations, the proportion of dark-winged moths in the population increases

Key terms in this question

natural selection

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