A population of peppered moths (Biston betularia) lives in a woodland area. Before industrialisation, most moths had pale, speckled wings, with very few having dark wings. After industrialisation, dark-winged moths became much more common in the population. Explain why the proportion of dark-winged moths increased in this population after industrialisation.

Pearson Edexcel A-Level Biology A (9BI0) — Topic 7 Genetics, Populations, Evolution and Ecosystems · Explain · 5 marks · View as Markdown

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

Before industrialisation, tree bark in the woodland was pale and covered with lichens. After industrialisation, soot from factories killed the lichens and darkened the tree bark.

Model answer (5 marks)

Before industrialisation, pale moths were better camouflaged on the light, lichen‑covered bark and were less likely to be eaten by predators, so they survived in greater numbers. After industrialisation the bark became darkened by soot, making pale moths more visible to predators while dark‑winged moths were better camouflaged and therefore survived more often. Dark‑winged moths were more likely to reproduce, passing the allele(s) for dark wing colour to their offspring. Consequently the frequency of the dark‑wing allele increased in the population. Over many generations natural selection produced a higher proportion of dark‑winged moths in the woodland.

Examiner tips

  • Use the word "camouflaged" to link colour to predator avoidance. Explain the chain: better camouflage → higher survival → more reproduction → allele frequency change. Mention natural selection as the mechanism.
  • common_mistakes
  • :
  • Failing to explain why pale moths were initially favoured. Omitting the link between survival, reproduction and allele frequency. Using vague terms like "more common" without describing the evolutionary process.

Mark scheme (5 marks)

  1. Before industrialisation, pale moths were better camouflaged on pale, lichen-covered bark and so were less likely to be eaten by predators / survived in greater numbers
  2. After industrialisation, the bark became dark / soot darkened the bark, so dark-winged moths were better camouflaged and pale moths were easier for predators to spot and eat
  3. Dark-winged moths were more likely to survive and reproduce / had greater reproductive success
  4. The allele(s) / genes for dark wing colour were passed on to offspring in greater numbers / frequency of the dark wing allele increased in the population
  5. Over many generations, natural selection resulted in a higher proportion of dark-winged moths in the population

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