Explain how the peppered moth (Biston betularia) population in industrial areas of England changed in colour frequency during the Industrial Revolution through the process of natural selection.

IB DP Biology Standard Level (2023 syllabus) — D4.1 Natural selection · Explain · 4 marks · View as Markdown

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

Before the Industrial Revolution, light-coloured peppered moths were more common than dark (melanic) forms in England. As industrial pollution increased, tree bark became coated with soot, darkening the environment. By the late 19th century, dark moths had become the dominant form in heavily industrialised regions.

Model answer (4 marks)

Variation existed in the moth population – some were light coloured and some were dark (melanic). Soot from industrial pollution darkened tree bark, making light‑coloured moths more visible to birds while dark moths were better camouflaged. Consequently, dark moths had a higher survival rate and reproduced more successfully, passing the allele for dark colouration to their offspring. Over successive generations the frequency of the dark allele increased, so dark moths became the dominant form – an example of natural selection.

Examiner tips

  • Mention the two colour morphs and that they are a genetic variation.
  • Explain how the environment (soot) changed predator visibility.
  • Show that differential survival leads to differential reproductive success.
  • State that allele frequency shifts over generations, giving the observed change.

Common mistakes

  • Failing to identify both light and dark morphs as a variation in the population.
  • Confusing camouflage with predation risk or ignoring the role of differential survival.
  • Not linking increased survival to increased reproductive success and allele frequency change.

Mark scheme (4 marks)

  1. Variation existed in the moth population, with individuals differing in body colour (light and dark/melanic forms).
  2. Soot darkened tree bark, meaning light-coloured moths were more visible to predators (birds) on dark bark, and dark moths were better camouflaged.
  3. Dark moths had a higher survival rate and reproduced more successfully, passing the allele(s) for dark colouration to offspring (differential reproductive success).
  4. Over successive generations, the frequency of the dark allele increased in the population, so dark moths became more common — this is natural selection.

Key terms in this question

natural selection

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