Explain how natural selection could lead to a population of peppered moths becoming darker in colour over many generations, following an increase in air pollution that darkens tree bark in their habitat.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The peppered moth exists in two forms: a pale, speckled form and a dark form. Before industrialisation, pale tree bark was common. After industrialisation, soot darkened tree bark across large areas.
Model answer (4 marks)
Variation in colour exists in the moth population due to different alleles.
Dark moths are better camouflaged on soot‑darkened bark and are less likely to be eaten by predators.
Consequently, dark moths survive and reproduce more successfully, passing the allele for dark colouration to their offspring.
Over many generations the frequency of the dark allele rises, so the population becomes predominantly darker.
Dark moths are better camouflaged on soot‑darkened bark and are less likely to be eaten by predators.
Consequently, dark moths survive and reproduce more successfully, passing the allele for dark colouration to their offspring.
Over many generations the frequency of the dark allele rises, so the population becomes predominantly darker.
Examiner tips
- Use the word ‘variation’ and mention alleles; link camouflage to survival; state that surviving moths reproduce; show allele frequency change over generations
Common mistakes
- Failing to mention alleles or genetic basis of colour; confusing natural selection with mutation; not linking survival to reproduction
Mark scheme (4 marks)
- There is variation in colour within the moth population, with some moths being darker than others, due to different alleles.
- Dark moths are better camouflaged on darkened tree bark and are less likely to be eaten by predators, so they have a greater chance of survival.
- Dark moths are more likely to survive and reproduce, passing on the allele for dark colouration to their offspring.
- Over many generations, the frequency of the allele for dark colouration increases in the population, so the population becomes predominantly darker.
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Selection questions
- Explain why variation within a population is important for natural selection to …
- Describe how natural selection leads to a population of bacteria becoming resist…
- A dog breeder wants to produce guide dogs that are calm in temperament and easy …
- Selective breeding has been used to produce wheat plants with a higher grain yie…