A species of ground-dwelling beetle lives in a woodland. The beetles vary in shell colour, ranging from pale brown to dark brown. Owls are the main predator of these beetles and hunt by sight during the day. Over many generations, the average shell colour of the beetle population becomes significantly darker. Explain how natural selection could lead to this change in the beetle population.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A species of ground-dwelling beetle lives in a woodland. The beetles vary in shell colour, ranging from pale brown to dark brown. Owls are the main predator of these beetles and hunt by sight during the day. Over many generations, the average shell colour of the beetle population becomes significantly darker.
Model answer (5 marks)
There is variation in shell colour within the beetle population.
Darker beetles are better camouflaged against the woodland floor, so they are less easily seen by owls.
Pale brown beetles are more likely to be eaten by owls, giving them a higher predation rate.
Darker beetles survive and reproduce more successfully, passing on the allele(s) for dark colouration to their offspring.
Over many generations the proportion of darker beetles in the population increases.
Darker beetles are better camouflaged against the woodland floor, so they are less easily seen by owls.
Pale brown beetles are more likely to be eaten by owls, giving them a higher predation rate.
Darker beetles survive and reproduce more successfully, passing on the allele(s) for dark colouration to their offspring.
Over many generations the proportion of darker beetles in the population increases.
Examiner tips
- Use the word ‘variation’ to show the starting point. Explain the selective advantage of dark colouration. Show the link between survival, reproduction and gene transmission. Mention the change in allele frequency over generations.
Common mistakes
- Failing to mention that variation is a prerequisite for selection. Confusing ‘survival’ with ‘reproduction’. Omitting the idea that the change occurs over many generations.
Mark scheme (5 marks)
- There is variation in shell colour within the beetle population
- Darker beetles are better camouflaged against the woodland floor / less easily seen by owls
- Pale brown beetles are more likely to be eaten by owls / have a higher predation rate
- Darker beetles are more likely to survive and reproduce, passing on the allele(s) / genes for dark colouration to offspring
- Over many generations the proportion / frequency of darker beetles in the population increases
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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