A population of rabbits lives in a field. Over several years, a new strain of a pathogen spreads through the rabbit population. Some rabbits survive the infection whilst others die. Explain how natural selection could lead to most rabbits in the field eventually being resistant to this pathogen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Rabbits in a field are affected by a pathogen. Within the population there is variation in resistance to this pathogen. Some rabbits survive infection; others do not.
Model answer (5 marks)
There is variation in resistance to the pathogen within the rabbit population.
Rabbits with greater resistance are more likely to survive the infection.
Surviving rabbits are more likely to reproduce and pass on the resistance alleles to their offspring.
Non‑resistant rabbits are less likely to survive and reproduce, so resistance alleles become more common.
Over many generations most rabbits carry the resistance, so the majority are resistant to the pathogen.
Rabbits with greater resistance are more likely to survive the infection.
Surviving rabbits are more likely to reproduce and pass on the resistance alleles to their offspring.
Non‑resistant rabbits are less likely to survive and reproduce, so resistance alleles become more common.
Over many generations most rabbits carry the resistance, so the majority are resistant to the pathogen.
Examiner tips
- Use the exact wording from the mark scheme (e.g., "variation in resistance", "more likely to survive", "pass on the alleles"),
- Show the causal chain clearly: variation → differential survival → differential reproduction → allele frequency change → population change.
- Keep the answer concise – one sentence per point to hit all 5 marks.
Mark scheme (5 marks)
- There is variation in resistance to the pathogen within the rabbit population
- Rabbits with greater resistance to the pathogen are more likely to survive
- Surviving rabbits are more likely to reproduce and pass on the alleles/genes for resistance to offspring
- Non-resistant rabbits are less likely to survive and reproduce, so resistance alleles/genes become more common in the population over generations
- Over many generations, most of the rabbit population carries the resistance, so the majority of rabbits are resistant to the pathogen
Key terms in this question
natural selection · pathogen · population
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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