Explain how natural selection can lead to a population of bacteria becoming resistant to an antibiotic over several generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A patient is prescribed a course of antibiotics to treat a bacterial infection. After several weeks, the antibiotic no longer works against the bacteria causing the infection.
Model answer (5 marks)
1. Bacterial populations are genetically variable; random mutations create differences in their DNA.
2. Some of these mutations produce an allele that confers resistance to the antibiotic.
3. The antibiotic acts as a selection pressure, killing bacteria that lack the resistance allele.
4. Resistant bacteria survive, multiply and have a selective advantage over susceptible ones.
5. The resistance allele is passed to the next generation, so its frequency rises in the population over successive generations.
2. Some of these mutations produce an allele that confers resistance to the antibiotic.
3. The antibiotic acts as a selection pressure, killing bacteria that lack the resistance allele.
4. Resistant bacteria survive, multiply and have a selective advantage over susceptible ones.
5. The resistance allele is passed to the next generation, so its frequency rises in the population over successive generations.
Examiner tips
- Use the exact wording from the mark scheme (e.g., "selection pressure", "selective advantage").
- Show the causal chain: variation → mutation → selection → survival → inheritance.
- Keep each point concise and separate with numbers or bullet points.
Common mistakes
- Confusing mutation with selection; forgetting to mention the antibiotic as the selective agent.
- Using vague terms like "different" instead of "variation" or "allele".
- Failing to state that the allele frequency increases over generations.
Mark scheme (5 marks)
- There is variation in the bacterial population / some bacteria have random mutations
- Some bacteria have an allele / characteristic that gives resistance to the antibiotic
- The antibiotic acts as a selection pressure / non-resistant bacteria are killed by the antibiotic
- Resistant bacteria survive and reproduce / have a selective advantage
- The allele for resistance is passed on to offspring / the frequency of the resistance allele increases in the population over generations
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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