Explain how the process of natural selection can lead to the development of antibiotic resistance in a population of bacteria.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Natural selection acts on the genetic variation that exists within a bacterial population. When a new antibiotic is introduced, it kills the susceptible bacteria, but any bacteria that carry a mutation or gene that confers resistance survive. These resistant bacteria then reproduce, passing the resistance trait to their offspring. Over successive generations the proportion of resistant bacteria in the population increases, so the overall population becomes resistant to the antibiotic. Thus, antibiotic resistance develops as a direct consequence of natural selection favouring the resistant variants.
Examiner tips
- Use the word ‘selects’ to link antibiotic use to natural selection.
- Show the sequence: variation → selective pressure → survival of resistant strains → increase in frequency of resistance genes.
- Mention that the outcome is a resistant population, not just a single bacterium.
Common mistakes
- Confusing mutation with natural selection; mutation is the source of variation, selection is the process. Failing to explain that the antibiotic creates a selective pressure that removes susceptible bacteria. Using vague terms like ‘bacteria become resistant’ without showing the stepwise increase in frequency.
Mark scheme (4 marks)
- Description of natural selection acting on variation in bacterial population
- Explanation of how antibiotic use selects for resistant bacteria
- Description of the outcome of natural selection on bacterial population over time
- Link to the development of antibiotic resistance as a consequence of natural selection