Explain how natural selection leads to a population of bacteria becoming resistant to an antibiotic over many generations.

OCR GCSE Biology A: Gateway Science (J247) — B5.2 Natural selection and evolution · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A hospital notices that a species of bacteria that was once easily treated with a common antibiotic is no longer being killed by that antibiotic. Scientists suggest this change has come about through natural selection.

Model answer (4 marks)

1. Random mutation in some bacteria creates an allele that confers resistance to the antibiotic.
2. When the antibiotic is used, bacteria without the mutation are killed, while those with the mutation survive.
3. The surviving bacteria reproduce, passing the resistance allele to their offspring.
4. Over many generations the proportion of bacteria carrying the resistance allele rises, producing a resistant population.

Examiner tips

Use the exact sequence of steps: mutation → selection → reproduction → frequency change.
Show the causal link between each step.
Keep each point brief and use exam‑specific terms like "allele" and "frequency".
Avoid unnecessary detail such as mechanisms of mutation.

Common mistakes

Mixing up the order of events (e.g., saying reproduction occurs before selection).
Using vague language like "bacteria become resistant" without explaining the mutation or selection process.

Mark scheme (4 marks)

Random mutation occurs in some bacteria, producing an allele that gives antibiotic resistance
When the antibiotic is present, non-resistant bacteria are killed / resistant bacteria survive
Resistant bacteria reproduce and pass the advantageous allele to offspring
Over many generations the frequency of the resistant allele increases in the population

Key terms in this question

natural selection