Explain why a population of bacteria can become resistant to an antibiotic through natural selection.

AQA GCSE Biology (8461) — 4.6.2 Variation and evolution · Explain · 4 marks · View as Markdown

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

MRSA is a type of bacteria that is resistant to the antibiotic methicillin. It has become a serious problem in hospitals worldwide.

Model answer (4 marks)

1. A random mutation occurs in the DNA of some bacteria.
2. The mutation gives those bacteria an advantage by making them resistant to the antibiotic.
3. When the antibiotic is used, the resistant bacteria survive and the non‑resistant ones are killed.
4. The resistant bacteria reproduce, passing the resistance allele to their offspring, so the frequency of the resistance gene rises in the population over time.

Examiner tips

Use the exact terms "mutation", "allele", "survive", "reproduce". Show the causal chain: mutation → advantage → survival → inheritance. Keep each point brief and separate with numbers or bullet points.

Common mistakes

Mixing up the order of events (e.g., saying resistance appears after the antibiotic is used). Using vague language such as "change" instead of "mutation" or "gene". Failing to mention that the resistant bacteria reproduce and pass on the allele.

Mark scheme (4 marks)

A mutation occurs (randomly) in the DNA of some bacteria
The mutation gives some bacteria an advantage / makes them resistant to the antibiotic
Resistant bacteria survive and reproduce / non-resistant bacteria are killed by the antibiotic
The allele / gene for resistance is passed on to offspring, so the frequency of resistance increases in the population over time

Key terms in this question

natural selection