Explain how antibiotic-resistant bacteria can develop in a population of bacteria through natural selection.

Edexcel GCSE Biology (1BI0) — 4.1 Evolution by natural selection · Explain · 4 marks · View as Markdown

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 the course is completed, some bacteria remain in the body and are resistant to the antibiotic.

Model answer (4 marks)

A random mutation in the DNA of some bacteria gives them resistance to the antibiotic.
The antibiotic acts as a selection pressure, killing the non‑resistant bacteria.
The resistant bacteria survive and reproduce, passing the resistance gene to their offspring.
Over time the frequency of the antibiotic‑resistance trait increases in the population.

Examiner tips

Use the word ‘mutation’ and ‘selection pressure’ – these are key terms. Show the causal chain: mutation → survival → reproduction → change in frequency. Keep the answer concise, matching the four points of the mark scheme.

Common mistakes

Forgetting to mention the mutation as the source of resistance. Describing the antibiotic as ‘beneficial’ rather than a selection pressure. Failing to explain that the trait is passed to offspring, not just that the bacteria survive.

Mark scheme (4 marks)

A random mutation occurs in the DNA of some bacteria, giving them resistance to the antibiotic
The antibiotic acts as a selection pressure, killing non-resistant bacteria
The resistant bacteria survive and reproduce, passing on the resistance gene to offspring
Over time, the frequency of the antibiotic-resistance trait increases in the population

Key terms in this question

natural selection · antibiotic-resistant bacteria