Describe how natural selection leads to a population of bacteria becoming resistant to an antibiotic over time.

Cambridge International IGCSE Biology (0610) — 18.3 Selection · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

A mutation arises in a bacterial cell that confers resistance to the antibiotic.
When the antibiotic is applied, the susceptible bacteria are killed while the resistant cell survives.
The surviving bacterium reproduces, passing the resistant allele to its progeny.
After many generations the frequency of the resistant allele rises, so that most or all bacteria become resistant.

Examiner tips

Use the word "mutation" and "allele" to show genetic basis.
Show the sequence: mutation → survival → reproduction → increased frequency.
Mention that the antibiotic kills non‑resistant cells.
Keep the answer concise and to the point.

Common mistakes

Failing to mention the mutation or allele. Not explaining that the antibiotic kills susceptible bacteria. Using vague terms like "change" instead of "mutation". Over‑expanding the answer with unnecessary detail.

Mark scheme (4 marks)

A mutation occurs in a bacterial cell that makes it resistant to the antibiotic
When the antibiotic is administered, non-resistant bacteria are killed but the resistant bacterium survives
The resistant bacterium reproduces, passing on the resistant allele to offspring
Over many generations the frequency of the resistant allele increases in the population so that most or all bacteria are resistant

Key terms in this question

natural selection · population