# Explain how natural selection can lead to a population of bacteria becoming resistant to an antibiotic over several generations.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 7 Genetics, Populations, Evolution and Ecosystems · Explain · 5 marks

> A patient is prescribed a course of antibiotics to treat a bacterial infection. After several weeks, the antibiotic no longer works against the bacteria causing the infection.

## Mark scheme (5 marks)

1. There is variation in the bacterial population / some bacteria have random mutations
2. Some bacteria have an allele / characteristic that gives resistance to the antibiotic
3. The antibiotic acts as a selection pressure / non-resistant bacteria are killed by the antibiotic
4. Resistant bacteria survive and reproduce / have a selective advantage
5. The allele for resistance is passed on to offspring / the frequency of the resistance allele increases in the population over generations

## Key terms

- [natural selection](https://www.gradenine.co.uk/glossary/natural-selection)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-natural-selection-can-lead-7b05b24a) · Published by Druglandscape Ltd.