# A population of bacteria lives in soil that does not contain any antibiotics. A farmer begins using antibiotics on the soil to treat a crop disease. Over several generations, the population of bacteria becomes resistant to the antibiotics. Explain how natural selection leads to this change in the bacterial population.

> Pearson Edexcel A-Level Biology A (9BI0) — 2.6 Adaptation and natural selection · Explain · 5 marks

> Antibiotics are chemicals that kill bacteria or prevent them from reproducing. Within any population of bacteria, there is natural variation in the level of resistance to antibiotics.

## Mark scheme (5 marks)

1. There is variation / genetic variation in the bacterial population (in resistance to antibiotics)
2. Bacteria that are resistant to the antibiotic are more likely to survive / are better adapted to survive in the presence of the antibiotic
3. Surviving bacteria reproduce and pass on the alleles / genes for antibiotic resistance to offspring
4. Over many generations, the proportion / frequency of resistant bacteria in the population increases
5. This is natural selection — the environment (antibiotic) acts as a selection pressure that selects for the resistant characteristic

## 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/a-population-of-bacteria-lives-in-bee5c3e7) · Published by Druglandscape Ltd.