# Explain why the development of antibiotic resistance in a bacterial population is considered evidence for evolution by natural selection.

> OCR A-Level Biology A (H420) — 4.3 Classification and evolution · Explain · 4 marks

> A hospital trust recorded a significant increase in infections caused by methicillin-resistant Staphylococcus aureus (MRSA) over a ten-year period. During this time, the proportion of S. aureus isolates showing resistance to methicillin rose from 2% to 67% in the trust's wards.

## Mark scheme (4 marks)

1. Variation in resistance exists within the bacterial population due to random mutation
2. The antibiotic acts as a selection pressure, meaning non-resistant bacteria are less likely to survive and reproduce
3. Resistant bacteria survive, reproduce and pass the resistance allele/gene to offspring via inheritance
4. Over generations the frequency of the resistance allele increases in the population, changing the allele frequency — this change in heritable characteristics over time is evolution

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-why-the-development-of-antibiotic-91098b73) · Published by Druglandscape Ltd.