# Explain how antibiotic resistance can develop in a bacterial population through natural selection.

> IB DP Biology Standard Level (2023 syllabus) — D4.1 Natural selection · Explain · 4 marks

> Penicillin was introduced as a clinical antibiotic in the 1940s. Within years of its widespread use, strains of Staphylococcus aureus resistant to penicillin were identified in hospitals. Today, methicillin-resistant Staphylococcus aureus (MRSA) poses a significant global health threat.

## Mark scheme (4 marks)

1. Variation exists within the bacterial population, with some individuals carrying heritable mutations that confer resistance to the antibiotic.
2. The antibiotic acts as a selection pressure, causing differential survival — resistant bacteria survive and reproduce while non-resistant bacteria die.
3. Resistant bacteria reproduce and pass the resistance allele(s) to offspring, increasing the frequency of the resistance trait in the population over successive generations.
4. Over many generations, the population becomes predominantly resistant, illustrating an evolutionary change driven by natural selection.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](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-antibiotic-resistance-can-develop-80b70a48) · Published by Druglandscape Ltd.