Explain why the development of antibiotic resistance in a bacterial population is considered evidence for evolution by natural selection.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Variation in resistance exists within the bacterial population due to random mutation.
The antibiotic acts as a selection pressure, so non‑resistant bacteria are less likely to survive and reproduce.
Resistant bacteria survive, reproduce and pass the resistance allele/gene to offspring via inheritance.
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.
The antibiotic acts as a selection pressure, so non‑resistant bacteria are less likely to survive and reproduce.
Resistant bacteria survive, reproduce and pass the resistance allele/gene to offspring via inheritance.
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.
Examiner tips
- Use the word ‘variation’ and ‘mutation’ to show genetic basis. Explain the role of the antibiotic as a selective pressure. Show that resistant bacteria reproduce and pass on the allele. Link the change in allele frequency to evolution.
Mark scheme (4 marks)
- Variation in resistance exists within the bacterial population due to random mutation
- The antibiotic acts as a selection pressure, meaning non-resistant bacteria are less likely to survive and reproduce
- Resistant bacteria survive, reproduce and pass the resistance allele/gene to offspring via inheritance
- 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 in this question
antibiotic resistance · natural selection · population
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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