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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
There is genetic variation in the population for resistance to antibiotics. When the farmer applies the antibiotic, bacteria that are resistant survive while susceptible ones die. The surviving resistant bacteria reproduce, passing the resistance genes to their offspring. Over many generations the frequency of the resistance genes rises, so the proportion of resistant bacteria increases. This is natural selection: the antibiotic creates a selection pressure that favours the resistant trait.
Examiner tips
- Show the step‑by‑step chain: variation → differential survival → differential reproduction → change in gene frequency
- Use the words ‘survive’, ‘reproduce’, ‘frequency’ and ‘selection pressure’
Common mistakes
- Failing to mention the initial genetic variation
- Confusing ‘survival’ with ‘reproduction’ or omitting the increase in frequency
Mark scheme (5 marks)
- There is variation / genetic variation in the bacterial population (in resistance to antibiotics)
- Bacteria that are resistant to the antibiotic are more likely to survive / are better adapted to survive in the presence of the antibiotic
- Surviving bacteria reproduce and pass on the alleles / genes for antibiotic resistance to offspring
- Over many generations, the proportion / frequency of resistant bacteria in the population increases
- This is natural selection — the environment (antibiotic) acts as a selection pressure that selects for the resistant characteristic
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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