Antibiotic resistance in bacteria is an example of natural selection in action. A population of bacteria was treated with an antibiotic. After several generations, most of the bacteria in the population were resistant to the antibiotic. Explain how natural selection resulted in the majority of bacteria in the population becoming resistant to the antibiotic.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Antibiotic resistance is a major global health concern. When a population of bacteria is exposed to an antibiotic, some individuals may survive and reproduce whilst others die.
Model answer (4 marks)
A random mutation produced a resistant allele in some bacteria.
Bacteria lacking the allele were killed by the antibiotic and could not survive.
The resistant bacteria survived and reproduced, passing the allele to their offspring.
After many generations the frequency of the resistant allele rose, so most bacteria became resistant.
Bacteria lacking the allele were killed by the antibiotic and could not survive.
The resistant bacteria survived and reproduced, passing the allele to their offspring.
After many generations the frequency of the resistant allele rose, so most bacteria became resistant.
Examiner tips
- Use the word "mutation" and "allele" to show understanding of genetics. Show the sequence: mutation → survival → reproduction → allele frequency change. Keep each point short and directly linked to the marks.
- Avoid vague phrases like "some bacteria survived" – specify the resistant allele and its advantage.
- Include the idea of increased allele frequency over generations to demonstrate evolution.
Common mistakes
- Not mentioning the mutation that creates resistance. Confusing the antibiotic killing with natural selection. Failing to explain the increase in allele frequency over time.
Mark scheme (4 marks)
- A random mutation occurred in some bacteria, producing a resistant allele / variant that gave resistance to the antibiotic
- Bacteria without the resistant allele were killed by the antibiotic / were less well suited to the environment
- The resistant bacteria survived and reproduced, passing the advantageous allele to their offspring
- Over many generations the frequency of the resistant allele increased in the population, so most bacteria became resistant
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Evolution and natural selection questions
- Explain how variation within a population of organisms can lead to some individu…
- Describe how the fossil record provides evidence for evolution.
- Explain how a mutation could lead to a change in the characteristics of an organ…
- Explain how natural selection can lead to evolution in a population of organisms…