Explain antibiotic resistance by natural selection
Trace how resistant variants survive treatment and become more frequent in bacterial populations.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the antibiotic do in the selection process?
What you will learn
- Explain that resistance variation exists before antibiotic selection
- Explain how antibiotic exposure changes survival and transmission of resistance alleles
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Before | 10 resistant + 90 susceptible = 100 |
| Stipulated survivors | 8 resistant + 9 susceptible = 17 |
| Resistant share | 10% before; 8/17 ≈ 47.1% after |
| Absolute resistant count | 10 → 8, so it falls |
Build the evolutionary explanation
A bacterial population may contain rare resistant variants before treatment, often because of mutation. An antibiotic kills or inhibits susceptible bacteria; resistant bacteria are more likely to survive and reproduce. Their descendants inherit resistance, so the resistance allele becomes more frequent. Use of an antibiotic creates a selection pressure, not a purposeful mutation in bacteria responding to need.
Interpret data with care
Antibiotics do not directly make each bacterium develop a targeted resistance mutation. Resistance can spread between bacteria by gene transfer as well as reproduction, but distinguish that from selection of variants within the population.
Pause and explain
A resistant strain transfers a resistance plasmid to another strain. How does gene transfer differ from selection?
Worked example
Why does the proportion of resistant bacteria rise after an effective antibiotic treatment?
Show the worked solution
- Susceptible bacteria are reduced.
- Resistant survivors reproduce and pass on resistance.
Answer Resistant bacteria survive treatment more often and reproduce, increasing the proportion carrying resistance in later generations.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how repeated antibiotic use can increase resistance in a bacterial population.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Some bacteria already carry resistance.
- They survive and transmit the allele to descendants.
Antibiotic exposure kills or inhibits susceptible bacteria, while resistant bacteria are more likely to survive and reproduce. Repeated treatment can therefore increase resistance-allele frequency.
Look for these in your work
- Population process identified
- Cause and consequence linked
A bacterial culture becomes resistant even though no antibiotic was present during mutation. Does this contradict natural selection? Explain.
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- Mutation can generate variation without the antibiotic.
- Selection changes the frequency when the antibiotic is later applied.
No. Resistance variation can arise before exposure. When antibiotic is later applied, it selects for resistant bacteria by reducing susceptible ones.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A resistant strain transfers a resistance plasmid to another strain. How does gene transfer differ from selection?
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Transfer moves genetic material between cells.
- Selection changes the relative reproduction or survival of variants.
Plasmid transfer moves resistance genes between bacteria. Selection is the differential survival and reproduction that changes how common resistant bacteria become; both can contribute to resistance spread.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Saying bacteria become resistant because they try to adapt
- Claiming treatment makes every bacterium mutate in the useful direction
What can you explain now?
What does the antibiotic do in the selection process?
Compare with the explanation
It creates a selection pressure against susceptible bacteria
Resistant variants are more likely to survive and reproduce.
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Make it stick
How does antibiotic exposure select for pre-existing resistant variants?
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