Explain how natural selection can lead to evolution in a population of organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A population of bacteria is exposed to an antibiotic. Over many generations, the proportion of bacteria that survive antibiotic treatment increases.
Model answer (4 marks)
Mutation produces genetic variants – some bacteria acquire a random change in DNA that gives resistance to the antibiotic.
Bacteria with the advantageous trait (antibiotic resistance) are more likely to survive and reproduce.
The allele for resistance is passed on to offspring, so the frequency of the advantageous allele increases in the population.
Over many generations the inherited traits of the population change, so the population evolves to become resistant.
Bacteria with the advantageous trait (antibiotic resistance) are more likely to survive and reproduce.
The allele for resistance is passed on to offspring, so the frequency of the advantageous allele increases in the population.
Over many generations the inherited traits of the population change, so the population evolves to become resistant.
Examiner tips
- Use the word 'mutation' to start the answer – examiners look for this term. Show the causal chain: mutation → survival advantage → increased allele frequency → evolution. Keep each point short and use the exact wording from the mark scheme.
- Remember to mention that the change is inherited – this is the key to evolution.
Common mistakes
- Confusing mutation with natural selection – students sometimes say the antibiotic causes mutation. Failing to state that the change is passed to offspring. Using vague terms like 'change' instead of 'mutation' or 'allele'.
Mark scheme (4 marks)
- Mutation produces genetic variants / some bacteria have a random change in DNA that gives resistance to the antibiotic
- Bacteria with the advantageous trait (antibiotic resistance) are more likely to survive and reproduce
- The allele for resistance is passed on to offspring / the frequency of the advantageous allele increases in the population over time
- Over many generations the inherited traits of the population change / the population evolves to become resistant
Key terms in this question
natural selection · evolution · population
Related
- Revision notes: Variation and evolution →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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