A2 2 natural selection — the highest-frequency application context · 5 marks
Explain how a population of bacteria can become resistant to an antibiotic.
Marked instantly, no account needed
A2 · A2 2 Biochemistry, Genetics and Evolutionary Trends
Natural selection: the five-step explanation examiners want
The short answer
Explain natural selection in five steps: variation exists within a population due to mutation; a selection pressure acts; individuals with advantageous alleles survive and reproduce; they pass those alleles on; over generations the allele frequency in the population increases.
Applying it to an unfamiliar example
Exam questions rarely use a species you have studied. Keep the five steps but name the specifics of the case: the actual variation, the actual selection pressure, and the actual advantage. Antibiotic resistance, for example, needs 'a random mutation gave some bacteria resistance', 'the antibiotic is the selection pressure', 'resistant bacteria survive and reproduce', 'the resistance allele is passed on', 'the frequency of the resistance allele in the population rises'.
Three types of selection
| Type | What it favours | Example |
|---|---|---|
| Directional | One extreme of the range | Antibiotic resistance |
| Stabilising | The mean, against both extremes | Human birth mass |
| Disruptive | Both extremes, against the mean | Beak size in variable seed years |
From selection to speciation
- Allopatric speciation: a geographical barrier separates populations, so gene flow stops.
- Different selection pressures act on each population, so allele frequencies diverge.
- Over many generations the populations become so different that they can no longer interbreed to produce fertile offspring.
- Sympatric speciation happens without a physical barrier, through reproductive isolation such as differences in breeding season or behaviour.
Hardy–Weinberg in one line
p² + 2pq + q² = 1 and p + q = 1. Use it to calculate allele or genotype frequencies, and remember its assumptions: a large population, random mating, no migration, no mutation and no selection. If allele frequencies change between generations, at least one assumption has been broken.
Phrases that earn the marks
- variation caused by random mutation
- named selection pressure
- individuals with the advantageous allele survive and reproduce
- alleles passed to offspring
- allele frequency in the population increases over generations
- reproductive isolation prevents gene flow
Where marks get lost
- Saying organisms 'adapt' or 'want' to change — individuals do not adapt; populations evolve.
- Using 'survival of the fittest' as the whole answer.
- Talking about individuals evolving rather than allele frequencies changing.
- Forgetting to name the selection pressure in the given context.
Try these natural selection a level biology questions yourself
Write your answer, then see it marked against every mark scheme point before the model answer is revealed.
1/2
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Exam questions on natural selection a level biology, marked
Two A2-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
A2 2 natural selection — the highest-frequency application context · 5 marks
Explain how a population of bacteria can become resistant to an antibiotic.
Command word: Explain — how to answer itMark scheme — 5 creditworthy points
- Random mutation in a bacterium produces an allele giving resistance
- The antibiotic acts as a selection pressure
- Non-resistant bacteria are killed; resistant bacteria survive
- Survivors reproduce and pass the resistance allele to their offspring (by binary fission / vertical transmission)
- The frequency of the resistance allele in the population increases over generations
Full-mark answer
A random mutation in the DNA of one bacterium produces an allele that gives resistance to the antibiotic. When the antibiotic is used it acts as a selection pressure: the non-resistant bacteria are killed, while the resistant bacterium survives. The survivor reproduces by binary fission and passes the resistance allele to its offspring. Because resistant bacteria have greater reproductive success in the presence of the antibiotic, the frequency of the resistance allele increases in the population over successive generations, and the population becomes resistant.
Examiner insight: The mutation must be described as random and as occurring before exposure. Any suggestion that bacteria 'become' resistant because of the antibiotic, or 'adapt' to it, loses the first two marks outright.
A2 2 comparison question · 4 marks
Distinguish between directional selection and stabilising selection, giving one example of each.
Mark scheme — 4 creditworthy points
- Directional selection favours individuals at one extreme of the range, shifting the mean in that direction
- Example: antibiotic resistance in bacteria / peppered moth melanism
- Stabilising selection favours individuals near the mean and selects against both extremes, reducing variation
- Example: human birth mass
Full-mark answer
In directional selection, individuals at one extreme of the range of variation have the greatest reproductive success, so the mean of the population shifts towards that extreme over generations; antibiotic resistance in bacteria is an example. In stabilising selection, individuals close to the existing mean are favoured and both extremes are selected against, so variation around the mean is reduced and the mean stays the same; human birth mass is an example, since very low and very high birth masses both have lower survival.
Examiner insight: This is a 'distinguish' question, so the two statements must be linked in the same sentence structure. Marks are lost for examples given without saying which type of selection they illustrate.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- How do I explain natural selection in an exam?
- Variation from mutation, a named selection pressure, differential survival and reproduction, alleles passed on, and a rise in allele frequency over generations.
- What is the difference between directional and stabilising selection?
- Directional selection favours one extreme and shifts the mean; stabilising selection favours the existing mean and reduces variation.
- What causes speciation?
- Reproductive isolation — geographical or behavioural — followed by divergent selection until the two populations can no longer produce fertile offspring together.
Test yourself on this topic
Generate 10 spec-worded questions on natural selection a level biology, or photograph your written answer and see it marked line by line against the mark scheme.
Open natural selection a level biology in the app
These open straight into BioCCEA with the right unit and topic already picked — no setup, no searching.