A species of freshwater fish lives in a river. The fish vary in body size. Larger fish require more food but are better at escaping predators, whilst smaller fish require less food but are more frequently caught by predators. Explain how natural selection could lead to a change in the average body size of this fish population over many generations, if the river experiences a long-term reduction in food availability.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A species of freshwater fish lives in a river. The fish vary in body size. Larger fish require more food but are better at escaping predators, whilst smaller fish require less food but are more frequently caught by predators. Over several decades, the amount of food available in the river decreases significantly due to changes in the surrounding environment.
Model answer (5 marks)
1. There is variation in body size within the fish population.
2. With reduced food availability, smaller fish are better adapted because they require less food to survive.
3. Larger fish are less likely to survive because they cannot obtain enough food and are outcompeted for the limited resource.
4. Smaller fish are more likely to survive and reproduce, passing on the alleles for smaller body size to offspring.
5. Over many generations the proportion of fish with smaller body size increases, so the average body size of the population decreases.
2. With reduced food availability, smaller fish are better adapted because they require less food to survive.
3. Larger fish are less likely to survive because they cannot obtain enough food and are outcompeted for the limited resource.
4. Smaller fish are more likely to survive and reproduce, passing on the alleles for smaller body size to offspring.
5. Over many generations the proportion of fish with smaller body size increases, so the average body size of the population decreases.
Examiner tips
- Use the word 'selective advantage' when describing smaller fish. Show the causal chain: food reduction → selective advantage → survival → reproduction → allele frequency change. Keep each point short and directly linked to the mark scheme.
Common mistakes
- Mixing up which size has the advantage; students often say larger fish survive. Failing to mention that the change is over many generations. Using vague terms like 'change' without specifying 'average body size decreases'.
Mark scheme (5 marks)
- There is variation in body size within the fish population
- With reduced food availability, smaller fish are better adapted / have a selective advantage because they require less food to survive
- Larger fish are less likely to survive because they cannot obtain enough food / they are outcompeted for the limited food resource
- Smaller fish are more likely to survive and reproduce, passing on the alleles / genes for smaller body size to offspring
- Over many generations the proportion of fish with smaller body size increases / the average body size of the population decreases
Key terms in this question
natural selection · population
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Populations and evolution questions
- A species of moth lives in a forest. The moths vary in wing pattern — some have …
- A species of Arctic fox lives in a tundra ecosystem. The foxes have white fur an…
- A species of ground-dwelling beetle lives in a woodland. The beetles vary in she…
- A population of rabbits lives in a field. Over several years, a new strain of a …