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A-Level Biology October/November 2024 Q7(a)(ii): In pond 2, the scientists observed: • a low density of tadpoles • sufficient food avail…
A-Level Biology · Paper 9700/41 · October/November 2024 · Question 7(a)(ii) · [3 marks]
In pond 2, the scientists observed: • a low density of tadpoles • sufficient food availability for all tadpoles • that most of the tadpoles they counted were intermediates, with fewer detritus feeders or carnivores. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 2.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The type of natural selection acting is stabilising selection.
In this type of selection, the individuals with extreme phenotypes are selected against, while those with intermediate phenotypes are selected for. In pond 2, this means the specialist carnivore and detritus feeder tadpoles have lower survival rates and are less likely to reproduce. The intermediate tadpoles are selected for, leading to their higher frequency in the population.
An explanation for this is that the low density of tadpoles and sufficient food availability leads to very low intraspecific competition. In these conditions, the intermediate tadpoles, which can eat a wider variety of food types, may have a nutritional advantage that improves their growth and development, making them more successful than the specialists.
How the marks are awarded
- B1 — Correctly identifying the type of selection as 'stabilising (selection)'.
- B1 — Describing the selection process in context: the extremes (carnivores and detritus feeders) are selected against, while the intermediates are selected for.
- B1 — Suggesting a valid explanation based on the data, such as linking low density to low competition, or explaining that intermediates benefit from a wider range of food, which improves growth.
Common mistakes
- Incorrectly identifying the selection type as disruptive or directional, confusing which phenotypes are being selected for and against.
- Failing to use the specific information provided in the question (e.g., 'low density', 'sufficient food') to justify the explanation for why stabilising selection is occurring.
- Giving a generic definition of stabilising selection without applying it to the context of the different tadpole feeding types (carnivore, detritus feeder, intermediate).
- Incorrectly assuming high competition is the selection pressure, despite the question stating food is sufficient and density is low.
Examiner tip: Always use the specific data and observations provided in the question to support and contextualise your explanations of biological principles.
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