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A-Level Biology October/November 2024 Q7(a)(i): In pond 1, the scientists observed: • a high density of tadpoles • a low abundance of f…
A-Level Biology · Paper 9700/41 · October/November 2024 · Question 7(a)(i) · [3 marks]
In pond 1, the scientists observed: • a high density of tadpoles • a low abundance of food • that most of the tadpoles they counted were either detritus feeders or carnivores, with very few intermediates present. Describe and suggest explanations for the type of natural selection that appears to be acting in pond 1.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The type of natural selection occurring in pond 1 is disruptive selection.
This is because the two extreme phenotypes (carnivores and detritus feeders) are selected for and are therefore more likely to survive and reproduce, while the intermediate phenotypes are selected against.
The high density of tadpoles and low abundance of food creates intense competition for resources. This lack of food acts as the main selection pressure. The intermediate tadpoles are at a selective disadvantage because they are outcompeted by the specialist feeders; they are not as efficient at filter-feeding on detritus as the detritus-feeding specialists, and they are not as effective at hunting for small animals as the carnivorous specialists. Consequently, the intermediates fail to obtain sufficient energy, leading to their low survival rate and low abundance in the population.
How the marks are awarded
- B1 — Correctly identifying the type of natural selection as 'disruptive selection'.
- B1 — Describing the outcome of selection: the extremes (carnivores and detritus feeders) are selected for, while the intermediates are selected against.
- B1 — Providing an explanation, such as identifying the selection pressure (lack of food) or the mechanism (intense competition leading to intermediates being outcompeted).
Common mistakes
- Confusing disruptive selection with stabilising selection (where intermediates are favoured) or directional selection (where one extreme is favoured).
- Failing to explicitly name the type of selection as 'disruptive', even if the process is described correctly, thus losing the first available mark.
- Giving a vague explanation of 'competition' without linking it to why the intermediates are specifically disadvantaged (i.e., they are outcompeted for both distinct food sources by the specialists).
- Stating that tadpoles 'choose' or 'adapt' to a food source, which implies conscious choice or change within a lifetime, rather than a process of selection acting on pre-existing variation over generations.
Examiner tip: Always link the specific data or observations provided in the question (e.g., 'few intermediates', 'low food') directly to the biological principles you are explaining (e.g., 'selected against', 'selection pressure').
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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