QUESTION 151 (5 marks)
In a hypothetical plant–fly interaction, flowers have short or long tubes and flies have short or long mouthparts. Both traits are heritable. Equal numbers are tested for the same interval. The table gives mean plant seed production and fly egg production for each pairing. Assume these measures reflect relative reproductive success in the experiment.
| Flower tube | Fly mouthparts | Plant seeds | Fly eggs |
|---|---|---|---|
| Short | Short | 40 | 10 |
| Short | Long | 20 | 4 |
| Long | Short | 5 | 2 |
| Long | Long | 70 | 18 |
a) Identify the pairing with the highest reproductive success for both species. [1 mark]
b) Use the data to explain how reciprocal selection could occur. [3 marks]
c) Explain why this mechanism does not require flies to cause directed mutations for longer flower tubes. [1 mark]
Practice marking scheme
Answer
Long-tube/long-mouthpart pairings have the highest measured success for both species. Each species can impose selection on the other, providing a mechanism for coevolution. This does not imply that the other species directs specific DNA mutations.
Working
For plants, long tubes yield 70 seeds with long-mouthpart flies but only 5 with short-mouthpart flies. For flies visiting long tubes, long mouthparts yield 18 eggs compared with 2 for short mouthparts. Heritable variants with greater reproductive success can become more common, and changes in either species can alter selection experienced by the other. Mutations generate variation without being targeted to a needed trait.
Marking criteria
- Selects long tubes with long mouthparts. [1 mark]
- Uses plant reproductive differences between relevant pairings. [1 mark]
- Uses fly reproductive differences between relevant pairings. [1 mark]
- Links heritable differential success in both interacting lineages to reciprocal selection/coevolution. [1 mark]
- Distinguishes selection on variants from mutations directed by need or by the interacting species. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.