A species of freshwater shrimp has either a long tail spine or a short tail spine. Tail-spine length is inherited.
Two isolated ponds contained populations of these shrimp. Pond P contained predatory fish, which were more likely to catch short-spined shrimp. Pond Q contained no predatory fish. When fish were absent, long-spined shrimp produced fewer offspring than short-spined shrimp.
Scientists sampled each population at generation 0 and again after eight generations.
| Pond | Predatory fish present | Generation | Number of shrimp sampled | Number with long tail spines |
|---|---|---|---|---|
| P | yes | 0 | 200 | 60 |
| P | yes | 8 | 180 | 153 |
| Q | no | 0 | 250 | 75 |
| Q | no | 8 | 200 | 40 |
Calculate the change in the percentage of long-spined shrimp in each pond between generation 0 and generation 8. State whether each change is an increase or a decrease.
Explain why the percentage of long-spined shrimp changed in opposite directions in the two ponds.
After generation 8, all the predatory fish are moved from Pond P to Pond Q.
Suggest how the percentage of long-spined shrimp is likely to change in each pond over many more generations. Explain your answer.
122 exam-style questions on OCR GCSE Biology 5.2 Natural selection and evolution, covering 5.2.1 Genetic variation within a population, 5.2.2 Developments in classification systems, 5.2.3 How evolution occurs through natural selection, 5.2.4 Evolution as change over time, 5.2.5 Evidence for evolution, and 5.2.6 Darwin and Wallace. Each one has a worked solution and a mark scheme showing where the marks go.