QUESTION 147 (6 marks)
A heritable body-size trait has three categories. A selection episode produces the survival counts shown. No migration or reproduction occurs during the episode.
| Category | Size / units | Before | Survivors |
|---|---|---|---|
| Small | 4 | 20 | 18 |
| Medium | 6 | 60 | 12 |
| Large | 8 | 20 | 18 |
a) Calculate survival percentages for the three categories. [2 marks]
b) Identify the selection type and justify from the pattern. [2 marks]
c) Show that mean body size remains unchanged, and explain why this does not show that selection was absent. [2 marks]
Practice marking scheme
Answer
Small and large survival is 90%; medium survival is 20%. The data support disruptive selection. The mean size remains 6 units because the losses are symmetric, even though the distribution becomes more weighted towards both extremes.
Working
Survival percentages are 18/20 = 90%, 12/60 = 20%, and 18/20 = 90%. Both extremes are favoured relative to the middle. Before, mean = (20×4+60×6+20×8)/100 = 6; after, mean = (18×4+12×6+18×8)/48 = 6. An unchanged mean can conceal a large distributional change.
Marking criteria
- Calculates 90% for each extreme. [1 mark]
- Calculates 20% for medium individuals. [1 mark]
- Identifies disruptive selection. [1 mark]
- Uses favoured extremes and reduced intermediate survival. [1 mark]
- Calculates means of 6 units before and after. [1 mark]
- Explains that distribution/relative category frequencies can change while the mean stays constant. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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