Worked Solutions
Module 3: Biological Diversity — Worked Solutions (Preliminary Biology)
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Worked examples for Preliminary Biology Module 3 — Biological Diversity. Each shows where the marks are awarded, the key idea, and a full model answer in your choice of tutor — Stella, Ella or Cassie.
How to use these
Attempt each question under exam conditions first, then check your response against the model answers. Use the tutor tabs to read the solution in the style that suits you: Stella is direct and to the point, Ella is warm and explains the why, and Cassie is concise and uses bullet points.
Example 1 — Natural selection
Question
A population of beetles lives on dark tree bark. Most beetles are brown, but a few are green. Birds prey on the beetles by sight. Using the theory of natural selection, explain how the population could become predominantly brown over many generations. (5 marks)
Solution
Start with variation: the population already varies in colour, with brown and green forms. This variation is heritable, passed from parents to offspring.
Next, selection pressure: birds hunt by sight on dark bark. Green beetles stand out against the dark bark and are eaten more often, so brown beetles have a survival advantage — they are better camouflaged.
Because brown beetles survive longer, they reproduce more and pass on the alleles for brown colouring to more offspring. This is differential reproduction.
Over many generations, the proportion of brown beetles increases while green beetles decline, so the population becomes predominantly brown. Always run the chain: variation → selection pressure → differential survival and reproduction → change in allele frequency over generations.
Natural selection always follows the same logical chain, so let's walk it through with these beetles.
It begins with variation: the population isn't uniform — some beetles are brown, some are green — and this colour is inherited from parents.
Then there's a selection pressure, something in the environment that affects survival. Here it's the birds that hunt by sight on dark bark. Against that dark background, green beetles are easy to spot and get eaten, while brown beetles are camouflaged and more likely to survive.
Because the brown beetles survive, they go on to reproduce and pass the alleles for brown colour to their offspring. The green beetles, being eaten, leave fewer offspring.
Repeat this over many generations and the frequency of the brown allele rises while the green allele becomes rare — so the population gradually becomes predominantly brown. The crucial idea is that the environment "selects" which variations get passed on; the beetles don't choose to change.
- Variation: population has brown and green beetles; colour is heritable
- Selection pressure: birds prey by sight on dark bark
- Differential survival: green beetles conspicuous → eaten more; brown beetles camouflaged → survive
- Differential reproduction: surviving brown beetles reproduce, passing on brown alleles
- Over many generations: frequency of brown allele increases → population becomes predominantly brown
Where the marks go
- 1 mark: Identifies heritable variation in colour within the population
- 1 mark: Identifies the selection pressure (visual predation by birds against dark bark)
- 1 mark: Explains differential survival (brown camouflaged, green more easily eaten)
- 1 mark: Links survival to greater reproduction and passing on of favourable alleles
- 1 mark: States the change in allele/phenotype frequency occurs over many generations
Key idea
Natural selection acts on heritable variation: a selection pressure causes differential survival and reproduction, increasing the frequency of favourable alleles over many generations.
Example 2 — Structural adaptations
Question
Many Australian desert plants have small leaves or leaves reduced to spines, and thick waxy cuticles. Explain how these structural adaptations help the plants survive in an arid environment. (3 marks)
Solution
The challenge in an arid environment is water loss, which occurs mainly by transpiration through the leaves.
Small leaves or spines reduce the surface area exposed to the air, which reduces the area available for transpiration, so less water is lost.
A thick waxy cuticle is a waterproof barrier over the leaf surface that further reduces water loss by evaporation. Both adaptations conserve water, allowing the plant to survive where water is scarce. Tie each structure to reduced water loss — that is what earns the marks.
In a desert, the biggest problem a plant faces is losing precious water. Plants lose water by transpiration — evaporation from the leaf surface — so these adaptations are all about cutting that loss.
Reducing the leaves to small leaves or spines shrinks the surface area exposed to the dry air. Less surface area means less area for transpiration, so the plant loses less water.
The thick waxy cuticle adds a waterproof coating over the surface, blocking evaporation through the leaf. Together these features let the plant hold on to the little water it has, so it can survive long dry periods where other plants couldn't.
- Main problem in arid environments: water loss by transpiration from leaves
- Small leaves / spines → reduced surface area → less area for transpiration → less water lost
- Thick waxy cuticle → waterproof barrier → reduces evaporation from the surface
- Result: water conserved → survival in arid conditions
Where the marks go
- 1 mark: Identifies that the adaptations reduce water loss by transpiration
- 1 mark: Links reduced leaf surface area (small leaves/spines) to less transpiration
- 1 mark: Links the thick waxy cuticle to reduced evaporation/water loss
Key idea
Reduced leaf surface area and a waxy cuticle both cut water loss by transpiration, conserving water so the plant can survive in arid conditions.