Biology · Year 11 · Module 3 · Lesson 13
HSC Exam Practice
Biogeography
Short answer
1.Short answer
Define biogeography and explain how geographic isolation leads to evolutionary divergence.
Explain how the distribution of marsupials in Australia and South America constitutes evidence for evolution. Include the role of Gondwana in your answer.
Describe what Darwin’s finches demonstrate about evolutionary processes on isolated islands. Use the term adaptive radiation in your answer.
Explain what Wallace’s Line is and what it shows about the relationship between geographic proximity and evolutionary history.
Explain why islands often have a high proportion of endemic species despite sometimes having fewer total species than nearby continental areas.
Data response
2.Data response — island endemism
The table below shows endemism and total species counts for four island groups and one continental region. Use the data to answer the questions.
| Region | Total bird species | Endemic bird species | % endemic | Geographic isolation |
|---|---|---|---|---|
| Hawaiian Islands | ~70 native | ~56 | 80% | Very high (remote Pacific) |
| Galapagos Islands | ~57 native | ~27 | 47% | High (remote Pacific) |
| New Zealand | ~200 native | ~60 | 30% | High (South Pacific) |
| British Isles | ~600 native | ~5 | <1% | Low (near Europe) |
| Western Europe (mainland) | ~800 native | ~15 | ~2% | Minimal (connected) |
Describe the trend between geographic isolation and the percentage of endemic species. Refer to at least two specific rows.
Using the concept of isolation and gene flow, explain why the Hawaiian Islands have both fewer total species and a higher endemism percentage than Western Europe.
Identify one limitation of using this data alone to conclude that geographic isolation is the sole driver of endemism.
Extended response
3.Extended response
Evaluate biogeographic evidence for evolution, with reference to at least two named case studies from the lesson. In your response, explain the mechanism by which geographic isolation leads to divergence, assess the strength of distribution patterns as evidence, and reach a justified conclusion about the value of biogeography as evolutionary evidence.
Biology · Year 11 · Module 3 · Lesson 13
Answer Key & Marking Guidelines
3 marks · Band 3
Biogeography is the study of species distribution across geographic space [1]. Geographic isolation occurs when a barrier (ocean, mountain, glacier) reduces or stops gene flow between populations [1]. Without interbreeding, separated populations accumulate different mutations and experience different selection pressures, leading to divergence over generations into distinct lineages or species [1].
3 marks · Band 3
Related marsupial groups are found in Australia and South America — both formerly part of Gondwana — but not in other regions [1]. This suggests that the ancestral marsupial lineage was present on Gondwana before continental separation [1]. After the continents drifted apart, the marsupial lineages on each continent diverged independently from this shared ancestor, producing different groups in each region while retaining the shared marsupial reproductive strategy [1].
3 marks · Band 3
Darwin’s finches demonstrate adaptive radiation: a single colonising ancestral species diversified into multiple species with different beak forms on the Galapagos Islands [1]. Island isolation reduced gene flow between island populations and between islands and the mainland [1]. Different selection pressures on different islands (different food sources available) drove divergence in beak shape and diet, producing a range of specialist species from a common ancestor [1].
3 marks · Band 4
Wallace’s Line is a sharp biogeographical boundary in Indonesia that separates mainly Asian fauna to the west from mainly Australasian fauna to the east, even between geographically close islands [1]. It was caused by deep-water barriers that prevented terrestrial organism movement even during ice ages when sea levels were lower [1]. This demonstrates that geographic proximity does not automatically mean evolutionary connectedness — geological history of barriers matters more than current map distance [1].
2 marks · Band 4
Islands have fewer total species because colonisation is limited by distance and the barrier of surrounding water — fewer species manage to reach and establish on an island than exist on nearby continental areas [1]. However, the species that do colonise are reproductively isolated from source populations, allowing them to diverge into new lineages found nowhere else, producing high endemism [1].
7 marks · Band 4–5
(i) 2 marks. There is a clear positive relationship between geographic isolation and the percentage of endemic species [1]. Highly isolated regions like Hawaii (80% endemic) and Galapagos (47%) show much higher endemism than low-isolation regions like the British Isles (<1%) or Western Europe (~2%); as isolation increases, endemism percentage increases [1].
(ii) 3 marks. Hawaii is extremely remote (very high isolation), so almost no species colonise naturally, resulting in far fewer total species than Western Europe [1]. However, the species that did colonise were completely isolated from mainland populations — gene flow was essentially absent — so over time they diverged into unique endemic species found nowhere else [1]. Western Europe is connected to other landmasses, allowing continuous gene flow and colonisation that prevents the evolution of many unique local species; the low endemism reflects this openness [1].
(iii) 1 mark. One limitation: this data is correlational — it shows that isolation and endemism tend to co-occur, but does not control for other variables such as island age, climate, habitat diversity, or colonisation history. Other factors could contribute to endemism independently of isolation [1].
7 marks · Band 5–6
Marking criteria (1 mark each):
- Explains the mechanism: barrier → reduced gene flow → divergence through mutation, selection and drift.
- Uses marsupial distribution + Gondwana continental history as a named case study with correct explanation.
- Uses Darwin’s finches (or Wallace’s Line) as a second named case study with correct explanation.
- Explains what makes biogeographic patterns specifically consistent with evolutionary theory (fit to geological history, not just environment).
- Acknowledges one limitation of biogeographic evidence (e.g. distribution alone without geological context, or correlation not direct observation of evolution).
- Reaches a justified evaluative conclusion about the strength of biogeographic evidence.
- Quality mark: coherent, precise terminology, logical argument structure.