Biology • Year 11 • Module 3 • Lesson 13
Biogeography
Apply biogeographic reasoning to distribution maps and case studies, evaluate island endemism data, and use Wallace’s Line to assess claims about geographic proximity and evolutionary relatedness.
1. Interpret a distribution pattern
The table below summarises the distribution of marsupial orders across three regions. Use the data to answer the questions. 8 marks
| Marsupial order | Present in Australia? | Present in South America? | Present in Africa / Asia? |
|---|---|---|---|
| Diprotodontia (kangaroos, koalas, wombats) | Yes | No | No |
| Dasyuromorphia (quolls, Tasmanian devil) | Yes | No | No |
| Didelphimorphia (American opossums) | No | Yes | No |
| Paucituberculata (shrew opossums) | No | Yes | No |
1.1 Describe the pattern in this data and explain what it suggests about the evolutionary history of marsupials. 2 marks
1.2 Explain why the presence of marsupials in both Australia and South America, but not in other regions, is consistent with a Gondwana origin. 3 marks
1.3 Predict what would happen to the marsupial species composition of Australia if a permanent land bridge connecting it to Asia were formed. Use lesson reasoning about gene flow and isolation. 3 marks
2. Cause-and-effect chain — adaptive radiation on islands
Complete the chain below. Each cause is given; fill in the effect. 5 marks
Cause 1: A small group of birds from a mainland species colonises a set of remote, isolated islands. The islands have no competing bird species and multiple unoccupied ecological niches (seeds, insects, nectar, cacti).
Effect 1 / Cause 2:
Cause 3: Island isolation prevents gene flow from the mainland or between some island populations.
Effect 3 / Cause 4:
Cause 5: Scientists later survey the islands and find multiple distinct finch species with different beak shapes adapted to different food sources.
Effect 5 (Overall conclusion):
3. Case study — Wallace’s Line and island proximity
Read the scenario, then answer the questions. 6 marks
Scenario. An ecologist studying islands in the Indonesian archipelago notices that Bali (west of Wallace’s Line) has predominantly Asian-affiliated wildlife including Asian deer, wild pigs and tiger populations in historical records, while Lombok (only ~35 km east of Bali, east of the Line) has predominantly Australasian-affiliated wildlife including distinct honeyeaters, cockatoos and small marsupials. The ecologist notes that the islands are separated by a deep-water strait that remained a barrier even during ice ages when sea levels dropped significantly.
3.1 Explain why the faunal difference between Bali and Lombok is so dramatic despite the islands being only ~35 km apart. 2 marks
3.2 Explain what Wallace’s Line demonstrates about the relationship between current geographic distance and evolutionary history. 2 marks
3.3 A student says that because Bali and Lombok are so close, their animals must share the same evolutionary history. Evaluate this claim using lesson content. 2 marks
Q1.1 — Distribution pattern and evolutionary history (2 marks)
Marsupials are found in Australia and South America only, with different orders on each continent [1]. This suggests that the ancestral marsupial lineage was present on a shared landmass before these continents separated, and that subsequent divergence after separation led to different marsupial groups on each continent [1].
Q1.2 — Gondwana origin (3 marks)
Australia and South America were both part of the ancient supercontinent Gondwana, which began separating approximately 180 million years ago [1]. The ancestral marsupial lineage was present on Gondwana before separation occurred, so both now-isolated continents inherited related marsupial groups from this shared ancestor [1]. The absence of marsupials in Africa and Asia (other parts of Gondwana, or regions never connected to the original marsupial range) is consistent with the interpretation that marsupials diversified after the landmasses that became Australia and South America had already separated from the rest [1].
Q1.3 — Prediction: land bridge to Asia (3 marks)
A permanent land bridge would allow gene flow between Australia and Asia, introducing Asian placental mammal species into Australia [1]. Over time, increased competition from placental mammals could reduce the diversity of marsupials in Australia, as has been observed historically after placental species were introduced to isolated island systems [1]. Additionally, some hybridisation or interbreeding between closely related colonising and resident species could occur; alternatively, the gene flow from newly arriving populations would reduce the genetic distinctiveness of Australian lineages [1].
Q2 — Cause-and-effect chain (sample answers)
Effect 1 / Cause 2: The colonising birds spread across different islands and begin exploiting the available niches. Different populations on different islands begin adapting to different food sources, experiencing different selection pressures for beak shape and feeding behaviour.
Effect 3 / Cause 4: Populations on different islands accumulate different mutations and are shaped by different selection pressures without interbreeding. Over generations, the populations diverge genetically and morphologically — beak shapes, body sizes and behaviours diverge to match the local food sources.
Effect 5 (Overall conclusion): This is an example of adaptive radiation: a single colonising ancestor diversified into multiple distinct species through isolation and adaptation to different ecological niches. The distribution of finch species across islands, combined with their different beak forms, constitutes biogeographic evidence for descent with modification driven by island isolation.
Q3.1 — Why Bali and Lombok differ (2 marks)
Despite being only ~35 km apart, Bali and Lombok are separated by a deep-water strait that has remained a barrier to terrestrial organism movement throughout geological history, including during ice ages when sea levels were lower [1]. This deep-water channel prevented land dispersal of terrestrial mammals and other land animals, so the faunas on each side developed separately — western species (with Asian affinities) could not colonise Lombok, and eastern species (with Australasian affinities) could not colonise Bali [1].
Q3.2 — What Wallace’s Line demonstrates (2 marks)
Wallace’s Line demonstrates that geographic proximity does not automatically mean evolutionary connectedness [1]. The distribution of species is determined by geological history and the presence of barriers to movement, not just by current map distance — organisms on either side of the line have different evolutionary histories because they were separated for long periods, despite being geographically close [1].
Q3.3 — Evaluate the student’s claim (2 marks)
The claim is incorrect. The deep-water barrier between Bali and Lombok functioned as an effective barrier to terrestrial organism dispersal throughout geological time, preventing the faunas from sharing evolutionary history despite their proximity [1]. Wallace’s Line marks a sharp boundary where Asian-affiliated and Australasian-affiliated faunas differ dramatically. Geographic closeness alone is not sufficient to infer shared evolutionary history — the history of barriers to movement matters more than current distance [1].