Where organisms live is not random. Distribution patterns across islands, continents and tectonic boundaries can reveal shared ancestry, long periods of isolation, and evolutionary divergence that would be hard to explain if species were fixed and independent.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Make your first call before the examples start shaping your answer.
1. If Australia and South America are far apart today, why might scientists still compare their animal groups when studying evolution?
2. If two islands are close to each other, would you expect their species to be more similar or less similar than species on very distant islands? Why?
Write your first reasoning now. We will revisit it once the geographic case studies are in place.
Write your initial answer in your book, then return later to compare it with your final explanation.
Core Content
Why geography can become evolutionary evidence
Biogeography is not just map work. It asks why certain organisms occur in some places and not others, and whether those patterns make more sense under common ancestry and divergence than under fixed, unrelated creation.
When a population is split by an ocean, mountain range, glacier or desert barrier, individuals on either side can no longer interbreed freely. With gene flow reduced or stopped, mutations, natural selection and drift can push the two populations in different directions. Over long periods, the result can be distinct lineages or even separate species. Geographic isolation therefore becomes a mechanism that helps explain why related organisms are distributed in predictable ways.
Marsupials, finches and the logic of isolation
Some of the strongest biogeographical evidence comes from patterns that fit Earth's geological history.
Marsupial mammals occur mainly in Australia, with related forms in South America, but not across the rest of the world in the same way. That pattern makes sense if these landmasses were once connected as part of Gondwana and their faunas later diverged after separation. It is much harder to explain as a random independent distribution. Darwin's finches provide a second classic case. A single colonising ancestor on the Galapagos Islands appears to have diversified into multiple species with different beak forms adapted to different food sources. That pattern is adaptive radiation driven by isolation and different ecological opportunities.
| Case Study | Pattern Observed | Evolutionary Interpretation |
|---|---|---|
| Australian and South American marsupials | Related marsupial groups on formerly connected southern landmasses | Shared ancestry before continental separation, followed by divergence |
| Darwin's finches | Multiple finch species on isolated islands with different beaks and diets | Adaptive radiation from a common ancestor under different selection pressures |
| Island endemics | Many species occur nowhere else | Isolation allows unique lineages to evolve and persist |
Island biogeography therefore supports evolution in two linked ways. First, nearby islands tend to share more similarities than extremely distant islands because colonisation is easier across shorter distances. Second, once populations arrive and become isolated, endemism increases because new lineages evolve in place.
A sharp distribution boundary that points to deep isolation
Wallace's Line is a biogeographical boundary in Indonesia that separates mainly Asian fauna to the west from mainly Australasian fauna to the east.
This boundary is powerful because it cuts across islands that may appear geographically close, yet their faunas differ sharply. The reason lies in deep geological history and the collision zone of Asian and Australian tectonic plates. Even when sea levels changed, deep-water barriers limited movement of terrestrial organisms. As a result, lineages on either side experienced long periods of isolation and evolved separately.
| Biogeographical Pattern | What It Suggests | Why It Supports Evolution |
|---|---|---|
| Sharp faunal boundary at Wallace's Line | Long-term isolation between regional populations | Isolation allows divergence and separate evolutionary histories |
| High island endemism | Species evolved locally after colonisation | Shows divergence in isolated environments |
| Marsupial distribution across southern landmasses | Shared history linked to continental movement | Distribution fits descent with modification across geological time |
Activities
A map shows related marsupial groups in Australia and South America, with very different mammal distributions across other continents. Explain why this pattern supports evolution, and include the role of continental history in your answer.
Link the distribution pattern to shared ancestry and later isolation.
Map the continents in your book first, then summarise the explanation here.
A student says, "If islands are close together, their animals should always be the same." Use Wallace's Line and island biogeography to evaluate this claim.
A strong answer should mention barriers to movement, faunal regions and endemism.
Draft the evaluation in your book, then write your tighter final version here.
Biogeography becomes powerful evidence when distribution is treated as history, not just location. A species map matters because it can reveal old connections, barriers to gene flow, and the kinds of divergence expected under evolution.
If your original answer assumed nearby places must always share the same fauna, the key correction is this: geological barriers, colonisation history and long isolation can matter more than simple map distance.
Assessment
Answer first, then read the explanation
1. What is biogeography primarily concerned with?
What is NOT biogeography primarily concerned with?
2. Why does geographic isolation often lead to divergence?
3. Darwin's finches are a classic example of:
Darwin's finches are a classic instance of:
4. What does Wallace's Line show most clearly?
What is NOT does Wallace's Line show most clearly?
5. Which statement best describes island biogeography?
1. Explain how geographic isolation can lead to new lineages or species. (3 marks)
1 mark: barrier reduces gene flow | 1 mark: populations accumulate differences | 1 mark: divergence/speciation outcome
2. Use one example to explain how biogeography supports evolution. (3 marks)
1 mark: valid example | 1 mark: distribution pattern | 1 mark: evolutionary interpretation
3. Assess the statement: "Species distributions are mainly random, so biogeography is weak evidence for evolution." (4 marks)
1 mark: judgement | 1 mark: pattern is not random | 1 mark: use a case study | 1 mark: evaluative conclusion
Answers
SA1: Geographic isolation separates populations with a barrier such as an ocean or mountain range, reducing or stopping gene flow between them. Once isolated, the populations accumulate different mutations and experience different selection pressures and drift. Over many generations they can diverge enough to form distinct lineages or even new species.
SA2: One example is the distribution of marsupials in Australia and South America. Related marsupial groups occur on these formerly connected southern landmasses, which is consistent with a shared ancestry before continental separation. After the landmasses split, the populations diverged, so the distribution pattern supports evolution through isolation and descent with modification.
SA3: This statement is weak because species distributions are not mainly random. Biogeography reveals repeated, interpretable patterns that fit geological history and evolutionary processes. For example, Darwin's finches on the Galapagos Islands show adaptive radiation from a common ancestor after colonisation and isolation, while Wallace's Line shows a sharp faunal boundary caused by long-term separation. Therefore biogeography is a strong line of evidence for evolution because it connects distribution patterns to isolation, ancestry and divergence.
Say each answer aloud before moving to the next prompt