Biogeography
In 1876, Alfred Russel Wallace published The Geographical Distribution of Animals, documenting that the Lombok Strait — just 35 km wide — separates completely different faunas: Asian tigers, monkeys and elephants to the west; Australasian marsupials and cockatoos to the east. This boundary, now known as the Wallace Line, showed that geography and geological history, not just climate, determine which species live where.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Know
- What biogeography studies and why distribution patterns matter.
- How isolation can lead to divergent lineages.
- Key examples including marsupials, Darwin's finches and Wallace's Line.
Understand
- Why geographic barriers limit gene flow and allow populations to diverge.
- Why islands often have fewer species but higher endemism.
- How continental history helps explain present-day species distributions.
Can Do
- Use distribution patterns as evidence in an evolution argument.
- Explain how island isolation can lead to adaptive radiation.
- Interpret Wallace's Line as evidence for long-term biogeographical separation.
Core Content
Why geography can become evolutionary evidence
Stand on the western shore of the Lombok Strait in Indonesia — you are in the range of Asian elephants, tigers, and monkeys. Cross just 35 km of ocean to the eastern shore and every large mammal is different: Australasian marsupials, cockatoos, and fruit doves, with no elephants or tigers. Alfred Russel Wallace mapped this sharp faunal boundary in 1876 and called it what it is: geological history frozen in animal distributions. When a population is split by an ocean, mountain range, glacier or desert barrier, individuals on either side can no longer interbreed freely — and with gene flow stopped, mutations, natural selection and drift push the two populations in different directions.
Biogeography is the study of species distribution across geographic space. Geographic barriers (oceans, mountains, glaciers) reduce or stop gene flow between populations — allowing separated populations to diverge through mutation, selection and drift, eventually forming distinct lineages or species.
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. A species map, read carefully, is also an evolutionary map — distribution patterns that fit Earth's geological history provide strong evidence for common ancestry and descent with modification.
Barrier Forms
Oceans, mountains and glaciers can interrupt gene flow.
Isolation Matters
Separated populations experience different mutations, environments and selection pressures.
Divergence Follows
Enough time and separation can produce distinct species.
Biogeography turns physical separation into evolutionary evidence when isolated populations diverge over time.
Pause — copy the highlighted points into your book before continuing.
Biogeographical evidence for evolution comes from studying:
Marsupials, finches and the logic of isolation
We just saw that geographic barriers drive population divergence. That raises a question: what real-world examples show this happening at continental scale? This card answers it — marsupial distributions and Darwin's finches are the two classic cases.
Some of the strongest biogeographical evidence comes from patterns that fit Earth's geological history — shared ancestry on formerly connected landmasses, and adaptive radiation on isolated islands.
Australian and South American marsupials share a Gondwana origin: these landmasses were once connected, so related marsupial groups evolved from a shared ancestor before continental separation drove divergence. Darwin's finches on the Galapagos Islands demonstrate adaptive radiation — a single colonising ancestor diversified into multiple species with different beak forms adapted to different food sources, 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 |
Islands often have fewer species overall but a higher proportion of endemic species — isolation limits colonisation and promotes divergence in place. Island proximity alone does not guarantee faunal similarity — geological history of barriers matters more than current map distance.
Add the marsupial, finch and island endemism points to your notes before the check below.
Darwin's finches are a classic example of:
A sharp distribution boundary that points to deep isolation
We just saw how continental separation explains marsupial and finch distributions. That raises a question: is there evidence of a biogeographical boundary that is still visible today, even between geographically close islands? This card answers it — Wallace's Line.
Wallace's Line is a biogeographical boundary in Indonesia that separates mainly Asian fauna to the west from mainly Australasian fauna to the east — visible even between islands that appear geographically close.
Wallace's Line is a sharp faunal boundary in Indonesia, caused by deep geological history and deep-water barriers that prevented movement of terrestrial organisms even when islands were nearby. Lineages on either side experienced long periods of isolation and evolved separately — explaining why geographic closeness does not always mean evolutionary connectedness.
West of the Line
Species with stronger Asian affinities dominate.
East of the Line
Species with stronger Australasian affinities dominate.
Key Idea
Geographic closeness does not always mean evolutionary connectedness.
| 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 |
Add the Wallace's Line and biogeography reasoning points to your notes before the check below.
Geographic barriers reduce gene flow between populations, allowing them to diverge over time.
Wallace's Line separates mainly Asian fauna to the west from mainly Australasian fauna to the east.
Islands always have more species than nearby continents because they have unique habitats.
Activities
Explain the Pattern
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.
Use Wallace's Line
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.
Which statement best describes island biogeography?
Biogeography
- Biogeography studies how species are distributed across Earth.
- Distribution patterns can reflect evolutionary history and common ancestry.
Isolation
- Geographic barriers reduce gene flow between populations.
- Over time, isolated populations can diverge into distinct lineages or species.
Case Studies
- Marsupials support a Gondwana-linked history of shared ancestry and divergence.
- Darwin's finches show adaptive radiation on isolated islands.
Wallace's Line
- A sharp faunal boundary separates Asian and Australasian regions.
- It supports long-term isolation as a driver of evolutionary divergence.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Explain how geographic isolation can lead to new lineages or species.
1 mark: barrier reduces/stops gene flow · 1 mark: separated populations accumulate different mutations/selection pressures · 1 mark: diverge into distinct lineages or species over time
AnalyseBand 3–4(3 marks) 2. Use one example to explain how biogeography supports evolution.
1 mark: names a valid example (marsupials/finches/Wallace's Line) · 1 mark: describes the pattern · 1 mark: links to common ancestry/evolution
EvaluateBand 4–5(4 marks) 3. Assess the statement: "Species distributions are mainly random, so biogeography is weak evidence for evolution."
1 mark: distributions are not random · 1 mark: fit geological history · 1 mark: specific example (finches, marsupials, Wallace's) · 1 mark: judgement — biogeography is strong evidence because pattern + isolation + divergence
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Explain the Pattern
This pattern supports evolution because related marsupial groups occur on landmasses that were once connected as part of Gondwana. Before continental separation, a shared ancestral marsupial population lived across the connected landmass. After separation, the two isolated populations diverged through different selection pressures, mutations and drift. The distribution pattern — related groups on formerly connected continents but not elsewhere in the same way — fits descent with modification across geological time and is difficult to explain without shared ancestry and subsequent isolation.
Activity 2 — Use Wallace's Line
This claim is too simple. Wallace's Line shows that even geographically close islands can have sharply different faunas if deep-water barriers have prevented movement of terrestrial organisms over geological time. Faunas west of the line show Asian affinities, while faunas east show Australasian affinities — despite some islands being relatively close to each other. Island biogeography also shows that long isolation promotes endemism, so nearby islands can still develop their own distinct species if colonisation is rare and populations diverge over time. Physical proximity on a current map does not predict faunal similarity when geological barriers and isolation history are considered.
Short Answer Model Responses
SA1 (3 marks): Geographic isolation separates populations with a barrier such as an ocean or mountain range, reducing or stopping gene flow between them [1]. Once isolated, the populations accumulate different mutations and experience different selection pressures and drift [1]. Over many generations they can diverge enough to form distinct lineages or even new species [1].
SA2 (3 marks): One example is the distribution of marsupials in Australia and South America. Related marsupial groups occur on these formerly connected southern landmasses [1], which is consistent with a shared ancestry before continental separation [1]. After the landmasses split, the populations diverged, so the distribution pattern supports evolution through isolation and descent with modification [1].
SA3 (4 marks): This statement is weak because species distributions are not mainly random [1]. Biogeography reveals repeated, interpretable patterns that fit geological history and evolutionary processes [1]. 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 [1]. Therefore biogeography is a strong line of evidence for evolution because it connects distribution patterns to isolation, ancestry and divergence [1].
Isolation = divergence
Barriers reduce gene flow. Separated populations accumulate differences and can become distinct species.
Gondwana explains marsupials
Related marsupials in Australia and South America reflect shared ancestry before continental separation.
Wallace's Line = deep barrier
A sharp faunal boundary despite nearby islands — geological isolation matters more than map distance.
Most common exam trap
Assuming nearby must mean same fauna — proximity alone does not override geological history and barriers.
Rapid-fire questions on biogeography, isolation, Wallace's Line, island endemism and adaptive radiation. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaYou were asked why two islands just 35 km apart — separated by the Lombok Strait in Indonesia — have completely different faunas.
Alfred Russel Wallace's 1876 answer is now a cornerstone of evolutionary biology: the barrier is not the 35 km of present-day ocean. It is the deep-water geological trench that has separated Asian and Australasian tectonic plates for tens of millions of years — long enough to prevent all terrestrial animal movement throughout the entire period when both faunas were diversifying. Tigers, monkeys and elephants to the west; marsupials and cockatoos to the east. The same gene-flow interruption that drove speciation in L07 and L09 operated here at continental scale, and Wallace's Line is its sharpest visible boundary. Distribution reflects evolutionary history, not just present geography.