Biogeography: what distributions reveal
What biogeography is
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Biogeography is the study of where species are distributed, and why.
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It provides evidence for evolution because distributions are explained by history — where lineages arose, and what has since allowed or prevented their spread — rather than by present conditions alone.
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The core observation is this: species are often not found in every place that would suit them.
- If species simply appeared wherever conditions were suitable, similar habitats worldwide would hold the same species.
- Instead, similar habitats on different continents hold different, unrelated species that have converged on similar forms.
- That pattern is explained by common ancestry plus barriers to dispersal, and it is difficult to explain otherwise.
Continental drift and vicariance
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Vicariance is the splitting of a once-continuous population by a physical barrier arising, such as a continent breaking apart.
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Gondwana — the southern supercontinent — began breaking up roughly 180 million years ago. New Zealand separated from it about 80 million years ago.
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If lineages were present on Gondwana before it split, their descendants should now be found on several southern landmasses, related to each other despite the ocean between them.
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This is observed:
- Southern beech (Nothofagus) occurs in New Zealand, Australia, New Guinea and South America — a distribution that makes no sense for a tree with heavy, short-dispersed seed unless the landmasses were once joined.
- Ratites — moa, kiwi, emu, cassowary, ostrich, rhea — are flightless birds distributed across southern landmasses.
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Note the important complication: molecular evidence has shown that not all such distributions are vicariance. Some lineages arrived by long-distance dispersal across ocean, long after separation. Distinguishing the two requires dating the divergence and comparing it with the date the landmasses separated.
Islands
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Islands provide the clearest biogeographic evidence, because their biotas are predictably unbalanced:
- Oceanic islands — formed by volcanism, never connected to a continent — lack whole groups that cannot cross ocean. Typically no native land mammals, amphibians or freshwater fish.
- The groups present are those that can disperse across water: birds, bats, flying insects, and plants with wind- or bird-dispersed seed.
- Island species are usually most closely related to species on the nearest mainland, indicating they arrived from there and then diverged.
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This pattern is exactly what colonisation plus divergence predicts, and it is not what would be expected if species arose wherever conditions suited them.
New Zealand as biogeographic evidence
- New Zealand shows both signals, which is why it is used so often:
- Ancient Gondwanan lineages — Nothofagus, tuatara (the only surviving rhynchocephalians), and several invertebrate groups — consistent with vicariance at separation about 80 million years ago.
- More recent arrivals by long-distance dispersal, whose molecular divergence dates are far too young for vicariance.
- The absence of native land mammals other than bats is the key island signal: bats flew here, and no other terrestrial mammal lineage could cross the ocean.
- The Saint Bathans fossil mammal complicates the simple story, showing a terrestrial mammal was present around 16–19 million years ago. This is a good example of biogeographic inference from absence being revised by fossil evidence.
Selective advantage
- Biogeography is a pattern, not a response. But it shapes the selection organisms experience:
- A lineage arriving on an island encounters vacant niches and absent predators and competitors, so selection differs sharply from the mainland — which is what drives island radiations and repeated flightlessness.
- Where a barrier prevents a competitor or predator from arriving, traits that would be disadvantageous elsewhere can be favoured. New Zealand's flightless, ground-nesting, slow-breeding birds are the consequence.
- The same reasoning explains why those species are now so vulnerable: the selective regime they were adapted to has been changed by introductions that crossed the barrier artificially.
Worked Example
Worked Example
A plant genus occurs on New Zealand, Australia and South America, and nowhere else.
- Its seeds are large and heavy, with no adaptation for wind or bird dispersal.
- The three landmasses were joined as part of Gondwana until about 80 million years ago.
- Molecular analysis gives divergence times: NZ–Australia about 75 million years; NZ–South America about 82 million years.
- A second genus shows the same three-continent distribution, but its molecular divergence times are all under 8 million years, and its seeds are small with hooked barbs that attach to feathers.
Explain what each distribution shows, and evaluate the evidence.
Answer:
Genus 1: vicariance.
The distribution matches Gondwana, and three lines of evidence agree that this is vicariance — a once-continuous population split by the continents separating.
- The divergence dates match the geology. NZ–South America at about 82 million years and NZ–Australia at about 75 million years are close to the ~80 million year separation. The lineages diverged when the landmasses did, which is precisely what vicariance predicts.
- The order matches too. South America diverged earlier than Australia, consistent with the sequence in which the landmasses parted — a detail that would be an odd coincidence under any other explanation.
- The seeds could not have crossed. Large, heavy seeds with no wind or bird dispersal adaptation cannot cross thousands of kilometres of ocean. So dispersal is not a plausible alternative.
The ancestral population was therefore continuous across Gondwana. When the landmasses separated, gene flow stopped, and the isolated populations diverged by mutation, selection and drift until they became separate species — allopatric speciation on a continental scale, with continental drift providing the barrier.
Genus 2: long-distance dispersal.
The distribution looks identical, but the evidence points to a completely different history.
- The dates are far too young. Divergence under 8 million years is roughly ten times more recent than the separation of the landmasses. The lineages cannot have been split by continental drift, because they had not yet diverged when the continents parted — they did not exist as separate lineages then.
- The seeds can cross oceans. Small seeds with hooked barbs attach to feathers, so seabirds and migratory birds could carry them between landmasses. There is a plausible mechanism.
So this genus reached the three landmasses by long-distance dispersal across existing ocean, and diverged afterwards.
Why the comparison is the point.
The two genera have the same distribution but entirely different histories. This shows that a distribution pattern on its own cannot establish vicariance — the pattern is equally consistent with dispersal. What distinguishes them is dating the divergence and comparing it with the geological date, supported by whether a dispersal mechanism is plausible.
This has been a genuine and important revision in biogeography. Many southern distributions were assumed to be Gondwanan on pattern alone, and molecular dating has since shown a substantial proportion to be much more recent dispersal.
Evaluating the evidence.
Strengths. The conclusion for each genus rests on independent lines converging — molecular dating, geological dating, and seed dispersal biology. Each could be wrong alone, but they have unrelated sources of error, so their agreement is convincing. The two genera also act as controls on each other: the same pattern with different dates and different dispersal biology gives different answers, which shows the method is discriminating rather than just fitting a story.
Limitations.
- Molecular dates depend on clock calibration, which uses fossils. Rates vary between genes and lineages, so the dates carry real uncertainty — probably enough that 75 and 82 million years should be read as approximate.
- Absence of a dispersal adaptation does not prove dispersal was impossible. Rare, chance events over millions of years — rafting on floating vegetation, transport in mud on a bird's feet — can move organisms that appear incapable of crossing. Vicariance should be the conclusion because the dates fit, not merely because dispersal seems unlikely.
- Extinction distorts distributions. The genus may once have occurred elsewhere and been lost there, so the present distribution is an incomplete record of the past. Fossil evidence would help test this.