Biogeography

Species are not distributed according to where they would live best. They are distributed according to where their ancestors were and where they could reach.

This distinction is what makes geographic distribution evidence. A designer placing organisms in suitable habitats, or a single dispersal from one landing point, predicts a different pattern from descent constrained by geography.

Islands#

Oceanic islands — those that formed from volcanic activity and were never connected to a continent — have a consistent and unusual biota.

Hawaii, 3,200 km from the nearest continent, had no native land mammals except a bat and no native amphibians or freshwater fish. New Zealand had no native land mammals except three bat species.

The absent groups are those that cannot cross salt water. Amphibian skin is permeable and their eggs dry out; freshwater fish cannot osmoregulate in seawater. The habitats are entirely suitable — introduced frogs, rats, and trout thrive in all these places, which is why they are now ecological problems.

Distribution tracks ability to arrive, not suitability.

Endemism and radiation#

Island species are usually found nowhere else, and typically resemble species on the nearest mainland rather than species in similar climates elsewhere.

The Galápagos are volcanic and tropical. Their plants and animals resemble those of temperate South America, 900 km east, rather than those of ecologically similar islands elsewhere. Cape Verde, at a similar latitude off Africa, has species resembling African ones.

Under a suitability model, similar environments should hold similar organisms. They do not; they hold relatives of whatever was nearby.

Continental patterns#

Marsupials. Nearly all living marsupials are in Australia and the Americas. The fossil record traces the group across Antarctica, and marsupial fossils have been recovered from Seymour Island — consistent with a South America–Antarctica–Australia land connection before break-up.

Ratites. Ostriches in Africa, rheas in South America, emus and cassowaries in Australia, kiwis and extinct moa in New Zealand, extinct elephant birds in Madagascar. All flightless, all on fragments of Gondwana.

Freshwater fish across the Atlantic. Closely related families occur in South America and Africa. Marine fish, which can cross, show no such split.

Camels. The family originated in North America. Fossils trace dispersal into Asia via the Bering land bridge and into South America via the Isthmus of Panama, leaving camels in Asia, llamas in South America, and none in North America.

Glossopteris. A fossil seed fern found across South America, Africa, India, Australia, and Antarctica, with seeds too heavy to cross oceans. This was among Wegener's original evidence for continental drift, later confirmed by plate tectonics.

Common objections#

"Animals migrated from Ararat after the flood and settled where they were suited"

The standard young-earth account, developed in detail by Answers in Genesis.

It requires the distribution to be explained by dispersal from one point in the Middle East within a few thousand years.

The pattern does not fit. Marsupials would have to cross Asia to Australia leaving no fossils along the route, while placental mammals following the same path left abundant remains. Every sloth would have to reach South America without a single one stopping anywhere else. Lemurs would have to reach Madagascar while no monkey did, across the same water.

Island endemism is the sharper problem. Species found nowhere else, resembling mainland relatives, would have to have arrived and then diverged into hundreds of distinct species within a few thousand years — a rate far beyond anything biology proposes.

"Land bridges and floating vegetation rafts explain the distributions"

Both mechanisms are real. The Bering land bridge is well documented, and rafting is the accepted explanation for how monkeys and rodents reached South America and how lemurs reached Madagascar.

Mainstream biogeography uses these routes routinely, so they are not in dispute.

They are filtered rather than general. A raft carries small animals that tolerate weeks without fresh water; it does not carry an elephant, and amphibians die in salt water. That filtering produces exactly the observed pattern of which groups are present.

Land bridges also require the geography to have existed, which is established by plate tectonics and sea-level records — the same framework that requires deep time.

"A designer could place species wherever he chose"

Logically unassailable. Any distribution whatever is compatible with a designer's unstated preferences.

That is the difficulty. A hypothesis compatible with every possible observation predicts none of them, so the actual distribution provides no support for it.

Common descent constrained by geography predicts specifics that could fail: no native amphibians on oceanic islands, island species resembling the nearest mainland, marsupials on former Gondwanan fragments, camel fossils in North America. Each was checkable and each holds.

"Convergent evolution shows similar environments do produce similar organisms"

Correct, and the cases are striking. Australian marsupials converged on forms resembling wolves, moles, mice, and flying squirrels; cacti and euphorbias converged on the same body plan on different continents.

Convergence is convergence of form, not of ancestry. The thylacine resembled a wolf externally and was a marsupial in every anatomical and genetic respect, with a pouch and marsupial dentition.

The pattern therefore supports the argument rather than undercutting it. Similar environments produce similar-looking solutions from whatever lineage is locally available, which is why Australia's wolf-analogue is a marsupial and no placental wolf evolved there independently.

What the evidence shows#

Species distribution tracks ancestry and accessibility rather than habitat suitability. Groups unable to cross salt water are absent from oceanic islands whose habitats suit them, as demonstrated by introduced populations thriving there.

Island species resemble their nearest mainland relatives rather than organisms in comparable climates elsewhere, and radiate into locally available roles.

Continental distributions match the fossil record and the reconstructed positions of landmasses, with several groups confined to fragments of a former supercontinent.

The pattern is what descent with modification constrained by geography predicts, and it was among Darwin's original lines of evidence.