The geographic distribution of species across the planet reflects historical processes—dispersal, isolation, speciation, and extinction—that are best explained by evolutionary theory combined with geological change.
Islands as Natural Experiments
Oceanic islands provide some of the clearest evidence for evolution, showing how isolation leads to unique species found nowhere else.
Endemism and Mainland Affinity
Approximately 90% of Hawaiian native flowering plants are endemic (found nowhere else).
Madagascar has lemurs, New Zealand has kiwis and tuataras, Hawaii has honeycreepers—each lineage absent from other landmasses.
Island species are most closely related to organisms on the nearest mainland: Galápagos finches to South American finches, Hawaiian honeycreepers to North American finches.
Adaptive Radiation
Hawaiian honeycreepers diversified into over 50 species from a single colonizing ancestor, filling niches from nectar-feeding to seed-cracking to insect-eating.
Darwin's finches (~18 species) radiated to fill different ecological roles on the Galápagos.
These radiations demonstrate how a single ancestor can diversify rapidly when empty niches are available.
Dispersal Limitations
Large mammals, amphibians, and freshwater fish are rarely native to remote oceanic islands.
This pattern reflects the difficulty of crossing ocean barriers rather than unsuitability of habitat—when humans introduce such animals, they often thrive.
Galápagos Marine Iguanas
The marine iguana (Amblyrhynchus cristatus) is the world's only marine lizard, found exclusively in the Galápagos Islands.
Diverged from land iguanas approximately 5.7 million years ago.
Evolved laterally flattened tails for swimming, blunt snouts for grazing algae, and specialized salt glands to excrete excess salt.
Can hold breath for up to 30 minutes during dives.
Demonstrates how island isolation allows evolution of unique adaptations to exploit available resources.
New Zealand: 85 Million Years of Isolation
New Zealand separated from Gondwana approximately 85 million years ago, creating what scientists call "Moa's Ark"—an isolated evolutionary experiment.
Before human arrival: 245 breeding bird species, 71% endemic.
The tuatara (Sphenodon) is a "living fossil" with an independent evolutionary history spanning over 250 million years—the only surviving member of reptile order Rhynchocephalia.
Moa (giant flightless birds) filled herbivore niches typically occupied by mammals elsewhere.
Kiwis evolved nocturnal, burrow-nesting lifestyles with keen sense of smell—traits more typical of mammals than birds.
Absence of mammalian competitors allowed birds to diversify into niches usually occupied by mammals, resulting in an unusually high number of flightless species.
Continental Distributions and Gondwana
The distribution of species across southern continents reflects the breakup of the ancient supercontinent Gondwana, which began fragmenting approximately 180 million years ago.
Gondwana Breakup Timeline
~180 Ma: Western Gondwana (Africa/South America) separated from Eastern Gondwana (Antarctica/Australia/India/Madagascar).
~132 Ma: India began separating from Australia-Antarctica.
~90-100 Ma: Africa and Madagascar split; Australia and Antarctica began separating.
~45-33 Ma: Australia fully separated from Antarctica, allowing circumpolar ocean currents and triggering Antarctic glaciation.
Marsupials
Marsupials are concentrated in Australia and South America, with fossils linking them via Antarctica.
Molecular evidence indicates a single dispersal event from South America to Australia via the Antarctic land bridge during the Early Eocene (~50 million years ago).
The dispersing ancestor was likely related to South America's monito del monte (Dromiciops).
Marsupial crown group dates to approximately 87 million years ago.
Ratites (Flightless Birds)
Ostriches (Africa), rheas (South America), emus/cassowaries (Australia), and kiwis (New Zealand) share a common ancestor.
Molecular evidence reveals a surprising finding: ratites are not a simple vicariant group separated by continental drift.
Ancient DNA shows elephant birds (Madagascar) are closest relatives of kiwis (New Zealand), not nearby African ostriches.
Nuclear DNA indicates tinamous (flying birds) nest within ratites, making the group polyphyletic.
This demonstrates multiple independent losses of flight from flying ancestors who dispersed across oceans, then convergently evolved flightlessness.
Fossil Evidence of Past Connections
Glossopteris (seed fern), Lystrosaurus (therapsid), and Mesosaurus (aquatic reptile) fossils occur across now-separated southern continents.
Lungfish and southern beech (Nothofagus) distributions track Gondwanan connections.
These patterns would be inexplicable if continents had always been in their current positions.
The Great American Interchange
The formation of the Isthmus of Panama approximately 2.7-3 million years ago connected North and South America, triggering a massive exchange of fauna between two continents that had evolved in isolation for over 100 million years.
Only three South American lineages achieved widespread success in North America: porcupines, Virginia opossum, and nine-banded armadillo.
Before 12,000 years ago, South America had approximately 25 herbivore species over 1,000 kg; most went extinct.
Research indicates disproportionate extinction of South American mammals—particularly when North American carnivores with more specialized teeth arrived around 5 million years ago.
This natural experiment demonstrates how geographic barriers create isolated evolutionary theaters, and their removal leads to competition, predation, and differential survival.
The Wallace Line and Wallacea
One of biogeography's sharpest boundaries runs through Indonesia, separating Asian and Australian fauna despite close proximity and similar climates.
The Three Lines
Wallace's Line (1859): Runs through the Makassar Strait (between Borneo and Sulawesi) and Lombok Strait (between Bali and Lombok). Marks the edge of Asian fauna.
Weber's Line: East of Wallace's Line, between Sulawesi and the Maluku Islands. Represents roughly equal balance of Asian and Australian species.
Lydekker's Line (1896): Runs along the Sahul Shelf between the Banda Islands and New Guinea. Marks the edge of Australian fauna.
The Wallacea Transition Zone
The region between Wallace's and Lydekker's lines encompasses 346,782 km², including Sulawesi, the Lesser Sunda Islands, and the Moluccas.
Contains unique endemic fauna including Komodo dragons, pygmy elephants (Stegodon), and giant rodents.
Species Distributions
Bali shares approximately 97% of bird species with Java (west of the line).
Lombok (just east of the line) shares only about 50% of bird species with Bali, despite being only 35 km away.
Most Asian mammals stop at Borneo and Java; very few reach Sulawesi.
Only one species of marsupial (flying phalanger) reaches Sulawesi from the Australian side.
Why the Barrier Persists
The Sunda Shelf connected mainland Asia to Borneo, Java, and Bali when sea levels dropped during ice ages.
The Sahul Shelf connected Australia to New Guinea during low sea levels.
Between them: deep ocean trenches (including Makassar Strait and Timor Trough) that remained submerged even when sea levels dropped over 100 meters during glacial maxima (~18,000 years ago).
This permanent deep-water barrier prevented terrestrial animal dispersal for millions of years.
Komodo Dragons
The Komodo dragon (Varanus komodoensis) provides a case study in Wallacean biogeography.
Endemic to only five Indonesian islands: Komodo, Rinca, Flores, Gili Dasami, and Gili Motang.
Originated in Australia approximately 3.8 million years ago.
Migrated from Australia to eastern Indonesia approximately 900,000 years ago.
Last representative of a relic population of large lizards that once lived across Indonesia and Australia.
Population today: approximately 5,700 individuals.
Convergent Evolution Across Continents
Isolated landmasses with similar environments independently evolve organisms with similar forms and lifestyles—a pattern predicted by evolution but difficult to explain otherwise.
Placental and Marsupial Equivalents
Marsupials in Australia and placental mammals elsewhere last shared a common ancestor over 160 million years ago, yet independently evolved remarkably similar forms:
Wolf / Thylacine: The thylacine (Tasmanian tiger) and gray wolf developed nearly parallel skull shapes and body plans. Studies show their crania develop along almost identical growth trajectories despite 160+ million years of independent evolution.
Mole / Marsupial mole: Both independently evolved reduced eyes, powerful digging forelimbs, and streamlined bodies for subterranean life.
Flying squirrel / Sugar glider: Both independently evolved patagia (gliding membranes), large eyes for nocturnal activity, and tree-dwelling lifestyles. Recent research shows they reused ancient genetic toolkits (like the Wnt5a gene) that existed since before the dinosaurs.
Significance
Similar environmental pressures produce similar adaptations independently.
Marsupials in Australia filled every ecological role that placentals filled elsewhere: carnivores, herbivores, gliders, burrowers.
This pattern demonstrates evolution's predictability: given similar selection pressures, similar solutions evolve.
Molecular Evidence
DNA analysis and molecular clocks now allow scientists to test biogeographic hypotheses by dating when lineages diverged.
Confirming Continental Drift Timing
Molecular dating of marsupial divergence (~87 million years ago for the crown group) aligns with the geological timeline of Gondwana's breakup.
The four Australasian marsupial orders diverged 67-64 million years ago, closely associated with the K-Pg boundary.
Divergence times between African and South American lineages match the timing of Atlantic Ocean formation.
Most invertebrate lineages colonized before the current high islands formed (>5.1 million years ago).
Lineages follow the "progression rule": older lineages on older islands, younger lineages on younger islands.
Hawaiian Drosophila (fruit flies) underwent explosive radiation into hundreds of species, each typically endemic to a single island.
Summary
Biogeographic patterns—island endemism, continental distributions matching geological history, sharp boundaries at persistent barriers, convergent evolution on isolated landmasses, and molecular confirmation of divergence timing—consistently support the conclusion that species distributions result from evolutionary processes operating over geological time.