Patterns in Place

Biogeography examines the distribution of life. Why are kangaroos only in Australia? Why are cacti primarily in the Americas? These patterns aren't random; they reflect evolutionary history shaped by continental drift, migration, speciation, and extinction.

Islands: Natural Laboratories

Oceanic islands (never connected to mainlands) are crucial. Their inhabitants arrived via long-distance dispersal (wind, water, wings) and evolved in isolation.

Key Observations:

  • Endemism: High percentage of unique species found nowhere else. ~90% of Hawaiian native flowering plants are endemic. Madagascar has endemic lemurs, tenrecs, and fossa. New Zealand has kiwis, tuataras, and no native land mammals (except bats).
  • Mainland Resemblance: Island species are typically most related to species on the *nearest* mainland, not ecologically similar but distant lands. Galapagos finches resemble a South American mainland finch.
  • Ecological Gaps & Adaptive Radiation: Colonizers often radiate to fill niches unoccupied by mainland competitors. Hawaiian honeycreepers evolved >50 species with diverse beak shapes from a single finch-like ancestor. Darwin's finches (~18 species) on the Galapagos adapted beaks for different food sources.
  • Missing Groups: Native large mammals, amphibians, and freshwater fish are usually absent from oceanic islands, reflecting their poor dispersal ability across salt water.

Example: Flightless Birds: Flightlessness evolved independently in birds on islands lacking predators (Kiwi in NZ, Dodo in Mauritius, Galapagos Cormorant).

Continental Connections

Plate tectonics continuously rearranges continents. The distribution of related organisms often matches past continental configurations.

Evidence from Drifting Continents:

  • Marsupials: Concentrated in Australia, but also found in South America. Fossils link them across Antarctica, reflecting their origin and spread across the southern supercontinent Gondwana before its breakup. North American opossum is a later migrant from South America.
  • Ratites (Flightless Birds): Ostrich (Africa), Rhea (S. America), Emu/Cassowary (Australia), Kiwi (NZ) are related. Their distribution mirrors Gondwanan fragmentation.
  • Fossil Flora/Fauna: Fossils of the plant *Glossopteris* and reptiles like *Lystrosaurus* and *Mesosaurus* are found across now-separated southern continents, dating to when they were joined (Permian/Triassic).
  • Lungfish: Found only in Australia, South America, and Africa – another Gondwanan distribution pattern.
  • Southern Beeches (*Nothofagus*): These trees are found in southern South America, New Zealand, Australia, and New Caledonia, tracing Gondwanan connections.
  • Camelids: Originated in North America (fossils), migrated to South America (llamas, alpacas) and Asia (camels via Bering land bridge), went extinct in North America.

The Wallace Line: A Deep Divide

Alfred Russel Wallace noted a sharp boundary in the Malay Archipelago separating Asian fauna (west) from Australian fauna (east), despite geographical proximity and similar climates.

  • Faunal Contrast: West side has placental mammals like tigers, rhinos, monkeys, apes. East side has marsupials like kangaroos, possums, cuscus, tree kangaroos, and monotremes (echidnas, platypus - platypus is only mainland Australia). Bird families also differ significantly.
  • Geological Basis: The line corresponds to a deep-water trench (Lombok Strait) that remained a barrier even when lower sea levels during ice ages connected islands on either side via land bridges.

Significance: Demonstrates that distribution is governed by historical barriers and evolutionary lineage, not just current environment.

Conclusion: Life's Geographic Story

Biogeographic patterns provide powerful, large-scale evidence for evolution:

  • Island endemism and mainland affinities point to colonization and divergence.
  • Adaptive radiations on islands (e.g., Galapagos tortoises, Canary Island lizards) fill available niches.
  • Distributions matching continental drift (e.g., Gondwanan fauna/flora) reveal ancient connections.
  • Sharp boundaries like the Wallace Line highlight the role of historical barriers.
  • Systematic absence of certain groups from isolated landmasses matches dispersal limitations.

These distributions make sense only in light of species originating in specific locations and spreading, diverging, or going extinct over geological time, driven by evolutionary processes.