Geographical Distribution

“Neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions.” — Charles Darwin

Why do kangaroos live only in Australia? Why are the Galápagos finches found nowhere else? The distribution of species across the globe poses questions that cannot be answered by climate or habitat alone. Darwin shows that biogeography—the study of where organisms live—provides some of the strongest evidence for evolution and common descent.

The Puzzle of Distribution

If species were created to fit their environments, we’d expect similar environments to have similar species. But they don’t:

Climate Doesn’t Explain Distribution

• African and South American tropics have similar climates but very different species
• Australian deserts resemble African deserts but have completely different animals
• Mountains in different continents have different alpine species despite similar conditions

Barriers and Affinity

Darwin identifies the real pattern: species distributions are explained by barriers and ancestral connections.

What Barriers Do

• Oceans separate land faunas (Australia vs. Asia)
• Mountain ranges divide populations (Andes, Himalayas)
• Deserts isolate communities
• Climate zones prevent tropical species from reaching temperate regions

Species that could thrive in an area often don’t occur there simply because they can’t get there.

Meanwhile, species that can reach each other tend to be related. The closer the connection, the more similar the fauna.

Single Centers of Creation

Darwin argues that each species originated in one place and spread from there:

Single Center of Origin

Each species arose in one geographic location and dispersed from that center. Its current distribution reflects how far it could spread given available routes and barriers. The same species isn’t independently created in multiple places.

This principle explains why species on different continents differ even in similar environments—they evolved in isolation from different ancestors, not because those environments required different creatures.

Means of Dispersal

How do species cross barriers? Darwin catalogs various mechanisms:

Ways Species Disperse

• Land bridges: Continents were sometimes connected (Bering Strait, Panama)
• Rafting: Animals and plants can float on debris across water
• Birds: Carry seeds in mud on feet, in gut, on feathers
• Wind: Spores, small seeds, and insects can be blown great distances
• Ice ages: Changed sea levels and climate zones allowed new routes

Darwin conducted experiments to test these ideas—seeing how long seeds remained viable in salt water, examining birds’ feet for transported seeds, and tracking ice age glaciation patterns.

Oceanic Islands

Oceanic islands (those that rose from the sea, never connected to continents) provide crucial evidence:

Patterns on Oceanic Islands

• Oceanic islands lack amphibians and native land mammals (can’t cross salt water)
• They have birds, insects, and plants (which can disperse)
• Island species resemble those of the nearest mainland
• Yet they’re not identical—they’ve diverged since arrival
• Islands often have many unique (endemic) species found nowhere else

The Galápagos Islands show this perfectly: species resemble South American relatives (the nearest mainland) but have evolved into unique forms on each island.

Affinity of Species Within Continents

Species on the same continent, even in different environments, tend to be related:

Continental Affinities

• Australian mammals are mostly marsupials (across all Australian environments)
• South American mammals include unique groups like armadillos and sloths
• African species share features distinguishing them from Asian equivalents

This makes sense under evolution: species on a continent share common ancestors that lived there. Under special creation, why should a desert kangaroo resemble a forest kangaroo more than it resembles a desert antelope?

The Galápagos Evidence

Darwin’s visit to the Galápagos in 1835 profoundly influenced his thinking:

What the Galápagos Revealed

• Each island has its own species of giant tortoise
• The famous finches differ from island to island
• Mockingbirds vary between islands
• All these species resemble South American forms—but are distinct

The islands are recent, close together, with similar climates. Why should each have unique species? Because colonizers from the mainland evolved separately on each island.

Key Takeaways

• Similar environments on different continents have different species
• Barriers (oceans, mountains, deserts) explain breaks in distribution
• Each species originated in one place and dispersed from there
• Species on the same continent are related regardless of habitat
• Oceanic islands have species resembling the nearest mainland—but modified
• The Galápagos finches and tortoises demonstrate evolution in action
• Biogeography makes sense under evolution; it’s puzzling under special creation