Biogeography is the study of the geographical distributions of biological organisms. For scientists who study evolution, biogeography is often an important part of their analysis, because it provides compelling proof for their theory. This is because many geographical features, such as oceans, rivers, mountains and islands, provide barriers to species, allowing scientists to observe how they evolve separately from one another.

One of the most significant pieces of proof for evolution comes from the study of island or continental biogeography. Many of Charles Darwin's most important discoveries occurred on remote islands, such as the Galapagos. In these remote locations, Darwin noticed that there were unique species not found anywhere else.

His observation that these animals were not found in similar climate zones elsewhere on Earth was especially important. This insight yielded much of evolution's most important biogeographical proof emanates. Darwin sought to answer the question, "Why do animals on distant and isolated landmasses appear related, but distinct?" Evolution was his answer.

Since the beginnings of the theory of evolution, Charles Darwin used remote oceanic islands to show how isolated environments seemed to give rise to new species. For example, Darwin asked the question of why the Galapagos and the Cape Verde Islands, which are off the coast of northwestern Africa, have such different species, despite having nearly identical climates.

Darwin observed that the species on both islands appeared to be closely related to the species on the nearest continent. He concluded that the animals on these isolated islands must have been originally from the nearby continent, but because they were separated from the other species on the continent, they gradually evolved into something different over thousands of years.

The marsupials of Australia are another famous example of how an isolated region seems to produce unique animals that are nonetheless clearly related to animals on the nearest larger landmass. While the exact lineage of marsupials is still being debated, marsupials in South America and Australia appear to be related, despite being thousands of miles apart.

While Darwin didn't understand the concept at the time, the answer is probably related to plate tectonics. When Australia and South America were united in a single continent, an "original" marsupial species lived there, and then as the two continents separated, the marsupials on each continent gradually evolved into different species to better adapt to their new environments.

For Darwin, one of the most significant pieces of biogeographical evidence in favor of evolution was the fact that mammals - except when introduced by humans - were almost never naturally present on islands that were more than 300 miles from the nearest landmass. Why were there no mammals on islands like the Canary Islands or the Galapagos? Darwin’s explanation for the absence of mammals on islands like the Canary Islands or the Galapagos was how difficult and unlikely it would be for large terrestrial animals to travel over hundreds of miles of water to reach such isolated islands. As such, the lack of mammals on islands supports Darwin’s assertion that mammals all originally branched off at a particular point far down an evolutionary tree, on the continents, instead of arising separately on various landmasses across the planet.