Biogeography: Polar Bears & Penguins

Why does the Arctic have polar bears and Antarctica penguins?

Polar bears live in the Arctic, but not the Antarctic. Penguins live in the Antarctic, but not the Arctic. This is part of a pattern of organisms around the globe: the absence of some species from Environments which would suit them, and closer relationships between species which inhabit similar environments. This is evidence of the evolutionary origin of biodiversity.

19th century naturalists who travelled widely were fascinated by the distribution of animals and plants in their habitats around the world. One of these was Darwin.

Patterns  of biodiversity impressed Darwin. To him, the world was divided into areas by physical boundaries such as oceans, rivers, mountain chains, deserts... He found that species that had developed in one such area were different from those that developed in other places, and they were limited in living space to where they could migrate.

The distribution of flora and fauna of the oceanic islands provided Darwin with some of his strongest arguments. The islands contain a small number of species because immigration from the mainland was difficult, he said. Some groups were absent altogether, such as frogs, toads, and newts, even though they would seem to be adapted for such habitats. The reason? They are killed by saltwater, so could not reach the islands by migration. Land mammals aren't found on oceanic islands more than 300 miles from the mainland, but there are many bats, as a result of their long-distance flying ability.

Aside from the islands, Darwin was intrigued by unusual distributions of animals and plants across the continents. He concluded that changes in locations of climatic zones over time, for example, the advance and retreat of glaciers, could explain some of the patterns in animals' habitats.

Just as intriguing to Darwin, and even more apparent now, is the fact that fossils of possible ancestors of living species are often found in the same parts of the globe where their descendants live today. Darwin observed this in the South American fossils he collected, relatives of today's capybaras and armadillos. Apes today live only in Africa and Asia, and that is where the fossils most resembling modern apes are also found. There are no apes, fossil or living, known from anywhere in America.

These same patterns of biodiversity are just as impressive today, and since Darwin's day advances in science have shown how accurate his conclusions were.

So, back to the question: Why does the Arctic have polar bears and Antarctica have penguins? Well, it's a result of biodiversity: the habitats are suitable for both species, but the animals cannot migrate to the place where the other lives.