The facts: Biogeography

The theory of descent with modification accounts for some species living only in certain parts of the world even though they could have flourished just as well in other places. Plants and animals in isolated locales such as Australia, for example, tend to be especially exotic.

Generally, the more physically connected two geographic regions are, the more homogeneous its species are. Comparing similar climatic and geological environments (deserts with deserts, snowy mountains with snowy mountains, jungles with jungles, etc.), there is a certain amount of variation among North American species. There is a greater variation between North American and South American species. There is still greater variation between South American species and African species.

The same patterns holds for similar environments that are physically close but separated by natural barriers such as mountain ranges.

Some examples:

• Penguins could live just as well in the Arctic as the Antarctic, but they are found only in the Antarctic and a few places close to it.
• No frogs or other amphibians are native to isolated ocean islands, but they have thrived there when brought in by humans.
• Many South American monkey species inhabit only one side of the Amazon River.
• The Pacific and Caribbean coasts of Panama are environmentally identical, but the offshore marine communities are very different. With rare exceptions, no species of fish found off one coast is also found off the other.

This is the sort of thing we would expect if organisms tend to change over time. Interbreeding would tend to keep contiguous populations within a region homogeneous at any given time, while populations that were physically close but prevented from actual contact would almost inevitably diverge.

The fossil record, furthermore, shows that this pattern has persisted over time. Fossils of species that look closely related are found in geographic proximity, and the more they look alike, the closer their habitats were.

For one example, every fossil ever found of a creature that looks like a horse or a potential ancestor of horses has been found in North America.

Other examples include:

• Fossils of creatures that look ancestral to whales are clustered along the coast of India.
• Fossils of bipedal dinosaur-like creatures with feathers have nearly all been found in China.
• No fossil that looks intermediate between apes and humans has been found outside of Africa, with two exceptions, both of which are the two most recent apparent relatives: Homo erectus and Neanderthal man. However, Homo erectus fossils are also found within Africa, and so are the oldest Homo sapiens fossils.
• Africa is also the only habitat of gorillas and chimpanzees. According to DNA and anatomical evidence, if we are related to any other species, chimpanzees and gorillas are our nearest relatives.

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(This page last updated on August 6, 2010.)