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:

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:

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