Earth is a patchwork of highly varied terrestrial and aquatic environments, and throughout geologic time these environments have undergone periodic separation and re-connection. Such changing contacts have often allowed animals and plants to reach areas that are now separate. For example, modern North America and Eurasia share many terrestrial mammal species despite the presence of the Bering Strait. Scientists hypothesize that many of these animals originated in Asia and reached North America during the Pleistocene Ice Ages, when lower sea levels created a land bridge across this strait.

Biogeography is the study of the geographic distributions of organisms. These distributions are usually uneven - different organisms live in different regions, and some species are common in places where others are entirely absent. Physical and climatic barriers prevent organisms from moving freely between areas. The distribution of each lineage reflects both its evolutionary history and the geologic history of the Earth.

Animals and plants achieve their geographic distributions through several processes. The simplest explanation for the presence of an organism is vicariance. Under this scenario, the distribution of a species reflects its area of origin. This original distribution may have been fragmented subsequently by the breakup of continents, with the result that the distributions of its descendants reflect the history of this continental breakup. Other natural barriers to dispersal can include mountain ranges, deserts, inland seaways and rivers.

A second key process is dispersal. Organisms can often disperse from one area to another despite the presence of a barrier. Land animals can reach distant islands by rafting on floating mats of vegetation, or enter new continents when lower sea levels create land bridges. Aquatic animals can reach new bodies of water through periodic flooding events or during periods of sea level rise. Dispersal events are rare, but they have occurred many times over the immense stretches of geologic time.

Finally, extinction also plays an important role in the distribution of organisms. Whether organisms achieve their distribution through vicariance, dispersal, or both, this distribution can be affected at any time by extinction. For example, a species may become extinct in some locations but not others, resulting in a patchy distribution.