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