Encyclopedia of Evolution.pdf - Online Reading Center

A map showing the approximate extents of the eight world biogeographical realms
Global Patterns of Vicariance and Dispersal
When geographical barriers split a previously unified population
or species, allowing speciation, the resulting geographical
pattern is due to vicariance. Alternatively, individuals of
a species can migrate to a new location. The resulting geographical
pattern is due to dispersal. Since each dispersal
event is independent, it is unlikely that broad-scale biogeographic
patterns are due to dispersal.
Often, biogeography on a higher taxonomic level (such
as class or family; see Linnaean system) may need to invoke
more ancient continental movements and climatic changes,
while biogeography on a lower taxonomic level (such as
genus or species) may be attributed to more recent continental
movements and climatic changes. Consider the example of
the oak family, Fagaceae:
The oak family Fagaceae apparently evolved soon after the
breakup of the supercontinent Pangaea at the end of the
Mesozoic era. Since the northern and southern continents
separated, they have remained largely separate,
except for the connection of Africa to Eurasia, the col-
biogeography
lision of the Indian subcontinent into Eurasia, and the
recent formation of the isthmus of Panama. Because
the family Fagaceae was already in existence before the
Northern and Southern Hemispheres had separated very
far, both hemispheres contain members of this plant
family.
The modern genera within the Fagaceae family evolved after
the separation of northern from southern continents.
Therefore oaks (genus Quercus), chestnuts (genus Castanea),
and beeches (genus Fagus) are restricted to the northern
continents, where they evolved (see table), while South
America and Australia have the genus Nothofagus, where
it evolved.
Eighty million years ago, North American and European forests
were not widely separated from one another. Thereafter,
climates began to change and the northern continents
drifted apart. Asian, North American, and European oaks
were separated from one another. Within North America,
the development of the Rocky Mountains and the southwestern
desert separated the western oaks (especially in
California) from the oaks of the eastern deciduous forest.