Encyclopedia of Evolution.pdf - Online Reading Center

iogeography
Biogeography of Selected Species of Angiosperms
(Excluding Tropical Areas and the Southern Hemisphere)
Plant group North America Europe and Western Asia Eastern Asia
Fagaceae: Fagus Fagus grandiflora Fagus sylvatica Fagus crenata
Fagaceae: Castanea Castanea dentata Castanea sativa Castanea crenata
Fagaceae: Quercus
Section Lobatae Quercus rubra
Section Quercus Quercus alba Quercus ilex Quercus lanata
Section Cerris Quercus suber Quercus acutissima
Ulmaceae: Ulmus Ulmus americana Ulmus glabra Ulmus parvifolia
Betulaceae: Betula Betula nigra Betula pendula Betula utilis
Betulaceae: Alnus
Section Alnus Alnus serrulata Alnus cordata Alnus japonica
Section Clethropsis Alnus maritima Alnus nitida
Salicaceae: Salix Salix nigra Salix alba Salix tetrasperma
Salicaceae: Populus
Section Aegiros Populus deltoides Populus nigra
Section Populus Populus tremuloides Populus tremula Populus adenopoda
Aceraceae: Acer Acer saccharum Acer platanoides Acer palmatum
Europe and North America had apparently already separated
by the time the red oak subgenus (section Lobatae)
and cork oak subgenus (section Cerris) had evolved; red
oaks are found only in America, and cork oaks only in
Eurasia. Separate species within the white oak subgenus
(section Quercus) are found in America, Europe, and eastern
Asia (see table).
Further speciation occurred after the separation of North
America and Europe. California, separated by mountains
and deserts, has a completely separate set of oak species
from eastern North America.
Many plant families are largely tropical, and they have
remained in the tropical areas of the world as the continents
have moved. Some of them have a pantropical distribution,
in tropical forests all over the world, while others are found
only in the Old World or in the New World.
There are numerous examples of families, genera, or subgenera
of plants that are found only in the Northern Hemisphere,
for example, the oaks, beeches, and chestnuts; elms
(genus Ulmus); birches (genus Betula); cottonwoods (genus
Populus section Aegiros); aspens (genus Populus section Populus);
and maples (genus Acer) (see table). There are fewer
examples of families, genera, or subgenera restricted to the
temperate regions of the Southern Hemisphere in the manner
of Nothofagus. This may be because the cold climates of the
Southern Hemisphere are not as cold as those of the Northern
Hemisphere. There is almost no land surface in the Southern
Hemisphere climatic zone that corresponds to the tundra
of the Northern Hemisphere, for example. Furthermore, the
Southern Hemisphere is mostly ocean, and water does not
change in temperature as readily as does land. The gentler
temperature conditions of the Southern Hemisphere temperate
zone has apparently allowed many otherwise tropical plant
families to extend throughout South America and Africa.
In the above examples, vicariance seems to explain most
of the patterns of plant biogeography. In some cases, dispersal
appears to have also played a role. For example, most willows
(genus Salix) are found in northern continents (see table)
but a few species are found in the southern continents: Salix
humboldtiana in South America and Salix cinerea in Australia.
These may be the evolutionary descendants of willows
that dispersed southward.
Dispersal is also the major explanation for the similarity
of Eurasian and North American mammal genera. The
major modern genera of mammals (such as bears, raccoons,
deer, horses, bison, etc.) evolved after North America and
Eurasia had separated. These animals dispersed across the
Bering land bridge (Beringia). This dispersal occurred more
readily in periods between ice ages. This explains why
North America and Eurasia share many genera but not
many species of mammals. Dispersal may explain the biogeography
of groups of animals more readily than of plants.
Birds, some of which migrate thousands of miles, disperse
best of all.
Some groups of organisms are cosmopolitan or worldwide
in their distribution. This may be the result of both
vicariance and dispersal. The major examples appear to be
groups of organisms that disperse very well or live in disturbed
habitats: rodents among mammals, and plants in the
mustard (Brassicaceae), composite (Asteraceae), bean (Fagaceae),
and grass (Poaceae) families.
Placental mammals proliferated in the northern continents,
while marsupial mammals proliferated in the south-