Geology Reference
In-Depth Information
resulting in a desert east of the southern Andes that is virtu-
ally uninhabitable.
The distribution of continents and ocean basins not
only infl uences wind and ocean currents, but also affects
provinciality by creating physical barriers to, or pathways
for, the migration of organisms. Intraplate volcanoes, is-
land arcs, mid-oceanic ridges, mountain ranges, and sub-
duction zones all result from the interaction of plates,
and their orientation and distribution strongly infl uence
the number of provinces and hence total global diversity.
Thus, provinciality and diversity will be highest when
there are numerous small continents spread across many
zones of latitude.
When a geographic barrier separates a once-uniform
fauna, species may undergo divergence. If conditions on
opposite sides of the barrier are suffi ciently different, then
species must adapt to the new conditions, migrate, or be-
come extinct. Adaptation to the new environment by vari-
ous species may involve enough change that new species
eventually evolve.
The marine invertebrates found on opposite sides of
the Isthmus of Panama provide an excellent example of di-
vergence caused by the formation of a geographic barrier.
Prior to the rise of this land connection between North
and South America, a homogeneous population of bot-
tom-dwelling invertebrates inhabited the shallow seas of the
area. After the rise of the Isthmus of Panama by subduction
of the Pacifi c plate approximately 5 million years ago, the
original population was divided. In response to the chang-
ing environment, new species evolved on opposite sides of
the isthmus (
Figure 2.28 Plate Tectonics and the Distribution of Organisms
Caribbean
Sea
Isthmus of
Panama
Pacific
Ocean
0
500 km
(a)
a The Isthmus of Panama forms a barrier that divides a once-
uniform fauna of molluscs.
Pacific species
Caribbean species
Figure 2.28).
The formation of the Isthmus of Panama also in-
fl uenced the evolution of the North and South American
mammalian faunas. During most of the Cenozoic Era,
South America was an island continent, and its mamma-
lian fauna evolved in isolation from the rest of the world's
faunas. When North and South America were connected
by the Isthmus of Panama, most of the indigenous South
American mammals were replaced by migrants from North
America. Surprisingly, only a few South American mammal
groups migrated northward.
(b)
b Divergence of gastropod and bivalve species after the
formation of the Isthmus of Panama. Each pair belongs to the
same genus but is a different species.
 
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