Evolution__3rd_Edition

512 PART 4 / Evolution and Diversity
Biogeographic distribution
Ecology History
The formation of the Isthmus of
Panama ...
. . . led to an encounter between
North ...
Vicariance Dispersal Figure 17.13
The relationship between different explanatory dichotomies.
by speciation within each island, but that their distributions within each island do not
fit with vicariance. Also, many of the frutifly species in the Hawaiian archipelago
evolved after dispersal between islands. We can be sure that speciation was not simply
by splits in the range of a larger species, because the younger islands did not even exist
while the flies were on the older islands.
In summary, dispersal and vicariance form two historic alternatives (Figure 17.13).
Much the same point can be made about the distinction between them as was made for
ecological and historic factors. In any particular case, either dispersal or vicariance may
have been exclusively at work. The area cladogram of Brundin’s midges was likely generated
by vicariance, but the area cladogram of the Hawaiian fruitflies and tarweeds was
likely generated by dispersal among an emerging archipelago of volcanic islands. The
two processes can also operate together. The challenge is to work out the relative contributions
of the two.
17.8 The Great American Interchange
The processes of plate tectonics and dispersal have both contributed to the events that
take place when two previously separate faunas come into contact. These events are
called biotic interchanges, and several are known from the history of life. The most
famous is the Great American Interchange. Its deep geological cause is probably connected
with the tectonic processes that have been raising the Andean mountains for the
past 15 million years or so. The rate of this mountain building has varied from time to
time, but during a period between 4.5 and 2.5 million years ago it intensified. At the
same time a maybe 3 million years ago a the modern Isthmus of Panama rose out of
the sea and the South and North American continents were reconnected. The connection
had dramatic repercussions for the fauna, most noticeably the mammalian fauna,
of the southern continent.
North and South America had been connected before, over 50 million years earlier.
They may have had similar mammalian inhabitants, but the Cretaceous mammals of
South American are too poorly known to be sure. Then, likely in the late Paleocene, the
two halves of the American continent drifted apart. At that time, the modern orders of
mammals a the groups such as horses, dogs, and cats that are still the dominant land
vertebrates a evolved in North America, Africa, and Europe; however, South America
..