Evolution__3rd_Edition

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Plate tectonics causes vicariance
events
Vicariance and dispersal are two
biogeographic processes
CHAPTER 17 / Evolutionary Biogeography 505
no reason why the saber-tooth should be created in Australia with arbitrary similarities
to marsupials but in North America with arbitrary similarities to eutherians. (The
argument can be recognized as a geographic special case of the general argument for
evolution from homology a Section 3.8, p. 55.) We could now add results such as those
for the Caribbean lizards and Hawaiian fruitflies to strengthen Darwin’s case.
17.7 Geographic distributions are influenced by vicariance
events, some of which are caused by plate tectonic
movements
A second factor influencing geographic distributions is plate tectonics (informally
known as continental drift). The continents have moved over the surface of the globe
through geological time. The positions of the main continents since the Permian have
been reconstructed in some detail (Figure 17.7), and these maps immediately suggest
the reason for many biogeographic observations. For example, when we looked at the
faunal regions of the world (see Figure 17.2), we saw the difference between the faunas
of the northern and southern Indonesian Islands known as Wallace’s line. It turns out,
as can be more or less seen in Figure 17.7, that the two regions have separate tectonic
histories and have only recently come into close contact. The patterns of faunal similarity
are therefore what we should expect, given plate tectonics.
Let us look at one of the main modern research programs that studies the relation
between biogeography and plate tectonics. It is called vicariance biogeography. The
drifting apart of tectonic plates is the sort of event that could cause speciation (Section
14.2, p. 382). If the splitting of the land and of the species on it coincide, it results in two
or more species occupying complementary parts of a formerly continuous area that
was occupied by their common ancestor. This is an example of a vicariance event.
(Vicariance means a splitting in the range of a taxon.) In theory, tectonic movements
are just one process that could split a species’ range; others could include mountain
building or the formation of a river. According to the theory of vicariance biogeography,
the distributions of taxonomic groups are determined by splits (or vicariance
events) in the ranges of ancestral species.
We can contrast this idea with another, that distributions are determined more by
dispersal. Before plate tectonics was known about, or at any rate accepted, the main
process believed to alter biogeographic distributions was dispersal. Taxonomic groups
were thought to originate in one confined area, called the center of origin, and then
descendant populations dispersed away from it. Thus the geographic history of a group
could have been either a series of splits within formerly larger ancestral ranges, or a
series of dispersal events, or some mixture of the two (Figure 17.8). The events hypothesized
by the dispersal and vicariance theories took place in the past, but they are
not beyond study. Vicariance biogeographers test their idea by two methods.
One is to see whether the pattern of splitting in one group matches the geological
history of the region where it lives. The first major piece of vicariance biogeographic
research, by Brundin in 1966, was of this kind. He studied the Antarctic chironomid