Evolution__3rd_Edition

510 PART 4 / Evolution and Diversity
The test of vicariance biogeography
requires cladistic classification
Evidence from marsupials illustrates
vicariance ...
for taxa 2 and 3 are congruent, and that of taxon 4 is incongruent, with taxon 1. If the
land area A + B + C had first split into A + B and C and then into A and B, the phylogeny
of taxa 1–3 would all fit with it and the phylogeny can be understood as a series of
vicariant events. Taxon 4 does not fit. If its common ancestor occupied the whole area,
its first split suggests that the land first divided not into A + B and C, but into A + C and
B. The congruence of taxa 1–3 conform with the ideas of vicariance biogeography, but
taxon 4 does not.
Before these methods were developed a indeed before plate tectonics was widely
accepted a the Venezuelan biogeographer Léon Croizat had established that different
taxonomic groups often show correlated distributions. Croizat called them “generalized
tracks,” the distribution of any one species being its “track.” He argued that if
different species independently dispersed from centers of origin, they would not end
up with correlated distributions. Correlated distributions are more likely to result from
common vicariance events, such as plate tectonics, that split the ranges of several taxa
in the same way. Modern vicariance biogeography adds to Croizat’s ideas in two ways.
One is that we now know more details about plate tectonics. The other is the importance
of using a realistic phylogeny when testing whether different taxa have congruent
distributions.
The analyses in Figures 17.10 and 17.11 are only possible for taxa that are monophyletic
in the cladistic sense (Figure 16.4, p. 480). If a set of phylogenetic groups have
been classified into a mixture of mono-, para-, and polyphyletic groups, then even if
they have experienced the same sequence of range subdivisions, their area cladograms
need not be congruent. Look at Figure 17.11 again. The area cladograms of taxon 4 and
taxon 1 are incongruent. If taxon 4 has the phylogeny of Figure 17.11b a that is, if the
groups there are monophyletic a then the incongruency between taxa 1 and 4 implies
there must have been some dispersal events in the past (Figure 17.11d). But if the
classification of taxon 4 was paraphyletic or polyphyletic, the theory of vicariance biogeography
no longer predicts that the area cladograms of the taxa will be congruent.
There is no reason to expect different paraphyletic or polyphyletic groups to have congruent
biogeographic patterns with one another, or with monophyletic ones. It is
therefore essential for vicariance biogeography that taxa are classified cladistically, to
reflect the order of phylogenetic branching. If the classifications contain a mixture of
phenetic and cladistic taxa, any general biogeographic study is liable to become meaningless.
Let us now turn to an example from part of a larger study by Patterson (1981).
His starting point was a probable area cladogram for the marsupials (Figure 17.12a).
Recent marsupials live in Australia and New Guinea, and South and North America
(where they are represented by the opossum Didelphis). Fossil marsupials can also be
found in Europe, making five areas in the complete area cladogram for marsupials.
Now, marsupials have evolved on the same globe as all other species. If the modern
distributions of vertebrates result from a history of range splitting, they should all share
much the same geological history and their area cladograms should therefore all be
more or less congruent. What do the area cladograms for the other vertebrates look
like? Figure 17.12b reveals, for five other vertebrate groups, that the vicariance prediction
is upheld: their area cladograms are congruent. The result could in theory be
because all the taxa have dispersed in the same order and the same direction, but that
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