Geography Reference
In-Depth Information
phyletic groups. The three assumptions show an inclusion relationship be-
cause topologies obtained under assumption 0 are included within those ob-
tained under assumption 1, and those obtained under assumption 1 are in-
cluded within those from assumption 2 (
fig. 5.2
).
Are there any biogeographic processes implicit in the assumptions?
Under assumption 0, we assume that the pattern is due exclusively to vicari-
ance. When we apply assumption 1, in addition to vicariance, we recognize
the possibility that some species have not responded to it with speciation or
that some species have became extinct. Finally, assumption 2 adds the pos-
sibility of dispersal to the processes implicit in the previous assumptions.
Some authors prefer assumption 2, especially to deal with widespread
taxa (Humphries 1989, 1992; Humphries et al. 1988; Morrone and Car-
penter 1994; Nelson and Platnick 1981), considering that widespread taxa
are a source of ambiguity, because a future analysis can show that a wide-
spread taxon really represents two or more different taxa, not necessar-
ily related, and inhabiting different areas; a taxon may have a widespread
distribution due to dispersal; and a taxon may have a wide distribution be-
cause it did not respond with speciation to a vicariance event. Other authors
accept the informative value of widespread taxa, thus preferring assump-
tion 0 (Brooks 1990; Enghoff 1996; Wiley 1988a; Zandee and Roos 1987).
Enghoff (1995) and van Veller et al. (1999) considered assumption 2 to be
less informative because it offers more solutions than assumptions 0 or 1.
Zandee and Roos (1987), Wiley (1988a), Enghoff (1996), and van Veller
et al. (1999, 2000) argued that assumptions 1 and 2 distort the phylogen-
etic relationships between the terminal taxa of the taxon-area cladogram.
However, Page (1989a, 1990a) indicated clearly that assumptions 1 and
2 are interpretations about relationships between areas, not between taxa.
The main criticism of assumption 0 is that it is too restrictive, not consider-
ing the possibility of dispersal to explain the distributions of widespread taxa
(Page 1989a, 1990a).
