Geography Reference
In-Depth Information
pound areas would behave as species of hybrid origin, showing conflicting
relationships with different “paternal” areas.
Nodes are particularly interesting from the evolutionary biogeographic
viewpoint because they allow us to speculate on the existence of compound
or complex areas. On a global scale, some major nodes include Mesoamer-
ica, Chocó (Colombia), Fouta Djallon plateau (Guinea), Madagascar, New
Caledonia, and the area around Wallace's line in Indonesia (Heads 2004).
Nodes may represent the location of endemism, high diversity, distributional
boundaries,disjunction,anomalousabsenceoftaxa,incongruenceandcon-
vergence of characters, and unusual hybrids, among other features (Heads
2004).
In order to provide an objective procedure to identify nodes, Henderson
(1989) suggested that they may correspond to points with high density of
terminal track vertices, such as 1° vertices, which are endpoint vertices that
only have one connecting link to another point. A higher number of 1° ver-
tices is found at the periphery of a minimum-spanning tree, and the highest
density of 1° vertices occurs where different individual tracks come into con-
tact (Grehan 1991). Where 1° vertices of two individual tracks are in close
proximity, their highest density willbecloseto the boundary between the two
taxa. In cases of complex, overlapping distributions, this approach allows an
objective nodal analysis by testing a null hypothesis of random distribution
(Craw et al. 1999). In order to represent nodes graphically, Fortino and Mor-
Some authors (Craw et al. 1999; Grehan 2001a; Henderson 1989) also
considered as nodes the localities of intersection of two or more individual
tracks. These represent the geometric center of “form making” or the bound-
ary between two sister species (Henderson 1989). Nodes found at the inter-
section of generalized tracks therefore should be called generalized nodes.
The definitions and criteria for areas of endemism are complex issues
(Linder 2001; Morrone 1994b; Platnick 1991; Szumik et al. 2002; Viloria
2005). There are several definitions of areas of endemism:
• Fairly small areas that have a significant number of species that occur
nowhere else; areas delimited by the coincident distributions of taxa
that occur nowhere else (Nelson and Platnick 1981:390, 468).
