Geography Reference
In-Depth Information
group. In addition, the oldest known fossils of a group do not always
represent primitive or ancestral forms, and there are even examples
of recent species that show more plesiomorphic characters than fossil
species of the same taxon.
• The area where the most primitive taxa of a monophyletic group are
distributed. It is based on the progression rule of phylogenetic biogeo-
graphy (Brundin 1966; Hennig 1950), which assumes a parallelism
between the progression ofmorphological characters inthe cladogram
and the geographic distribution of the species, so that the ancestral
area is identified with the most primitive species. This criterion is as
aprioristic as the one that situates the center of origin in the area in-
habited by the most recent species of the group. On the other hand,
relictual distributions of ancient taxa would invalidate this criterion.
• The area where the largest organisms of the species are found. It as-
sumes that organisms reach their largest size in the area where condi-
tions are optimal, which occurs in the center of origin. It is similar to the
criterion that locates the center of origin in the area with more abund-
ance of organisms. Cases of gigantism on islands would invalidate this
criterion.
Common to all these criteria is the aprioristic assumption that some
areas are originally devoid of the taxa that eventually dispersed there
(Andersen 1982). In the second half of the twentieth century, some authors
(Briggs 1984; Müller 1973) reformulated this concept to refer to centers
where an important part—not necessarily the initial—of the evolutionary his-
tory of a taxon has taken place. These “centers of evolutionary radiation,”
“secondary centers of evolution,” or “centers of diversification” are identified
based on the congruence of the distributional ranges of several species. It
is interesting that the identification of these centers, at least from an op-
erational viewpoint, is similar to the discovery of areas of endemism (Mor-
rone 2002b). In fact, there are also parallels with the identification of Pleis-
tocene refugia and panbiogeographic nodes, which suggests that the same
patterns may be explained differently depending on the biogeographic ap-
proach (Contreras-Medina et al. 2001).
Briggs (1984, 1987) presented the most recent treatment of centers of
origin under a dispersalist framework. His main conclusions are as follows:
• Information available suggests that centers of origin usually are placed
in the tropics. They are large, are heterogeneous in topography, have
