Geography Reference
In-Depth Information
on the cladistic relationships between the taxa and their geographic distribu-
tion to postulate hypotheses on relationships between areas. If several taxa
show the same pattern, such congruence is evidence of common history. A
cladistic biogeographic analysis comprises three basic steps: (1) construct-
ing taxon-area cladograms from the taxonomic cladograms of two or more
different taxa by replacing their terminal taxa with the areas they inhabit, (2)
obtaining resolved area cladograms from the taxon-area cladograms (when
demanded by the method applied), and (3) obtaining a general area clado-
gram, based on the information contained in the resolved area cladograms.
Taxon-area cladograms are obtained by replacing the name of each ter-
minal taxon in the cladograms of the taxa analyzed with the area where it
is distributed. Their construction is simple when each taxon is endemic to
a single area and each area has only one taxon, but it is more complex
when taxonomic cladograms include widespread taxa, redundant distribu-
tions, and missing areas. In these cases, some methods require that tax-
on-area cladograms be turned into resolved area cladograms (Morrone and
Crisci 1995; Nelson and Platnick 1981). General area cladograms based on
the information from the different resolved area cladograms represent hy-
potheses on the biogeographic history of the taxa analyzed and the areas
where they are distributed.
Because the geographic distributions of taxa have limits, and these limits
are repeated for different taxa, they allow the recognition of biotic compon-
ents. Once they have been identified, they may be ordered hierarchically
and used to provide a biogeographic classification. This stage of the analys-
is takes place before cenocrons are elucidated and a geobiotic scenario is
proposed.
Biogeographic regionalization implies the recognition of successively
nested areas for which classically the following five categories have been
used: realm, region, dominion, province, and district. Sometimes it is more
difficult to determine the exact boundaries of two realms or regions, and
authors have described transition zones. These zones represent events of
biotic hybridization, promoted by historical and ecological changes that al-
lowed the mixture of different biotic components.
