Areas where the distributional areas of two or more taxa overlap are called areas of endemism. The congruence of the distributional areas of different taxa gives identity to areas of endemism. It is interpreted as primary biogeo-graphical homology, which means a conjecture on a common biogeographic history, namely, they belonged to the same ancestral biotic component.
If we map the distributional ranges of relatively well-known taxa, the substantial overlapping in their ranges determines an area of endemism. When dealing with few taxa, this represents an easy task, but when having a high number of taxa to analyze, difficulties may arise. In order to provide ways to choose objectively which taxa to map and maximize the number of taxa contributing to the areas of endemism recognized, several procedures have been developed during the last decades. The most popular is PAE, formulated originally by the paleontologist Brian Rosen. It comprises the following steps (Figure 1):
1. Draw grid cells on a map of the region to be analyzed, considering grid cells only where at least one locality of one taxon exists.
2. Construct an r x c data matrix, where r (rows) represent the grid cells and c (columns) the taxa. Entries are 1 if a taxon is present and 0 if it is absent. A hypothetical area coded 0 for all columns is added to root of the tree.
3. Perform a parsimony analysis of the data matrix; if several cladograms result, obtain a strict consensus cladogram.
4. Delimit in the cladogram obtained the groups of grid cells defined by at least two species.
5. Superimpose the groups onto the grid cells and map the species endemic to each group of grid cells, in order to delineate the boundaries of each area.
Although the extension of the grid cells to be considered in step 1 is difficult to assess a priori, an additional step could be added when the five steps have been completed. In this step, the original grid cells involved in conflictive relationships with more than one of the delimited areas, could be further subdivided in smaller units, and then the procedure restarted. When dealing with distributions of widespread species, they could overlap and generate many equally parsimonious cladograms. The strict consensus cladogram, however, preserves the most robust groupings of grid-cells, thus minimizing the influence of widespread species.
Areas of endemism are successively nested, which means that within larger areas of endemism smaller ones are recognized, and within the latter there are even smaller ones. This allows proposing a hierarchic biogeo-graphical classification that parallels the taxonomic Linnaean hierarchy, employing the following subdivisions (in decreasing size): realms (or kingdoms), regions, dominions, provinces, and districts (Figure 2).
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