lunes, 26 de mayo de 2008

On Historical Biogeography

Historical biogeography is the study of the distribution of biodiversity over space and long periods of time. Its task is to reveal and explain the history of biotas defined as ancestor-descendant relationships. The biological importance of finding area relationships is to answer evolutionary questions, for instance, to distinguish modes of speciation (Wiley & Mayden, 1985) and reconstruct patterns of dispersal (Fritsch et al., 2001). In addition, phylogenetic studies can address the biogeographical implications of their results relating their findings to hypotheses of geological connections among areas (Wiens & Donoghue, 2004).

In the study of the distribution of biodiversity, areas of endemism are the entities compared (Linder, 2001) which are defined as the congruent distribution of at least two species (Platnick, 1991). This congruence does not demand complete agreement of those limits at all possible scales of mapping, but does require relatively extensive sympatry (Morrone & Crisci, 1995). In the same way, methods for the identification of areas of endemism that implements an optimality criterion directly based on considering the aspects of species distribution that are relevant to endemism have been developed (Szumik et al., 2002; Szumik & Goloboff, 2004).

Among the multiple approaches the Parsimony analysis of endemicity [PAE] (Rosen, 1988) appears as a tool of historical biogeography that allows the discover of patterns of organism distribution using biota similarity, but indeed, the main concern of this method is to classify areas according to the occurrence of taxa. Furthermore, it not make assumptions about processes in a subject where processes such as vicariance and extinction need to be taking into account. While vicariance refers to the geographical separation and isolation of a subpopulation, resulting in the original population's differentiation as a new variety or species., the extinction describes the process of a species becoming permanently disappearing in a local population. The dispersal is also not a rare thing taking place, where the range of the ancestral population was limited by a pre-existing barrier, which was crossed by some of its members (Crisci, 2001). In the other hand, The Panbiogeography (Croizat, 1958), although it uses distributional data as the previous approach, follows a methodology where the inferences done allow us to identify ancestral biotas and explain the distributions by tectonic and/or climate change. The method indeed, assumes the possibility of dispersal, vicariance and extinction, and its main concern is the history of biotas.

Minding the above said once you can easily imagine the factors, influencing the evolution of the species. Researchers study distributions of taxa in relation to their physical environment, historical biogeography attempts to reconstruct the origin, dispersal and extinction of taxa and biotas.