Forum on historical biogeography: what is cladistic biogeography?

Abstract

J. J. Morrone: Several papers published recently in this journal have revised and discussed some current different approaches of historical biogeography. In my opinion, some of their conclusions referring to the objectives and methods of cladistic biogeography were incorrect and may lead to some confusion. I would like to begin our exchange of opinions by discussing the question, ‘What is cladistic biogeography?’. Among these contributions, I was particularly surprised when I read van Veller et al. (2003). These authors proposed that the available cladistic biogeographical methods have been developed to implement two different research programmes. A priori methods, which allow modification of the taxon–area cladograms to deal with dispersal, extinction or duplicated lineages, in order to obtain resolved area cladograms and provide the maximum fit to a general area cladogram, are intended to implement cladistic biogeography. A posteriori methods, which deal with dispersal, extinction or duplicated lineages after the parsimony analysis of a data matrix based on unmodified taxon–area cladograms, are intended to implement phylogenetic biogeography. The outcome means that cladistic biogeography, as most biogeographers usually conceive of it, is restricted by van Veller et al. (2003) to include only component analysis, reconciled tree analysis, three area statements analysis, and paralogy-free subtrees analysis, whereas other methods, namely Brooks Parsimony Analysis (BPA) (Wiley, 1987) and component compatibility, really belong to phylogenetic biogeography. In addition to finding this terminology confusing, because ‘phylogenetic biogeography’ has been used for decades to refer to Hennig’s (1966) and Brundin’s (1966) cladistic implementation of the dispersalist approach, I am unconvinced about the existence of two different research programmes. It seems that Ebach et al. (2003) also support the distinction between cladistic and phylogenetic biogeography. But is this view justified? M. C. Ebach: I find the distinction between cladistic and phylogenetic biogeography interesting as it does delimit a theoretical rather than a methodological division in historical biogeography. As has been pointed out, the phylogenetic biogeography of Lars Brundin was the beginning of interpreting trees and cladograms from a biogeographical perspective. The taxon–area cladogram, a result of Brundin’s work, was interpreted as two separate things, on one hand as the ‘phylogenetic diversification of geographically and ecologically associated clades coincide[d]’ (Brooks, 1990; p. 16), and on the other as historical connections, that is, relationships between biotas in time (Platnick & Nelson, 1978). The two interpretations of taxon–area cladograms can be stated as thus: 1. The geographical distribution of taxa on a phylogenetic tree. 2. The distribution of areas based on the relationships of taxa on a cladogram or an areagram. Taxon–area cladograms tell us of the evolutionary history of lineages. Areagrams provide a classification of area relationships. Taxon–area cladograms still retain nodes, that is, information about individual taxa and areagrams replace nodes with components that contain information about the terminal branches. Taxon–area cladograms and areagrams tell us different things and the methods that use them have different functions and therefore provide biogeographers with different aims. The differences between taxon–area cladograms and areagrams are theoretical and not methodological. The argument used by van Veller et al. to erect a new ‘biogeographical programme’ based on comparing the validity of methods, rather than on aims or intentions, is confusing. If we wish to ‘classify’ historical biogeography into groups or research programmes, as has been attempted previously by Spellerberg & Sawyer (1999) and Crisci (2001), surely we need to clarify the theory first? Can we compare different methods if they are applied from opposing or different theoretical intentions? One example of this is in your own work where you use the Parsimony Analysis of Endemicity (PAE) (Rosen, 1984) to conduct panbiogeography (Croizat, 1958; Craw et al., 1999). Can we compare the use of the same method under two different theories, such as the implementation of PAE in BPA to score the absence and presence of areas on a taxon–area cladogram (see Ebach & Humphries, 2003) and the use of BPA in ‘phylogenetic’ and comparative phylogeography (Riddle & Hafner, 2004)?