Searching for Ancestral Areas and Artifactual Centers of Origin in Biogeography: with Comment on East–West Patterns Across Southern Australia

Abstract

Stemming from acceptance of continental drift theory and development of cladistic systematics in the 1960s, interest in historical biogeography was revived. The field and methodology have since been reviewed several times (e.g., Nelson and Platnick 1981; Myers and Giller 1988; Morrone and Carpenter 1994; Humphries and Parenti 1999; Crisci et al. 2003; Ebach and Tangney 2007; Santos 2007; Parenti and Ebach 2009). In essence, early methods focussed on discovering relationships of areas based on phylogenies of taxa endemic to those areas and biogeographic patterns presented as summary area cladograms. Comparisons across different groups (clades) of organisms revealing congruent patterns were interpreted as evidence of simple vicariance, which could be compared with other data, such as geological breakup of Gondwana, whereas incongruence of patterns was interpreted as requiring a complex explanation or stochastic long-distance dispersal (Nelson and Platnick 1981). A plethora of methods to determine area cladograms arose to deal with widespread taxa (representing range expansions or a failure to differentiate), missing areas (extinctions), and other complications. But by the 1990s, there was no unifying accepted biogeographic method (Nelson and Ladiges 2001), and different methods applied to the same data gave different results, indicating that at least some of the results are artifacts of the method. What is the current trend in methodology? Most recent studies follow a similar procedure: reconstruct taxon relationships based on molecular data, estimate clade divergences using a molecular dating method (sometimes employing few or distant fossils or only secondary calibration points), and employ a method of ancestral area reconstruction to determine areas at the internal nodes of the phylogenetic tree. The Progression Rule (acknowledged or implied) is often invoked to explain the results with interpretation constrained by the estimated ages of divergences.