Goodbye Gondwana

The southern end of the world is where the two central ideas of historical biogeography – vicariance and dispersal – have had their most intense encounter. The geography itself promotes controversy. The splinters of the megacontinent of Gondwana are now separated by vast stretches of ocean, studded with isolated chains of volcanic archipelagos. Tracks and oceanic baselines have proliferated in the vast information void of the ocean basins, which are haunted by rumours of vanished landmasses, such as Pacifica, fragmented into any number of conveniently potent pieces. All explanations seem both possible and improbable at the same time in the south. It is no accident, therefore, that some of the most extreme views in this debate have involved the 2000 km of ocean that have separated Australia and New Zealand for the last 80 Myr. Vicariance biogeography has never explained the trans-Tasman relationship well. Many features of current New Zealand biogeography map perfectly onto those of true oceanic island archipelagos such as Hawaii, strongly suggesting a history of dispersal, as does the apparently recent derivation of many Australasian groups from the Northern Hemisphere. The Australasian fossil record is clearly in conflict with a pure vicariance interpretation for many taxa. Therefore, if non-dispersal explanations were the only ones permitted, something stronger than cladistic biogeography was required. This stronger medicine came in the form of panbiogeography, modified and extended by New Zealand practitioners from the late 1970s onwards. The question they addressed was how can widespread, post-Gondwana clades on isolated continental fragments be explained without recourse to that devil child, dispersal? The panbiogeographic answer came in two forms: denial and something close to special creation. Allegedly young post-Gondwana clades were in fact old, dating back to pre-Cretaceous times in some instances. More ingeniously, it was proposed that many apparently identical taxa in widely separated locations ‘crystallized’ out of a widespread ancestor complex (for a full discussion see Mayben, 1991). No credible evidence has been produced to support these suggestions and the panbiogeography programme has been well and truly deconstructed in recent times (Cox, 1998; McDowall, 2004). Perhaps it can now be decently laid to rest. While vicariance remains a respectable and likely explanation for many disjunct biotic distributions around the globe, recent developments have undermined the preeminence it has enjoyed since plate tectonics and cladistics elevated it in the 1960s and 1970s. We are now in the middle of a dispersalist counter-revolution (see De Queiroz, 2005), fuelled by an outpouring of molecular phylogenies. To the credit of those who stuck by it, dispersal biogeography continued to advance through the latter half of the last century, despite the jibes that it was mainly a producer of ‘just-so-stories’ and despite the exhausting and unproductive task of countering the constricted worldview of cladistic biogeography and the alternative universe of panbiogeography. The recent flood of evidence supporting transoceanic dispersal (Givnish & Renner, 2004) has put it in the driving seat in the South. Moreover, increasing recognition of the damaging effects that invasive species have wrought around the globe has sharpened interest in the science of dispersal and colonization. Half a century of relative neglect means that there are a number of underexploited topics. There has been much recent exciting work that fully encompasses the dispersal paradigm. To take a few recent examples: Clark et al. (2003) tackle the thorny issues of how to model long distance dispersal (LDD) and show that parameterization might well be beyond our grasp; Ackerly (2004) looks in detail at how plant trait selection occurs during the formation of a biota by longdistance dispersal; Jordan (2001) demonstrates that recent plant dispersal events can be interrogated to get an idea of how plant ecology and morphology relate to LDD probabilities; Sanmartin & Ronquist (2004) use a meta-analysis of plant and animal phylogenies to show that different patterns among groups reveal those for which dispersal has to be invoked to explain biogeographical patterns. And, in this issue, Cook & Crisp (2005) show how the sea change with regard to dispersal is leading to a rethink of the methodologies of historical biogeography, in particular the use of phylogenies. Cladistic biogeography is greatly affected by its origin in a methodology designed to yield an unambiguous, objective classification. As dispersal was thought to overwrite in a stochastic fashion the initial parsimonic biotic pattern established by vicariance, it was assumed it interfered with the derivation of informative area relationships. Two separate approaches were used to deal to this inconvenience. Dispersal, it was argued, is not capable of falsification. As dispersal could potentially explain any pattern, it could conclusively explain no pattern in particular. Hence, it was not amenable to the usual procedures of scientific discourse as codified by Popper, and could be disregarded. As Cook and Crisp show, this ‘baby and the bath-water’ approach is simply wrong: a rich array of evidence and probabilistic methodology can be brought to bear. However, even if dispersal was regarded as unsuitable for scientific examination, it was an unpalatable fact that it actually had occurred. How to deal with this? The response was to contract the scope of historical biogeography: dispersal was irrelevant noise and could be disregarded in the search for vicariance relationships. Why disjunctions created by vicariance were regarded as worthy of study, and those that are a product of dispersal were not, has never been adequately explained. Cook and Crisp investigate what effect taking the ‘noise’ seriously will have on our interpretation of taxon–area phylogenies. They argue that the current suite of methods (Fitch optimization, Subtree analysis, DIVA, Journal of Biogeography (J. Biogeogr.) (2005) 32, 739–740