THE EMERGENCE OF VICARIANCE BIOGEOGRAPHY: FROM WEGENER TO CROIZAT AND THE CLADISTS

One of the strongest arguments that Wegener put forward to support his continental drift hypothesis derived from biogeography. The conventional interpretation of the close taxonomic relationships of Mesozoic terrestrial organisms between the southern continents was of land bridges that had subsequently foundered beneath the South Atlantic and Indian Oceans. Wegener cited this biogeographic evidence in support of his hypothesis, pointing out that neither the geological evidence (absence of granitic rocks) nor the geophysical evidence (high density of ocean floor) supported the idea of foundered continents, and that the only plausible alternative was that the Atlantic and Indian Oceans had opened in the fairly recent geological past. Later last century this notion led directly to the concept of vicariance biogeography, following the work of Croizat, which focussed on the spatiotemporal analysis of distribution patterns of organisms and is different from phenetic biogeography, which investigates similarities between biotas in terms of numbers of taxa in common. Croizat's so called generalised tracks indicated the distribution pattern of an ancient biota before it vicariated. The tracks for terrestrial organisms may cross oceans and hence could not be explained by present-day biogeography. With the general acceptance of plate tectonics by the early 1970s, Croizat's work was used to create a new school of vicariance biogeography. The early work was characterised by a polemical approach that virtually denied any validity to the alternative dispersalist school. Many have subsequently reacted against this excessive polarisation, and dismissive attitudes towards dispersalist mechanisms, but without question a new scientific rigour has been introduced, with more emphasis being placed on testing models and with ad hoc hypothesising being discouraged. An important difference quickly emerged, however, between Croizat and the other vicariance biogeographers, who supported cladistic methods of taxonomic analysis, whereas Croizat favoured conventional phenetic methods.