Forum on Biogeography : Introduction

many champions. Originally, in pre-evolutionary Europe, biogeography was unified by the common aim of uncovering the centre of origin, a concept derived from biblical texts. Organisms either were created in the places they were found or they moved there from elsewhere (Buffon, 1766; Sclater, 1858). Whether or not the organisms evolved was not an issue in biogeography. Organisms had a centre of origin (either by creation or evolution) from which they moved, thus forming the strange distribution patterns both in living and fossil species. The advent of Darwinian evolutionary theory, a process (natural selection) proposed to explain biogeographical distribution, was seen to be a unifying theme. Organisms had one centre of origin. Ernst Haeckel, who was deeply influenced by Darwin’s work, proposed a centre of origin for mankind. At first he believed it was the lost island of Lemuria, sunk off the coast of Pakistan. In a later revision he moved it to present day Afghanistan (see Haeckel, 1876). Earth at this time was thought to be static, continents were set rigid and only oceans and climate were seen to be dynamic. The unifying theme of biogeography relied on the actions of ocean currents and climate to explain odd distributions of living and fossil taxa. Matthew (1915), Darlington (1957), Simpson (1965) and MacArthur & Wilson (1967) were champions of static Earth biogeography, a theme united by dispersals and centres of origin. But unity did not last long. The discovery of diverging plate margins after the Second World War was the final clinching argument for continental drift and a dynamic Earth (see Hess, 1962). The works of Taylor (1910), Wegner (1915), du Toit (1937) and Carey (1976) finally came to the forefront. The role of continental drift explained disjunct fossil distributions, but more importantly it highlighted the speed at which plates could move and topology could change. Léon Croizat was the first to champion the idea that Life and Earth evolved together as a unifying theme for biogeography (Croizat, 1958, 1964). Donn Rosen (1978), Gareth Nelson, Norman Platnick (see Nelson & Platnick, 1981), Robin Craw, Michael Heads and John Grehan (see Craw & al., 1999) developed Croizat’s ideas further. The search for centres of origin was a task that no longer unified biogeography. Earth was dynamic, older areas were impossible to find, and many living species had a poor fossil record. The cladistic revolution in systematics also highlighted the need for monophyletic groups in order to discover historical patterns of taxa (Williams & Ebach, 2004). Biogeography under the Croizatian unification was historical and focused on discovering patterns and then explaining them. Discovery for some, however, is not separate from explanation or mechanical processes (see Hull, 1988). Proponents of phylogenetic systematics are convinced that transformational optimizations in phylogenetic trees offer the best way to approach biogeography. All phylogenetic lineages have separate centres of origin which by way of discovery, offer a better explanation for distributions and diversity. Recently, Brooks (in press) and Donoghue & Moore (2003) have argued that Life and Earth, in fact, do not evolve together, thus leaving the pursuit of centres of origin and direction of dispersal once again open for debate. Naturally each author realises the impact of a dynamic Earth, but not as the main aim of biogeography. A similar reaction had occurred in molecular systematics. The advent of molecular data in systematics and its eventual focus on biogeography is the next and latest unification in biogeography. All unifications before were based on morphological data and aimed at species level and above. Molecular data in biogeography, championed by phenetists such as Sokal (1979), were concerned with comparing genetic with geographical distance. Unification came in the form of the most accurate measurement for genetic distance and genetic relationships. Phylogeography is now the leading molecular biogeographical theory. The island biogeographers stemming from MacArthur & Wilson (1967) relied on unification via statistical measurement of diversity and proposing accurate models with which to predict future and past distributions. The dynamic Earth had little effect on island biogeography, as it is still mostly concerned with ecology, or simply biological interactions. Walter (2004) states that unification can be achieved by integrating “all available