The distribution of organisms has, in addition to its ecological portion, a considerable historic component. Historical biogeography endeavors primarily to place the relationships of organisms, i.e. their phylogeny, in a geographic perspective. The biogeographic history of a group is deduced from their phylogenetic relationships. In historical biogeography, there are two contrasting schools resulting in the propagation of two approaches: vicariance and dispersal biogeography. Vicariance is the process of splitting up a continuous distribution range by e.g. geological events such as the separation of continents through tectonics. In contrast, dispersal refers to the active or passive crossing of distribution barriers. These approaches are not mutually exclusive and the importance of the relative contribution of both processes to the evolutionary history of a group in space and time has been stressed repeatedly. Scale and time frame are also important issues in studies aiming to unravel the history of distribution patterns seen today.
More and more methods in historical biogeography appear nowadays, which can be used to analyze datasets of taxon distributions. A most crucial aspect is certainly the advance of molecular systematics, which allows the fast and comparatively reliable reconstruction of the relationships of animal and plant groups. Only recently sophisticated techniques have become available to include phylogenetic information in analyses of biogeographical patterns.
It has been pointed out that most complicated and least testable are hypotheses that involve explanations of the origin of species. Among others, time and duration of the origination event of a species are usually unknown and that even DNA data, phylogenetic analyses and vicariance biogeography cannot easily solve this problem as they cannot provide an unambiguous absolute time scale. This is not a trivial problem as a precise understanding of the timing of evolutionary events is crucial for inferring biogeographical processes, particularly in areas with a sequence of rapid geological and ecological changes such as Wallacea. However, in recent years molecular clock approaches have become more and more sophisticated and previous problems with calibrations points, unequal substitution rates and the calculation of meaningful error rates for clock estimates have been mitigated.
Wallacea as a model region for the study of biogeography
The Indoaustralian archipelago is a hotspot of biogeographic research ever since it served as the ‘type locality’ of zoogeography through the studies of Alfred Russel Wallace. Wallace was most struck by what he perceived to be an abrupt boundary between the Asian and Australian bird faunas, later given the name ‘Wallace’s line’ by Huxley in 1868. The Wallace line essentially separates the Asian continental shelf zone from the oceanic islands in the East, thus separating Bali from Lombok, Borneo from Sulawesi, and Palawan from the remainder of the Philippines. Practically instantly the perceived biogeographic puzzle posed by this virtual faunal boundary prompted intense zoological research in the area aiming at finding evidence for or against the hypothesis of such line. While Wallace was not the first to propose a faunal boundary in the region - he was preceded by S. Müller in 1846 – his line started a proliferation of lines based on research on various taxa and with different aims but often little heuristic value. Best known among these lines is probably Lydekker’s line proposed in 1896, which is the eastern counterpart of Wallace’s line in following the Australian continental shelf.
The primary notion of a strict separation of Asian and Australian biota by a faunal boundary has long been abandoned through the recognition of the more complex nature of biogeographic relationships in the region. Various authors have pointed out that the fauna of the zone between the Asian and Australian continental shelves is of a transitional nature, as might be expected from an assemblage of oceanic islands. This area of ‘faunal admixture’ has been termed ‘Wallacea’ by Dickerson in 1928 and was suggested to represent a separate faunal region with unique properties, which are largely confined to an extremely high degree of species level endemism (species only occuring in this or parts of this area) in most land and freshwater organisms, though.
Map of the Malayan Archipelago showing proposed faunal boundaries and the extent of Wallacea. Murray’s line of 1866 has been omitted, it only differs from Wallace’s line in running between Java and Bali.
Origin of Wallacean biota – emerging patterns
In recent decades, new systematic techniques and even more recently the tools of molecular phylogenetics have provided some insights into the evolutionary and biogeographic relationships of many groups whose distribution extends to both sides of the Wallace line. For the vast majority of taxa examined, dispersal is the favoured explanation of present day distribution patterns (e.g. for all mammals). In some cases the ancestral areas of Wallacean taxa could be derived from phylogenetic data, and e.g. set in relation to sea-level lowstands during the Pleistocene, when distances across water were much shortened. Similarly, some areas in the region, such as e.g. Sulawesi and Borneo, were much closer to each other in the Miocene, albeit for geologic rather than climatic reasons. Some of the endemic mammals of Sulawesi are thought to have reached the island in that time, e.g. pigs, anoa, babirusa and baboons. All these taxa (or their assumed ancestors) either can survive periods of floating or are capable of swimming across shorter stretches of sea, as apparently some of the extinct species (e.g. Elephas celebensis or Stegodon sompoensis) whose fossil remains were found on Sulawesi.
For a handful of taxa, however, vicariance has been considered a more likely hypothesis than dispersal to explain their occurence in some parts of Wallacea. Not very surprisingly perhaps, these groups comprise e.g. some freshwater groups with very limited dispersal abilities, such as viviparous gastropods or shrimps.
Tectonic history of Wallacea – the link to biogeography
While it has been possible through geological work in the last few decades to determine the ages and sometimes also the regional origin of these terranes with an acceptable accuracy and precision, it is exceedingly difficult to make statements as to whether these continental fragments were above the sea-level or submerged during their westward journey. This particular aspect is of pivotal importance for biogeography, though, as submerged terranes can obviously be discounted for the transport of non-marine organisms. In the case of Sulawesi, this uncertainity has e.g. led to contrasting statements by biogeographically inclined geologists.
The Indomalayan region has an immensely complex geological and more specifically tectonic history, arising from the assemblage of the present-day configuration through the repeated accretion of continental slivers from the Australian Gondwana margin since the Devonian, with the more recent complication of an onsetting continent-continent collision (Asia – Australia). This tectonic instability is also responsible for a long history of volcanism. In this huge plate fragment puzzle, the most relevant aspect from a biogeographic perspective is the Cenozoic westward movement of Australian continental slivers. None crossed Wallace’s line, but at least three of these so-called terranes form part of Sulawesi today and could potentially have contributed to its fauna at different times. A few more terranes form part of the Moluccas and might also still harbour taxa that originated at the Australian margin.