A biogeographic view on Southeast Asia's history

Tropical Southeast Asia, and particularly the Indo-Australian Archipelago, is known for its tremendous floral and faunal biodiversity, mainly accumulated in four of the world’s biodiversity hotspots identified (Indochina, Sundaland, the Philippines, and Wallacea). Here, Southeast Asia’s biogeographic history is examined to identify areas being characterized by high levels of biodiversity (number of lineages, species richness) through time and to test whether the respective biota is mainly due to in situ diversification, immigration and/or emigration, or equilibrium dynamics. Moreover, this thesis focuses particularly on Sulawesi and its peculiar fauna to test if a vicariant origin appears plausible for certain groups and when the remarkable fish and snail radiations found in the Malili Lakes system started to diversify. To achieve this, meta-analytical and multi-disciplinary approaches are considered for an integrative historical biogeographic history of Southeast Asia and its biota by using molecular clock analyses and ancestral area estimations together with tectonic, palaeogeographic and climatic reconstructions to reveal potential causes for present-day distribution. Borneo and Indochina are identified as major ‘evolutionary hotspots’ for a diverse range of fauna and flora mainly caused by intra-island diversification and subsequent emigration since at least the early Miocene and more pronounced in the Plio-Pleistocene. In contrast, the remaining areas are rather characterized by both the accumulation of immigrants and in situ diversification, most evident for comparatively young volcanically active emergent islands such as Java and Sumatra. Sulawesi is of particular importance for testing biogeographic hypotheses as it is located between two major zoogeographic regions and has experienced a highly complex tectonic and palaeogeographic history. This includes the opening of the Makassar Strait and the consequent separation of West Sulawesi from Borneo (Sunda Shelf), the extension and westward movement of the Sula Spur and thus the separation from the Australian margin (Sahul Shelf), and the final fusion of present-day Sulawesi in the Miocene. This geological scenario allows testing for a vicariant origin of certain Sulawesi groups; however, most divergence time estimates postdate relevant vicariant events and hence suggest that the island was predominantly colonized by dispersal potentially triggered and intensified by periodic sea-level fluctuations in the Pliocene. Interestingly, a “tectonic dispersal” vicariance hypothesis from the Australian margin could not be refuted for three taxa. The Malili Lakes system (mainly Lake Matano, Lake Mahalona and Lake Towuti) in central Sulawesi is a hotspot of freshwater biodiversity, characterized by endemic species flocks such as the pachychilid snail, telmatherinid fish, atyid shrimp and gecarcinucid crab radiations. Sulawesi’s Telmatherinidae have most likely originated in the Sahul Shelf area, have probably reached the island by both marine dispersal and terrane-rafting (“tectonic dispersal”), and have colonized the lakes from rivers. Diversification within epibenthic sharpfins and the 7 predominantly pelagic to benthopelagic roundfins in Lake Matano clearly predates hybridization events with stream populations. For Lake Matano, a scenario of initial benthic-pelagic divergence after colonization of the lake by riverine populations can be assumed, followed by rapid radiation within both clades. Secondary hybridization of stream populations with sharpfins occurred more recently and hence most likely has not contributed to the initial divergence of this benthic species flock. Similarly, for the endemic freshwater snail Tylomelania a vicariant origin from the Australian margin (“out-of-Australia” hypothesis) is proposed. The BanggaiSula region of the Sula Spur is identified as a possible source area for the colonization of Sulawesi by proto-Tylomelania. Time-calibrated molecular phylogenies and parsimony network analyses show a rapid diversification of Tylomelania into eight major lineages and a strong spatial genetic structure. This is likely related to poor dispersal capabilities and altitudinal distribution limits, and prominent mountain uplifts in the late Miocene and Pliocene.