Abstract
Sundaland is one of the most threatened biodiversity hotspots, experiencing a fast increase of threat levels during last decades. Covering Malayan Peninsula, Sumatra, Java and Borneo, this hotspot has one of the highest species richness and endemism for vertebrates in SEA, including freshwater fishes. This level of biodiversity has long attracted the attention of evolutionary biologists, particularly by considering effects of Sundaland complex geological history. This study addressed it by exploring time frame of vicariance and dispersal during diversity build-up of freshwater fish species in Sundaland. To support this, we first aimed to assess the match between distribution of molecular lineages from multiple taxa with palaeoriver boundaries using metadata analysis of existing molecular dataset with representative biological and spatial coverage in Southeast Asia (especially in Sundaland). Second, we focussed on estimating clades’ age and geographic distribution of Rasbora lineages in relation to the Pleistocene Palaeoriver Hypothesis by utilising newly generated empirical data for Rasborinae, a widespread and extremely diversified group of primary freshwater fishes in Sundaland. On both steps, we questioned: 1) if palaeorivers served as corridors of dispersal between islands during Pleistocene sea levels low stands; 2) if palaeoriver watersheds initiated allopatric divergence across their boundaries; and 3) if Pleistocene climatic fluctuation increased rates of species diversification. Overall, this study detected high level of cryptic diversity. Ancestral area reconstructions revealed that Sundaland freshwater fish lineages originated from Mainland Asia, and further colonised the region since Oligocene. This result validated the pre-Pleistocene settlement hypothesis. These lineages entered Sundaland mainly through North Sunda palaeoriver in contemporary Borneo and dispersed to other parts of Sundaland via long distance dispersal, often followed by in situ diversification. These results suggest Bornean part of North Sunda palaeoriver is the most likely centre of origin for Sundaland freshwater fishes. Contrary to the initial hypothesis, we found that although lowered sea level during glacial periods reconnected watersheds within palaeorivers, it did not necessarily open up inter-island dispersal channels for freshwater fishes. Corridors of savanna and seasonal forest ecosystems in the interior of Sundaland served as barrier to dispersal. Also, permeability of the physical boundaries of palaeoriver’s watersheds as well as geomorphological and habitat variabilities within palaeoriver created respectively gene flow between palaeorivers and allopatric speciation within palaeoriver. Moreover, although significant proportion of Sundaland freshwater fish lineages originated during Pleistocene, we found that Pleistocene dynamics did not affect diversification rate as sea level-dependent diversification models poorly account for species proliferation patterns for all clades excepting Channa. Besides, none of the taxa examined has declining diversification rates as suggested by diversity-dependent diversification (DDD) model. It is suggested then that global Pleistocene eustatic fluctuation and regional paleoriver dynamics are not sole drivers for Sundaland freshwater fish diversification, but only a part of abiotic aspects affecting it. Pleistocene Climatic Fluctuations likely interacted with other factors such as: landscape geomorphology, local ecosystem/habitat variability and life history traits of organisms.