Abstract
The freshwater fauna of Southern Australia is primarily restricted to the southwestern and southeastern corners of the continent, and is separated by a large, arid region that is inhospitable to this biota. This geographic phenomenon has attracted considerable interest from biogeographers looking to explain evolutionary diversification in this region. Here, we employed phylogenetic and phylogeographic approaches to evaluate the effect of this barrier on a group of four galaxiid fish species (Galaxiella) endemic to temperate Southern Australia. We also tested if continental shelf width has influenced connectivity among populations during low sea levels when rivers, now isolated, could have been connected. We addressed these questions by sampling each species across its range using multiple molecular markers (mitochondrial cytochrome b sequences, nuclear S7 intron sequences, and 49 allozyme loci). These data also allowed us to assess species boundaries, to refine phylogenetic affinities, and to estimate species ages. Interestingly, we found compelling evidence for cryptic species in G. pusilla, manifesting as allopatric eastern and western taxa. Our combined phylogeny and dating analysis point to an origin for the genus dating to the early Cenozoic, with three of the four species originating during the Oligocene-Miocene. Each Galaxiella species showed high levels of genetic divergences between all but the most proximate populations. Despite extensive drainage connections during recent low sea levels in southeastern Australia, populations of both species within G. pusilla maintained high levels of genetic structure. All populations experienced Late Pleistocene-Holocene population growth, possibly in response to the relaxation of arid conditions after the last glacial maximum. High levels of genetic divergence and the discovery of new cryptic species have important implications for the conservation of this already threatened group of freshwater species.
Highlights
Southern Australia provides an excellent geological setting for studying biogeographic patterns
High support from the cytb analysis was found between G. pusilla species, bootstrap support was lower between G. munda and G. nigrostriata
Relationships between Brachygalaxias samples based on cytb were congruent with recognised species, B. gothei rendered B. bullocki paraphyletic in the S7 gene tree
Summary
Southern Australia provides an excellent geological setting for studying biogeographic patterns. The biota that occupy these two regions have been isolated since at least Mid-Miocene (Fig. 1) [1,2] This has resulted in extreme endemism (,75% of species) across most flora and fauna in southwestern Australia, an area recognized as one of 25 worldwide biodiversity hotspots [3]. In southeastern Australia, sea level changes have repeatedly connected and isolated Tasmania from mainland Australia, resulting in a vast, but temporary, increase in terrestrial habitat during low sea levels (over 83,000 km; Fig. 1). This has resulted in a close relationship between the fauna of northern Tasmania and southern Victoria [4,5].
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