Aspects of the evolutionary history of a pair of fish species (Arripidae: Arripis): On either side of a biogeographic barrier in Southern Australian seas

Pairs of closely related species on either side of a barrier provide an opportunity to test evolutionary hypotheses. In southern Australian seas, many species pairs occur on either side of the Bass Strait, which temporarily becomes an isthmus during glacial stages, yet few evolutionary questions have been explored using these species. This thesis used partial nucleotide sequences from mitochondrial cytochrome b and COI genes and length-polymorphism in several nuclear introns in one such species pair, Arripis trutta (east of Bass Strait) and A. truttaceus (west of Bass Strait), to address some evolutionary questions. Data from the two remaining members of the family Arripidae, A. xylabion and A. georgianus, were included to provide context for interpreting patterns in the focal species pair. This research found no evidence for population subdivision in each of A. trutta, A. truttaceus and A. georgianus across their Australian distributions. This likely reflects panmixia, maintained by highly migratory life cycles and restricted breeding distributions. The molecular data also supported the currently recognised four-species taxonomy of the family. Phylogenetic reconstructions did not support the view of a sister-species relationship between the east-west species pair of A. trutta and A. truttaceus but instead suggested that these two species, together with A. xylabion, likely arose as a trifurcation (some 700,000 years ago), and so can be considered as a trio of sibling-species. All Arripis species were characterised by low genetic diversity and there was evidence for a more recent and/or severe population contraction in A. truttaceus compared to A. trutta. These different demographic signals support the a priori hypothesis that environmental conditions to the west of the Bass Strait were more severe than to the east during glacial stages and, in particular, the last glacial maximum. The results are considered in the context of the historical marine biogeography of tem