Geology shapes biogeography: Quaternary river-capture explains New Zealand's biologically ‘composite’ Taieri River

• Geological and biological data indicate a composite origin for NZ's Taieri River.
• Landforms, fish biogeography, and genetic data together reveal a dynamic history.
• This study highlights the tight links between earth history and biological evolution.

Geological processes are hypothesised to strongly affect species distributions. In particular, a combination of geological and biological data has suggested that tectonic processes can drive vicariant isolation and speciation in freshwater-limited taxa. Here we synthesise geological and biological evidence to demonstrate a composite geological and biological history for New Zealand's 290-km long Taieri River. Specifically, we assess evidence from structural geology and petrology, combined with phylogenetic and biogeographic analysis of galaxiid fishes, to show that the modern Taieri River was formed via capture of the ancestral Kye Burn during the mid-late Quaternary. Molecular dating analyses support a late-Quaternary timeframe for the geologically-mediated divergence between formerly-connected sister taxa Galaxias depressiceps and G. ‘teviot’. Fish biogeography lends further support to the geological hypothesis, as there is a substantial biogeographic disjunction between the lower- (ancestral) and upper (captured) portions of the Taieri River. Geological and biological data are assessed independently yet yield consilient patterns and timeframes for the evolutionary events inferred. Broadly, this study highlights the interplay between physical and biological processes in a geologically dynamic setting.