Genes meet geology: Fish phylogeographic pattern reflects ancient, rather than modern, drainage connections

We used DNA analysis of the freshwater Galaxias vulgaris complex (Pisces: G alaxiidae) to test a geological hypothesis of drainage evolution in South I sland, New Zealand. Geological evidence suggests that the presently north-f lowing Nevis River branch of the Clutha/Kawarau River system (Otago) once R owed south into the Nokomai branch of the Mataura. system (Southland). The flow reversal is thought to have resulted from fault and fold activity asso ciated with post-Miocene uplift. Mitochondrial DNA sequence data (control r egion and cytochrome b genes; 76 individuals; maximum divergence 7.1%) corr oborate this geomorphological hypothesis: The Nevis River retains a freshwa ter fish species (Galaxias gollumoides; five sites; 10 haplotypes) that is otherwise restricted to Southland (nine sites; 15 haplotypes). There is no indication that the Nevis River lineage of G. gollumoides lives elsewhere i n the Clutha/Kawarau system (> 30 sites). Likewise, two widespread Clutha l ineages (G. 'sp D'; G. anomalus-G. pullus) are apparently absent from the N evis (> 30 sites). In particular, G. 'sp D' lives throughout much of the Cl utha (12 sites, 23 haplotypes), including a tributary of the Kawarau, bat i s absent from the Nevis itself. Conventional molecular clock calibrations ( based on a minimum Nevis-Mataura haplotype divergence of 3.0%) indicate tha t the Nevis flow reversal may have occurred in the early-mid Pleistocene, w hich is roughly consistent with geological data. The broad phylogeographic structure evident in the Clutha system is consistent with the sedentary nat ure of nonmigratory galaxiids. Our study reinforces the value of combining biological and geological data for the formulation and testing of historica l hypotheses.