Across the Southern Alps by river capture? Freshwater fish phylogeography in South Island, New Zealand.

We used DNA analysis of galaxiid fish to test a hypothesis of localized headwater capture in South Island, New Zealand. The restricted western, but widespread eastern, distributions of three nonmigratory freshwater fish species suggest that part of the east-flowing Waiau River has been captured by the west-flowing Buller River. However, mitochondrial control region (Kimura 2-parameter distance = 4.1-5.4%) and microsatellite flanking sequences do not support a relationship between Waiau (N = 4 fish sequences) and western populations (N = 8) of Galaxias vulgaris. Instead, the point of capture is probably to the north-east, perhaps the Nelson lakes region. Phylogenetic analysis indicates that western populations, along with populations in the north-east (N = 18), represent a previously unidentified monophyletic Evolutionarily Significant Unit, possibly a cryptic species. We suggest a general caveat for zoogeographic conclusions based on distributional data alone.