Phylogeography of the longnose dace (Rhinichthys cataractae) species group in northwestern North America — the origin and evolution of the Umpqua and Millicoma dace

The Umpqua and Millicoma dace are small cyprinid fishes endemic, respectively, to the Umpqua and Coos rivers on the central coast of Oregon. The origins and relationships of these dace are unclear; however, two hypotheses have been postulated that assume these dace had evolved from a longnose dace ( Rhinichthys cataractae (Valenciennes, 1842)) like ancestor, but from different modes of origin. The direct origin hypothesis postulates that each of these dace originated directly, but independently, from a common ancestor. In contrast, the indirect origin hypothesis postulates that the Umpqua dace originated from a R. cataractae like ancestor and that the Millicoma dace evolved from the Umpqua dace. We used mitochondrial (cytochrome b and control region) sequences to test the two hypotheses. Our maximum likelihood analysis supports the indirect origin hypothesis and argues that together the Umpqua and Millicoma dace form a distinctive Oregon coastal clade within the R. cataractae species group. We also attempt to reconcile this result with the observation that the geographic distribution of the morphologically divergent Umpqua dace is sandwiched between the geographic ranges of the morphologically similar Millicoma dace and longnose dace.