We mapped 112 restriction sites in the mitochondrial DNA genome of the Speckled Dace (Rhinichthys osculus), a small cyprinid fish broadly distributed in western North America. These data were used to derive a molecular phylogeny that was contrasted against the hydrographic evolution of the region. Although haplotypic variation was extensive among our 59 sampled populations and 104 individuals, their fidelity to current drainage basins was a hallmark of the study. Two large clades, representing the Colorado and Snake Rivers, were prominent in our results. The Colorado River clade was divided into four cohesive and well-defined subbasins that arose in profound isolation as an apparent response to regional aridity and tectonism. The Lower and Little Colorado River subbasins are sister to one another and (with the Upper Colorado River) form a large clade of higher-elevation populations that seemingly reflect postglacial recolonization from refugia in the Middle Colorado River. The latter subbasin is sister to the Los Angeles Basin and, thus, supports the hypothesis of an ancient connection between the two. A haplotype from the Northern Bonneville was sister to the entire Colorado River clade. The Snake River clade revealed a strongly supported Lahontan group that did not share haplotypes with surrounding basins. It contained instead scattered sites from former Pluvial Lake Lahontan, as well as from eastern California. It was, in turn, sister to the Owens River, whereas Rhinichthys falcatus was sister to this larger clade. The hypothesis of a southerly, “fishhook”-configured tributary associated with a westward-draining Pliocene Snake River was manifested by the relationship of this Lahontan clade to upper Snake and northern Bonneville localities. The Klamath/Pit and Columbia Rivers were sisters in a clade basal to all the above, which in turn supported the hypothesis of a pre-Pliocene western passage of the Snake River. Our data also suggested at least three separate ichthyofaunal invasions of California, as well as a Bonneville Basin fragmented by a north-south connection between southeastern Idaho and the Colorado River. The dual western and southern movements of R. osculus from southern Idaho argued for a northern origin, possibly associated with Tertiary Lake Idaho.
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Vol. 2004 • No. 2