Oncorhynchus is a genus of fish in the family Salmonidae; it contains the Pacific salmons and Pacific trouts. The name of the genus is derived from the Greek onkos ("hook") and rynchos ("nose"), in reference to the hooked jaws of males in the mating season (the "kype").
Salmon and trout with ranges generally in waters draining to the Pacific Ocean are members of the genus. Their range extends from Beringia southwards, roughly to Japan in the west and Mexico to the east. In North America, some subspecies of O. clarki are native to the landlocked Great Basin, while others are native to the Rio Grande and western tributaries of the Mississippi River Basin which drain to the Gulf of Mexico, rather than to the Pacific.
Unlike many trout species of the mainly European genus Salmo, many Oncorhynchus are anadromous (migratory) and die after spawning. Some species of char (Salvelinus genus) are native to Pacific waters and are also referred to as trout.
Several late Miocene (~7 m.y.a.) trout-like fossils appear in Idaho, in the Clarkia Lake beds appear to be Oncorhynchus—the current genus for Pacific salmon and some trout. The presence of these species so far inland established that Oncorhynchus was not only present in the Pacific drainages before the beginning of the Pliocene (~5-6 m.y.a.), but also that rainbow and cutthroat trout, and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split between Oncorhynchus and Salmo (Atlantic salmon) must have occurred well before the Pliocene. Suggestions have gone back as far as the early Miocene (~20 m.y.a.).
Speciation among Oncorhynchus has been examined for decades, and to this day, a family "tree" is not completely developed for the Pacific salmonids. Mitochondrial DNA (mtDNA) research has been completed on a variety of Pacific trout and salmonid species, but the results do not necessarily agree with fossil research, or molecular research. It is generally agreed that chum, pink and sockeye salmon lineages diverged in the sequence after other species. Montgomery (2000) discusses the pattern of the fossil record as compared to tectonic shifts in the plates of the Pacific Northwest America. The (potential) divergence in Onchorhyncus lineages appear to follow the uprising of the Pacific Rim. The climatic and habitat changes that would follow such a geologic event are discussed, in the context of potential stressors leading to adaptation and speciation.
One interesting case involving speciation with salmon is that of the Kokanee, sockeye that have been landlocked. Kokanee sockeye evolve differently from anadromous sockeye—they reach the level of "biological species". Biological species—as opposed to morphological species—are defined by the capacity to maintain themselves in sympatry as independent genetic entities. This definition can be vexing because it appears that it does apply only to sympatry, and this limitation makes the definition difficult to apply. There are examples in Washington, Canada and elsewhere where two populations live in the same lake but spawn in different substrates, at different times, and eat different food sources. There is no pressure to compete or interbreed (two responses when resources are short). These types of Kokanee salmon show the principal attributes of a biological species: they are reproductively isolated and show strong resources partitioning.