Many salmonid species can interbreed and even produce fertile offspring, but in natural circumstances such crossbreeding occurs only rarely . Reproductive isolation is normally assured by such things as different spawning timing and different preferences in spawning habitat. Cutthroat/rainbow hybridization, for instance, is not a serious problem between coastal cutthroat (Oncorhynchus clarki clarki) and coastal rainbow/steelhead (O. mykiss irideus) in the coastal rivers where they evolved together. In this case, although the spawning timing is similar, the preferred spawning locations are quite different. In areas where the rainbow is an introduced species (including most of the western US outside of the coast and the Columbia River basin) crossbreeding with the other cutthroat subspecies has become a very serious problem, bringing several of them near to extinction.