While steelhead/rainbows (Oncorhynchus mykiss) and coastal cutthroat (O. clarki clarki) can and do occasionally hybridize here, it is far less common here on the Pacific coast where the two species co-evolved (having diverged from a common ancestor a mere eight million years ago) than in the intermountain west where the rainbow is an introduced species and has, in many areas, swamped populations of the several cutthroat subspecies there. The principal mechanism of genetic separation here seems to be reproductive isolation; steelhead/rainbows preferring larger streams and rivers while the coastal cutthroat selects the smallest of tributary streams for its purposes.