I find it extremely interesting that a Coho and a Steelhead can look almost identical. Clearly they both evolved from the same relative. I wonder which one came from where? Did a Rainbow Trout decide that 10 million years ago it was going to go to the ocean to find a food source and then branch off into Pacific Salmon species, or was it the other way around?? Google time
Guess we don't fully know? They seem to have all come from the era of the early Miocene (about 20 million years ago) and the only fossil species from that era being O. rastrosus, the sabertooth salmon.
Crazy interesting!
Anadromy is a life survival strategy developed in the right conditions (access to the ocean) so given the opportunity, most coastal species have adapted it. A steelhead is not a species on its own, but a rainbow trout which possesses this life history. It did not go into the ocean to be "more like" salmon. Anadromous and exclusively freshwater varieties of the same species can be found in many species in Coastal BC - Sockeye/kokanee, landlocked coho and chinook, searun/lake coastal cutthroat trout, bull trout, dolly varden, steelhead/rainbow trout, some sculpin species.