Trout phylogeny

[DonCooksey]

Brian:

Any chance that there could have been migration/exchange between Pacific and Atlantic fish populations through the northwest passage during warm periods between ice ages?

[Digger]

I hope there's not going to be a test on this!?

[DonCooksey]

No worries. Finals are still a few weeks off yet.

[DocEsox]

Perhaps I was a bit rash to infer the continental divide didn't have anything to do with species and their radiation....it just had little to do with the branching of the salmonidae family of fishes. The CD was considered to be formed by about 60 to 70 million years ago.....long before the salmonids we know today started branching out. Came across a recent publication outlining the current thinking on separation of the genus Salmo from Oncorhynchus. Most believe that it was cooling of the Arctic Ocean about 15 million years ago (ma) that seperated the common progenitor of Salmo and Oncorhynchus into Pacific and Atlantic oceans and the fish were forced to migrate south.....this ultimately lead to their subsequent divergences in the Atlantic and Pacific. This means that each group developed anadromy independently.

I haven't found much information on exactly where it is felt that the genus Salvelinus branched from Oncorhynchus....only that it was after the separation of Salmo from Oncorhynchus and after pacific salmon branched off the rest of the Oncorhynchus genus leading to trout. Nearly all the recent, detailed phylogenetic studies show that Salvelinus is indeed the sister taxa to Oncorhynchus....not Salmo. Although it is interesting there are a few, though rare, natural Salvelinus X Salmo hybrids (which are all sterile) with tiger trout but no known Salvelinus X Oncorhynchus hybrids.

Salvelinus, as mentioned before, developed exclusively from a north to south direction as they are the most cold adapted salmonid. The common progenitor for the Charrs developed into 3 lineages, of which 2 are exclusively North American and monophyletic (only 1 full species)…this would be lake trout and brook trout. The third is arctic charr which has an uncertain lineage and is polyphyletic having numerous subspecies and strains….all of which have still not been delineated or organized completely…..these include arctic charr (3 main North American subspecies), dolly varden (a north and south subspecies) and bull trout. Brook trout evolved only in the northeast which is their entire native range. They exist farther south than any other charr as they have developed an ability to live in water which is warmer than any other charr, similar to the temperature range of rainbow trout.

The continental divide does have some issues with speciation during and since the last glacial age which ended 10,000 or so years ago. The original cutthroat progenitor first invaded the west coast probably in the Columbia River drainage. They spread deep into the Rocky Mountains and as time progressed they were cut off from the ocean. This lead to geographic isolation which then gave rise to the 14 subspecies of cutthroat. With only a few exceptions these developed without other salmonids around and is why they are so susceptible to replacement by nonnative species….they did not evolve in a competitive ecosystem. The only subspecies which did was coastal cutthroat trout (O. clarkii clarkii) and because they evolved with rainbow they were able to keep their reproductive isolation and their was little hybridization. But stocking with nonnative rainbows can break down the barriers of reproductive isolation…..we have really screwed things up with so many of these fish by not understanding what was going on when for decades we threw stocked trout everywhere. All subspecies of cutthroat have been reduced to just a fraction of their original native ranges….two are extinct and a couple of others pretty darn close to extinct.

Rainbows developed later beginning with redband trout and the most recent evolution being the coastal rainbow trout (Oncorhynchus mykiss iredeus).

Study hard….the test will be sometime next month…….

Brian