In a previous article in Fish Bytes, I wrote about the five different migration
patterns of bony fishes with emphasis on those having the anadromous
type as demonstrated by seven species of the Genus Oncorhynchus;
Chinook, coho, chum, pink, and sockeye (Pacific Ocean salmon) and
steelhead (rainbow trout) and sea-run cutthroat trout. Worldwide, native
species of the genus Oncorhynchus are associated with the North Pacific
Ocean and the coastal regions of North America and Asia. During the
Miocene epoch, about 15-20 million years ago, a common ancestor of all
the extant species in the genera Oncorhynchus and Salmo (e.g.,Atlantic
salmon and brown trout) diverged into two groups; one group evolved in
the North Pacific Ocean to become the genus Oncorhynchus and the other
group evolved in the North Atlantic Ocean to become the genus Salmo. By
10 million years ago an Oncorhynchus ancestor had diverged into two
lineages, one lineage undergoing speciation leading to today’s Pacific
Ocean salmon species. This was completed by the end of the Miocene, 5
million years ago. The five species of eastern Pacific Ocean salmon had 5
million years since the end of the Miocene for intraspecific diversification
much of which occurred from glacial refugia during the Pleistocene (from 2
million to 10,000 years ago) and in the constant changing ecosystems.
The other lineage became the North Pacific Ocean and Western North
America trout species. The trout lineage led to two evolutionary lines; one
became the rainbow trout lineage and and the other became the cutthroat
trout lineage. Through geographic and reproductive isolation, each of these
trout lineages has undergone further evolution and genetic divergence to
produce subspecies. An important feature of the five species of eastern
Pacific Ocean salmon is, to date, no species has diverged into subspecies.
This indicates anadromy provided each of the Pacific salmon species with
the opportunity for genetic variation into specific stocks and populations
within a species. However, sufficient reproductive isolation and natural
selection has not occurred within a Pacific salmon species’ geographical
range that would be necessary for the evolution of subspecies . Because
straying does occur during the spawning migration, a mature adult salmon
does not necessarily always undergo homing to its natal stream. The
2. common feature is the salmon life cycle involving anadromy with the North
Pacific Ocean providing food and room for growth and fresh water for
more effective spawning with protection of fertilized eggs and alevins in the
gravel of a redd. Although the species of Pacific salmon in the western
North Pacific Ocean and eastern Asia may intermix with the same species
of the eastern Pacific Ocean salmon, they segregate during the spawning
migration to the home stream.
Whether a Pacific salmon species is likely to evolve into subspecies during
the current Anthropocene epoch, positioned within the Holocene, remains
to be seen. This unofficial geologic time period, perhaps from the middle of
the 1800s, began with industrialization. There has been the increasing
global human population’s activities and its demands on the use of
ecosystem resources. The more recent significant impacts on the earth’s
climate and ecosystems could create conditions where subspeciation
might occur. Some salmon scientists consider the kokanee, a
non-anadromous sockeye salmon, as a subspecies. However, there is no
such official designation with a subspecific epithet for kokanee, i.e., the
kokanee remains with sockeye as O. nerka. Examples of subspecies are the
coastal rainbow trout/steelhead, O. mykiss irideus and the interior
watersheds’ redband trout/steelhead, O. mykiss gairdneri. Similarly, the
coastal cutthroat trout/sea-run cutthroat, O. clarki clarki and the west slope
cutthroat, O. clarki lewisi.
Clearly, the Pacific salmon have a long evolutionary history that is in parallel
with the geological history of their historical range. The future history of
these species now depends on the magnitude of changes to their
ecosystems during the Anthropocene. Salmon have proven to be resilient
organisms, but resiliency has its limits when an organism can no longer
adapt to relatively rapid changing features within otherwise life supporting
ecosystems.
December 22, 2021
David Beatty
