Phylogeny
The most recent hypothesis for the evolution of Salmonidae as a whole places the group as having originated as freshwater organisms, as evidenced by a deposit of fossilized salmon remains (genus Eosalmo) of all ages that was found in freshwater sediments. This means that an anadromous lifestyle likely evolved in response to salmon needing a safe place to lay eggs and have fry hatch, but the rivers where that occurred not have being productive enough to support adult fish.
Due to anadromy being a shared trait between many members of Salmoninae (the sub-family containing strictly salmon, trout, and char), it has been hypothesized that anadromy is ancestral to this subgroup. This is supported by members of both genus Salmo and genus Oncorhyncus having specialized modifications for an anadromous lifestyle (relatively-large eggs relative to female body size being primary among these, large eggs being necessary to fry survival in low-nutrient environments), despite not being sister groups.
Based on the most current genetic assays, genus Salmo split off from the rest of Salmoninae (the family containing all salmon and trout) during the mid-Eocene, approximately 50 million years ago. This is likely due to the rapid cooling of the Arctic ocean, which separated the populations of ancestral salmonids from each other. [14]
Oncorhynchus, the genus containing all Pacific salmon and Pacific trout, split off from its sister genus Salvelinus (chars and relatives), before speciating in the early Miocene, approximately 20 million years ago. [14]
The intensive speciation visible in Oncorhynchus is currently hypothesized to be due to the extraordinary amount of tectonic activity that occurs around the northern Pacific basin. In prehistory, the shifting of the Earth's crust, as well as volcanic activity, created new niches for the fish to exploit. Further shifting of the crust would then isolate these rivers from one another, providing physical barriers to genetic flow between populations. [13]
Due to anadromy being a shared trait between many members of Salmoninae (the sub-family containing strictly salmon, trout, and char), it has been hypothesized that anadromy is ancestral to this subgroup. This is supported by members of both genus Salmo and genus Oncorhyncus having specialized modifications for an anadromous lifestyle (relatively-large eggs relative to female body size being primary among these, large eggs being necessary to fry survival in low-nutrient environments), despite not being sister groups.
Based on the most current genetic assays, genus Salmo split off from the rest of Salmoninae (the family containing all salmon and trout) during the mid-Eocene, approximately 50 million years ago. This is likely due to the rapid cooling of the Arctic ocean, which separated the populations of ancestral salmonids from each other. [14]
Oncorhynchus, the genus containing all Pacific salmon and Pacific trout, split off from its sister genus Salvelinus (chars and relatives), before speciating in the early Miocene, approximately 20 million years ago. [14]
The intensive speciation visible in Oncorhynchus is currently hypothesized to be due to the extraordinary amount of tectonic activity that occurs around the northern Pacific basin. In prehistory, the shifting of the Earth's crust, as well as volcanic activity, created new niches for the fish to exploit. Further shifting of the crust would then isolate these rivers from one another, providing physical barriers to genetic flow between populations. [13]