September 21

The genetics and behavior of fish conventionally described as salmon and trout are straining the taxonomic bounds we have placed on them, Scott told me. I was over at his place using the internet to check email. Scott is a 41-year old student in my WFR class who was trained as an engineer but left the discipline to become a field biologist. Some coho salmon, he says, behave very much like trout. Certain members of the species will swim to the salt water a year or so after their birth, as any normal coho should, but then they make a remarkable decision. Defying the nature of their species, they turn back. Saltwater life is not for them, these rebels seem to decide, and they return upstream to spend their lives in fresh water. Recent research indicates that these fish, which never grow as large as their far more numerous oceangoing brethren, wait until more conventional coho fight their way upstream to breed and then, after the female lays her eggs, the male rebel darts into the mating melee and fertilizes a few of the eggs. A cunning sneak attack without parallel in the human life cycle.

I wonder, wouldn’t the progeny of the rebel fish have a greater propensity to remain in fresh water than the offspring of two conventional coho? And if enough of the eggs fertilized via sneak attack produce healthy offspring, wouldn’t the rebel population grow? And, assuming that the lifestyle of the rebel coho is sustainable, wouldn’t it be reasonable to assume that, at some point, rebel females will start laying their own eggs to be fertilized by male rebel coho, who at this point will seem far less rebellious? Here we would have divergent evolution, a split in the evolutionary line. Maybe the path of the rebel coho is the phylogenetic path of all freshwater fish – after all, they came from the sea somehow.

Steelheads, Scott continues, are anadromous fish (meaning that they live in the ocean but breed in freshwater streams and lakes) but are almost genetically identical to rainbow trout, which are non-anadromous. Rainbows spend their whole lives in fresh water, but there is often more genetic variation between coho that breed in different river systems than between steelheads and rainbows. And I think here, surely, we have an instance of divergent evolution. Assuming that steelheads mostly breed with other steelheads and rainbows mostly breed with other rainbows, evolution is bound to produce ontological differences between these two fishes since their different lifestyles make different traits more advantageous. Maybe, Scott says. You can’t be sure. And Jeb, what do you mean by “these two fishes?” They’re the same. “Oh,” I say.

But back home, I am still wondering. If steelheads and rainbows are almost identical genetically, they must have the same phylogeny (evolutionary history). And if their genetic past is the same, and now their behaviors differ, what explanation can we proffer but divergent evolution in action? Maybe today’s steelheads and rainbows are like yesterday’s traditional coho and rebel coho. I will ask Scott tomorrow in class. Maybe we can have an argument.

Who knows the answer? Certainly not me. But I do know this is a fascinating, fascinating world we inhabit.