TK,

I cringe everytime you mention Darwinism.

Evolution is a fact substantiated by a fossil record. Now, if you share the belief of many superstitious people that "God" created the fossil record to "test our faith", nothing more needs to be said.

Charles Darwin did not invent evolution. He wrote a book on the subject in the mid 19th Century, The Origin of Species, which pulled together a lot of ideas going around at the time and which was widely influential.

Calling evolutionary theory Darwinism is as inane as calling physics Newtonism.

One phrase from Darwin's book captured the popular imagination: "survival of the fittest"

Darwin never intended, and no serious student of evolution has ever claimed "the fittest" meant the most powerful or the strongest or the most dominant. If that were the case T-Rex and/or Saber Tooth Tigers would still be dominant predators.

Social Darwinism came into vogue in England in the late 19th Century as a perversion of the phrase "survival of the fittest" to justify the horrific conditions of the time for factory workers and to justify the dominant social order and English imperialism. It had nothing to do with any reputable scientific theories about evolution, and nothing whatsoever to do with "fitness" as Darwin used the term. Social Darwinism was, and is, crap, popularization of the lowest order.

So what is "fitness" in evolutionary terms?
Determining fitness only works in hindsight. A "fit" physical characteristic is one which has survived.

In order to predict what characteristic will prove to be fit in the future,i.e, will survive, one would need to be able to predict the future.

In general, and certainly in the modern era, when ecological systems have undergone rapid change, there has been a premium on characteristics which increase flexibility in a species.

Hazarding a guess, I would predict that catfish or their descendants will do better in the future than Steelhead or their descendants.

(The ability mammals have to form habits in lieu of hard wired behaviors increases flexibilty because it allows the mammal to deal with temporarily unchanging aspects of the environment on autopilot, freeing up attention for dealing with aspects which are changing.)

Highly specialized species (Steelhead?) are often short on flexibility and tend to go extinct quicker than less specialized species.

Humans have been here just an eyeblink on the evolutionary time scale. Hard to say whether we will survive in our present form for long. Maybe we will. Maybe we will evolve into something else. Maybe evolutionary descendants of rats will do better over the long run.

TK, as a personal favor, please stop expounding Social Darwinism. It's embarassing.