Consider the long held theory that trout feed opportunistically sometimes, and selectively at other times. The theory goes that when there are few insects of a specific species on the water, the trout will feed opportunistically - they feed on anything that drifts or floats past them. That is, if they are in the [mood] to feed and there is no hatch, they will feed on whatever is available.
When there is a hatch, it is said that trout feed selectively. The theory goes that there are many insects of the same species on the water, and therefore, the trout become accustomed to to them and will only feed on them - selectivity being that the trout will forsake all but the abundant (hatching) bug on the water. They are being "selective".
Who makes up this stuff?
I believe that trout feed opportunistically all the time. They can't afford to be selective. How can they? Their number one instinct is survival, so how could they afford to be selective? Yes, when they are in a feeding mode and a hatch is not taking place, they will feed on anything that passes by them that they sense is food. And yes, when they are in a feeding mode and an insect is hatching in abundance, they will feed only on the insects that are hatching, and ignore all others.
But it is not selective feeding, it is survival - when an insect is hatching and many of them are available, of course the trout will feed only on them. It speaks directly to their survival instinct - why take anything else but the bug that is hatching? They can identify it as food, it's easy to feed on (little energy is needed because the prey is a clear and obvious food item), and identifying anything else is a waste of time.
Trout don't waste time. They cannot afford to. When they identify a food source that is abundant (a hatch) they will feed on only that. It is an opportunity that minimalizes the need to waste any trial and error effort.
I used the term mode, as opposed to mood, to describe the trout's behavior when their bodies tell them they need to feed, intentionally. One of the problems I think with fly fishing theories is that we tend to anthropomorphize trout behavior. i.e. "trout think". They do not think, otherwise I might have said "mood" to describe a trout whose biological clock tells it that it needs to eat. As in, "You know, I'm damn hungry, and there are a load of Pale Morning Duns on the water surface that I am going to sip in like pretzels and beer."
Trout behave, therefore, they cannot be selective. Selectivity implies that they are making an intellectual choice.
Trout do not think. They behave, and that behavior has been evolving for eons.
I'm not done with this one.............more to come.