When breeding koi you weed out the deformities every week, along with the traits you don’t want, but equally keep the ones you do want, such as long finned fish from two short finned parents. I agree that genetics is mainly the issue in both the deformities we see and the different (such as long finned, albino, etc.) but I bet at some point we also find that there is slightly more to it, and that certain bad traits can also be induced by things like water quality, food, conditions, ph, etc. when fish are young.
The difference being that in man made fish these deformities don’t get weeded out, and in nature they get weeded out naturally where only the fittest survives. A long finned pike would not necessarily die young, as it is one of the predators but there is nothing to say that for every one caught 1000 have died, we just wouldn’t know (even though very unlikely) .
I’m with you guys though, there has to be a better, more scientific name for it than just long finned xxxx but I sure don’t know it!