If you seek understanding, you must decide upon a specific better question of your choice; if you leave it up to us to devise the question, we can do that, but it's overwhelming unlikely that our free choice will connect with whatever it is that led you to ask the question in the first place.
For example, I could ask you to consider a model of a fish where each bone length is controlled by some gene, and then evolution converting fins to legs is some function of the magnitude of reproduction selection pressure on those changes over multiple individuals and generations… and nothing at all to do with one individual's brain.
Or were you thinking of a specific fish species which develops legs when it reaches adulthood?
That's not even an example of a paradigm shift. Continuous transformation trivially gets us between superficial differences like mere body shape, there are many examples of us applying selection pressure to other species to make that category of thing happen, including our crops, livestock, and pets — no more than 10k years separated Chihuahuas and Corgis from Newfoundlands and Afghan Hounds. Likewise even more extreme changes between crops and their wild relatives, the wild versions are almost unrecognisable except to experts.
> You are mixing the two together and complaining the second one is non-sense, but is it?
What I'm calling nonsensical is your description. You're calling for the time complexity for a brain to invent changes that aren't caused directly by brains.
> Somehow fish had to learn to walk ("evolution from one organism to another") in a limited time including complex mechanics that our computers can't solve exactly
1) Exact solutions aren't necessary; 2) computers have solved walking; 3) the boundary between walking and floundering is an arbitrary one. And #4 after the next quote…
> I am waiting for any computational biologist to give us some mechanism behind it that is realistic,
4) One of the ways computers solve problems like this is simulated evolution.
If you don't consider the working demonstrations to be realistic, that's a "you" problem, not something the rest of us care to waste time on.
> So far nothing and people keep mentioning local adaptations only.
Do you also insist nobody has ever climbed Mount Everest on the grounds that nobody has legs long enough to do it in one step?
Or that motion pictures are impossible because each frame is stationary?
If neural spikes had anything to do with it, if any of this worked like that, bacteria wouldn't develop resistance to antibiotics (they can), and I would be able to shapeshift (I can't).
So that question is like asking for a route finding algorithm that's O(n) where n is the number of letters in the destination.