That's called adaption not evolution which is what evolution always confuses.
You may give it a new name, but this exact process is what is known as "evolution" to the rest of the world. A random mutation turns out to be an advantage for a species' survival, those who don't have it die and cannot reproduce, the random mutation becomes a common trait through inheritance. Pretty easy. As you said, even you recognize that this mechanism works fine.
What you seem to be having problems with is how this mechanism could turn a fish into a land animal. You are a classic believer, they usually accept micro-evolution, but reject macro-evolution (because it makes god as a creator much less interesting). I understand that it's hard to comprehend mutations on such large timescales, but in the case of the fish, can't you see how it could be an advantage for a fish to have slightly longer and thicker fins than other fish, so that he could better travel from one shallow pool to another, or reach out of the water further to get at that juicy snail that is so very close to the pool's edge but not quite in it? Now what happens, when food in the pool actually gets rare? Certainly, the fish better able to change the pool or get at other sources of food will survive, while the others will die. Hence longer, thicker fins, better muscles to power them, a shorter, lighter tail that doesn't slow you down so much and eyes that are better suited for vision in air instead of vision in water.
I know it doesn't have lungs yet, but my biology-fu is too limited to explain this transmutation. But I don't see why this process shouldn't be useful for ANY change in ANY species, given that the external circumstances make development into this direction beneficial. Especially since there are so many fossils and even still living species that fit right into the gaps that so many religious people claim cannot be bridged.
Here is a list that orders some of those intermediate species so that you can see evolution better:
http://en.wikipedia.org/wiki/List_of_transitional_fossils The fish to tetrapods part is relevant to what I just said.