There's plenty of easy-to-understand examples around that are, yes, short term -- but how isn't it logical to assume that in the long-term a lot of the same processes would occur naturally regardless of our involvement?
Are you really interested in the truth of the world, or are you just seeking confirmation bias?
If people actually change guppies or any other animal into a different animal, its a people creation. This doesn't have anything to do with nature doing it by accident.
If nature did it, but not by accident, then nature is sentient.
What way other than DNA tracking of many kinds of animals and plants, over hundreds of thousands of years, gives us a positive for evolution theory evolution? Anybody can guess evolution when a skeleton of some animal is similar to that of another. But we are talking about proof in nature, not guesswork about what might have happened.
So far we aren't even close to proof. We have all kinds of guesses, and all kinds of stories to suggest how those guesses might have worked, and maybe some genetic manipulation that shows that WE can do it... but where is the proof in nature?
So far, evolution in nature is a big fat hoax.