First question is the most obvious, so I'll get it out of the way. Has anyone observed corn change into something other than corn?
Has even human-manipulated selected breeding of (just) corn produced something other than corn? I'm also not talking about GMO, but even with GMO, that isn't genetic change of the type that creates new genetic material, it just mixes existing material around.
The problem with genetic change, whether selective breeding or natural selection or whatever, is that genetic information isn't created, it's just scrambled in a different pattern. But increase in genetic information would be necessary for "amoeba to amphibian to man" evolution.
It's the same reason that wolves CAN be selectively bred to eventually produce poodles, but poodles can't be selectively bred to ever produce wolves, no matter how long it would be tried, since poodles are at the "shallow end" of the gene pool, compared to wolves, and don't have the genes necessary to come up with wolf descendants, ever.