"we can't see it therefore it isn't real" gravity makes you look like an idiot if you say that. and we can see it, you just have to be willing to put on the time and hope some bacteria can develop a noticeable change. that's right, you can see evaluation happen with some Petri dishes and an environment suitable for bacteria to live (as long as the bacteria aren't already perfectly suited to that environment and they can pass on the traits).