Oh Steve. In the first minute of my 55-minute class, I will demonstrate experimentally the validity of "gravity." Then you can have 54 minutes to demonstrate that macroevolution occurs. Take as much time as you need. Need more time? An extra week, a month, several millenia? You get the point?
You can argue that you could demonstrate it, given enough time, but the fact is, you haven't, and neither has anyone else.
That's why you can't put "evolution" on the same level with "gravity."
Deal!
(Oops -- you've assumed that you get to pick and choose from the examples I gave. Wrong.)
What's he gonna demonstrate in 55 minutes which nobody was able to demonstrate in the decades of the fruit fly experiments? Fruit flies breed new generations every few days and they ran those experiments for decades, producing the equivalent of hundreds of thousands of years of human lifespans. They subjected the flies to everything known to cause mutations and then tried to recombine the mutations, and all they ever got was fruit flies, sterile individuals, and individuals which returned, boomarang-like, to the norm for a fruit fly. They never got anything but fruit flies.