The concept of evolution is not much more than a starement that variations occur in the germ line -- an observed fact -- and that individuals differ in the number of offspring they have. Over time, populations tend to have more of the genes that favor reproductive success.
That's pretty much it in a nutshell, except for the details.
Well, the devil's in the details - as they say.
Seriously, I haven't seen anyone argue with the observations that "variations occur" and that individuals differ in the number of offspring they have. Over time, populations tend to have more of the genes that favor reproductive success.
Beyond these observations of what's "there" how can we affirm the stuff that's "not there"...ie. one species transitioning into another. That's an inference that has not currently been observed.