[PeterisP]

If we were to observe that it does not happen in cases where we see it now?

Students can (and do) perform experiments on evolution+natural selection on bacteria in a lab in reasonable timeframes - put in bacteria, add feed, maybe add extra mutagens or radiation, and then observe the changes in bacteria when a stressor or a particular "poison" is added - the bacteria strain changes, evolving resistance. If one would see simply the strain suffering&not changing, then it would disprove Darwinian selection.

Inter-species evolution for larger organisms is more time-consuming to study (since many generations are needed), but the existing "ring-species" such as some bird groups are a good example - if inter-species evolution would be false, then we'd expect to see distinct species that can successfully interbreed within a species, but not between species; instead, we observe a "ring" where everyone is "the same" as their neighbour, but the opposite sides of the ring are "different species".