[forsnax5]

... has this actually been observed to happen as a testable hypothesis, or is it just inferred based on the fossil record?

Google "ring species." There are multiple observed examples of this.

[webstersII]

"Google "ring species." There are multiple observed examples of this."

Lots of info on this. Very interesting.

But this is another example of verification by observation of existing species. I was looking for some verification of this while under observation, i.e., a test which could be repeated and verified multiple times.