We have fruit flies turning into other fruit flies. And salamanders turning into other salamanders. But I question how much real speciation has been observed.
I think we have theory and assumptions and that those sometimes get confused with empirical fact.
A new species develops (often initiated by temporary environmental factors such as a period of geographic isolation) when sub-population acquires characteristics, which promote or guarantee reproductive isolation from the alternative population, limiting the diffusion of variations thereafter.
This is an empirical fact., it has been observed both in nature, and in the lab.