Observation: Interbreeding populations gradually change over multiple generations in response to pressures of natural selection in relationship to heritable (or genetic) differences of the individuals.
Theory: When an interbreeding population splits into two separate groups, both groups will gradually change over multiple generations in different ways until they are no longer the same.
Theory: When an interbreeding population splits into two separate groups, both groups will gradually change over multiple generations in different ways until they are no longer the same.
First, your observation is correct. However, is contradicted by your "theory", as the populations in your "observation", seem to be able to interbreed(at least there is no mention of them not interbreeding). Also, you will need to define "natural selection" as this seems to be the basis for the "change". This "natural selection" seems like more of a candidate for a "theory" than your theory.
Second, your "theory" does not match/explain your "observation" but is in essence a completely separate observation (and quite possibly hard to prove has been observed). Therefore this fails the Scientific Method and is not a theory at all.
W.K.