Natural selection theoretically acts on phenotype, not genotype. This clarifies how the supposed interspecies change is not the same as that of intra-species.
Please stay focused on the topic at hand.
The transitional species you claim exist are only a guess. Scientific validation requires many steps beyond guessing.
The topic on hand is that DNA variation is, despite your ignorant objections, introduced due to germline mutations passed on from parent to child. This is the EXACT SAME mechanism that leads to intra and inter species change.
There is no “micro” DNA change that only happens within species, and “macro” DNA changes that happen only during speciation.
When two populations are separated, changes would accumulate the longer the two populations remain apart. Eventually the differences can grow to such an extent that the two populations no longer look the same and can (in most cases) no longer reproduce fertile offspring.j
What is going to prevent such a process? If we see that a population is changed in DNA by 0.001% in 20 years, what is going to prevent a 1% change in 20,000 years? Wishful thinking?
The existence of Australopithocine is only a guess? Well yes, such a thing was “guessed at” by evolutionary theory, and the fossil data supports such a prediction.