[reasonisfaith]

“when two different lines that formerly could reproduce fertile offspring can no longer do so”

This is not an observation, but a speculation. Speciation remains a myth.

“What would preclude two separate populations of the same species from accumulating enough differences in DNA over time that they could no longer reproduce between groups after sufficient time and the accompanying INEVITABLE change in DNA?”

Answer: the fact that genetic changes do not result in additional morphology.

[allmendream]

It is an observation that a line of fruit flies, that previously could produce fertile offspring with other populations of fruit flies, now can only do so within its own population.

That is not speculation. That is a fact. If your faith were dependent upon that being a “myth” it seems your faith is fragile.

Genetic changes within a population are the only thing that DO result in changes in morphology of a population.

What do YOU suppose determines our inherited morphology if not our DNA?

Besides, my question didn't ask about “additional morphology”, I asked about fertility between populations. Fertility between populations IS based upon DNA similarity, in case you were wondering and that was going to be your next wrong answer.

Here is the question again, in case you want to take another stab at it....

“What would preclude two separate populations of the same species from accumulating enough differences in DNA over time that they could no longer reproduce between groups after sufficient time and the accompanying INEVITABLE change in DNA?”