[templar]

Suppose I crossbreed plant A and plant B, yielding plant C. Suppose I further crossbreed plant C with other plants, yielding D, E, and F, until I finally get an example that will not crossbreed with either A or B. Is this a new species, or would we then be forced to conclude that not all members of a species can interbreed with one another?

Well, don't just speculate about it, Do it then present the results. That way we can deal with facts in the real world, not imaginary speculation.

Coffea Arabica is one such example - it cannot interbreed with either its closely related species, or with its ancestral species.

That is because they are different, and independent, species. What is the ancestral species, and how do you demonstrate that the 'ancestral' species produced it ( a new species)? Being in the same family does not prove that one species of that family gave rise to another of that family. Again, don't just make speculative statements, demonstrate it.

[MikeJ]

Well, don't just speculate about it, Do it then present the results. That way we can deal with facts in the real world, not imaginary speculation.

I can take the time to research just such an example, but I won't bother if you will simply dismiss it. Let's agree on the rules in advance:

If I find an example of a plant, repeatedly crossbred by humans, that can no longer crossbreed with one of its ancestral species, will you agree that this represents a new species of plant?