Whether two specimens can produce a viable offspring in a lab is a good and simple definition. That is your boundary.
Why it exists: because random mutations are more likely to break an existing genome than create another viable genome: see “Hamlet” randomly becoming “Othello” through random copying errors.
That the experiment will take too long: not my problem, and with genetic engineering you might find ways to accelerate it. If you cannot stage any such experiment then you do not have a solid scientific proof. Too bad — stop then pretending that you do.
Your “peer-reviewed work” does not seem to make logical sense and can only defend itself by demeaning the opponents. It’s a cult, unlike real science.
Nature is not the same as a lab.
A "boundary" which exists in nature can be broken under laboratory conditions.
Species which do not interbreed in nature can be forced in a lab.
And are those offspring "viable"? What is your definition of "viable".
Viable-in-the-lab is not the same as viable-in-nature.
Indeed, genetic science advances every day, and interbreeding which was not possible, say, 25 years ago might today be routine.
So your alleged "boundary" keeps moving -- your goal-post is in constant motion.
That's why your alleged "boundary" is a total mirage.
And why do you even posit it?
What possible reason could you have for imagining something which clearly doesn't exist, and has no reason to even want to exist?
It makes no sense -- certainly not scientifically.
Theologically, of course I understand it.
Theologically it makes perfect sense, since your imaginary "boundary" could define the biblical idea of "kinds".
But there's no scientific evidence to support it.
The scientific reality is not some hard-and-fast "boundary", but rather a sliding scale of increasing difficulty to interbreed.
As I've explained now many times here, most "breeds" (i.e., of dogs) interbreed enthusiastically, while many "sub-species" interbreed only reluctantly, and related species do not interbreed except under forced conditions (i.e., a lab).
Moving up the scale, most genera cannot even be forced to successfully interbreed (i.e., African & Asian elephants), although in a few cases they do -- i.e., "beefalo".
The relevant point is this: a new "species" is created -- in your warped language "a species boundary is crossed" -- whenever previously interbreeding sub-species become so different they no longer interbreed in nature.
Simulating such a descent with modifications in a laboratory would be as simple as changing some critical DNA to prevent interbreeding of sub-species, and so by definition make a new species.
Whether such an "experiment" has ever been done I couldn't say, but what would be the point?
In nature such changes can take millions of years, and we can already see exactly what happened by comparing & contrasting DNA of different species.
annalex: "Your “peer-reviewed work” does not seem to make logical sense and can only defend itself by demeaning the opponents.
It’s a cult, unlike real science."
I accept your claim here as being nothing more than a woe-filled cry of angst against a science that you loathe and totally misunderstand.
But, if you can produce real examples of actual scientific reports which do what you claim, I'll try to be a good therapist and explain them logically.