Not sure what you mean, but if the population of a single species gets seperated (say by geology -- earthquake or flood or something), and are seperated long enough with no cross-breeding, the seperated species often eventually change into two species.
This can be chance --- like, say if the population was 50% black and 50% white --- then the "event" (say flood with a new river) occurs, and you end up with a population that is 60% white on one side and 60% black on the other --- you'd generally get two distinct populations of black and white whatevers.
(They recently found a population of frogs where this happened that recently go re-mixed --- they generally don't cross breed, although physically very similar, apparently b/c cross-bred offspring are not very viable -- croaks being the only ready distinction)
Or, to use the more typical form of evolition, if one type of predator is one one side and not on the other, that population would start to pre-dispose whatever trait helped avoid that preditor (assuming any live, that is).
As far as "how far can they go" and not be the same species? (which I think is the question) --- the answer is, it depends.
The simplified definition of a species is usually: can they have viable children?
And the answer to that is: just depends on how far the DNA changes, on a case-by-case basis.
Too simplified to be operational; it doesn't apply to two men but it does to two earthworms.
What you stated is basically how I unserstand evolution, I was just amazed by the article's statement: We want to differentiate between the genetic capacity in each species genome that permits it to change with the environment as being different from changing to some other creature.
And was curious how they could define a "genetic capacity" when one of the things evolution could do is change the "genetic capacity", and what would regulate such "permission to change" if such permission is contained in DNA and thusly could itself be changed ...