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The one gold-standard way of distinguishing one species from another is when they diverge so far as to be incapable of interbreeding. Unfortunatly, this usually takes a great deal of time, many thousands of years at a minimum, so it is going to be hard to find a concrete example.

You are stating speculation as fact. There is no example that can be given of a 'new' species arising form an original parent population. Cows and Buffalo are adaptations of the same animal to two very different environments (all species have a fairly wide variety of traits that can become dominant to allow for surviving climatic and other environmental changes. That doesn't mean they become new species as they adapt since they, as in the guppies example, will revert to their original state if the environmental changes are reversed), both can interbreed, both would (probably) be able to interbreed with their parent population if that animal were known and around for breeding purposes. You might as well claim that African humans are a different species from European and Oriental humans if you are going to use the Buffalo/Cow example.

There is no actual example of one species arising from another, even with laboratory manipulation. Any claim that this can happen is speculation.

Well, if you consider cows and bison to the same species, we'll just have to agree to disagree. I presume that a similar example, that of a potentially fertile cross between a llama and camel, would lead you to suggest that camels and llamas are the same species, too.

Many plants can crossbreed, even if they are of different genera. These are all the same species too?

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?

Coffea Arabica is one such example - it cannot interbreed with either its closely related species, or with its ancestral species. Is this not a new species of plant?