Interesting. Why do you think @Heyo is correct, and correct about what exactly? What ring species show us (as with your excellent Netherlands example is that neither species, nor languages are strictly discrete, but are a spectrum of overlapping (ahem) dna. That one might potentially be a member of multiple species if one's ability and normative behavior is to mate with both outside of one's aggregate alleles.You are correct. I was thinking about extant populations only which indeed is too simplistic, especially in light of extinct populations.
In my second definition, the problem is actually made clear: 2 populations that could interbreed and produce viable off spring, but don't (for whatever reason). These are on a path of inevitable diversion to the point where interbreeding will no longer produce viable off spring (or off spring at all). This is what occurs in ring species.
It's also what occurs in lineages. Every creature ever born was of the same species as its parents. But go back by skipping 100.000 generations and the ancestor you meet there... hardly the same species as the current extant descended.
A point I was thinking about adding to my post (and in retro-spect, I should have...) is that the concept of "species" really only makes sense as a "snapshot" of the "current state of life".
Species indeed is a very dynamic / fluid thing which is already hard enough to pin down for extant life only. It only gets more complex if we include extinct life also.
As a useful analogy, I really like the concept of "language" as the development / evolution of language follows the same gradual pattern. At what point did spanish became spanish? When did it stop being Latin? And if we would go to the middle of that evolution, what would we call that? Spatin?
Or take the extreme dialects of any language. At which point does it "stop" being the original language and when is it a new language?
In the netherlands, there is a great example of this. All the way in the north is Friesland.
Start in the south and work your way north. In the south, I understand every word they say: it's just dutch. The same language I speak (even though our dutch is called "flemish"... which imo is just an accent / dialect difference). The further north we go, the less I understand of it. By the time we get to Friesland, I literally do not understand a single word of it!
So where does it stop being "dutch"? I have no idea.
So in summary.... I stand corrected.
Although I think that for extant life, the two definitions I gave are very workable.
Now, if humans actually met Vulcans, and Minbari and Cylons, and all were both able to procreate and DTF, that would be interesting. But the fact of the matter is that here on Earth, we don't have that situation afaik.
BTW, I heard last month that the cycle time on a species is thought to be about 200 - 300k years. Which would make us a different species from the original sapiens sapiens. Or there abouts. I will see if I can scare up the reference.