Time means nothing - species can stay stable for very long periods of time (e.g. coleocanth), and more to the point it makes no sense to call two animals different species if they can still successfully interbreed, since there then still remains the possibility that they could recombine. NOT being able to interbreed successfully (donkey & horse) marks the point of no return where they are now bound to genetically drift further apart over time.
I guess you missed the point that M. ibericus and M. structor are at this point of no return: like donkey and horse, they can't interbreed successfully, as hybrids are all workers and can't mate.
Regarding coleocanths... we have no data on how much drift has happened in their DNA. Our only real data is that they are morphologically very similar to their ancestors over a long range.
Your last sentence correctly points out the frailty of our definition of "species". However, this is not the only time our data has confounded our artificial, if often useful, definition of species boundaries.
True - I guess examples like these ants, or lions and tigers, where we have DNA available for both, give a better idea of the speed of genetic drift, or at least some datapoints. We can compare the DNA, and estimate how long those numbers of changes took to accumulate, without yet having got to the point of no return.
I wonder what are the most visually, or structurally, or genetically, different animals that can still interbreed. Things like lions & tigers, polar bears & grizzlies, and zebras & horses, come to mind ... what else ?!
The American paddlefish and the Russian sturgeon is a pretty wild one. They're in different families (your examples at least share genuses). As far as looking really different (but actually being pretty recently related) beluga whales and narwhals can hybridize.
The definition of speciation is more complicated than your highschool bio class lead you to believe. There's a dozen definitions, and if you choose one you end up with at least a couple of exceptional cases.
For example, American bison and domesticated cattle can interbreed to produce fertile female beefalos, but the males are sterile. Are domesticated cattle the same species as buffalo?
Then there's ring species: populations of animals where population A can interbreed with populations B and D, but not with C, but C can interbreed with B and D. (often the rings are larger than that). For example, the genus Ensatina salamanders here in California can interbreed with neighboring populations as you go around the mountains, but if you drove one from one side of the central valley to the other it couldn't interbreed. We've mostly decided in that case to call them a bunch of different species, but it's a weird case.
Shit gets even weirder when you leave the animal kingdom. All varieties of pepper will cross pollinate. Bacteria just sort of spread their genetic material to anything that's nearby. Don't even get me started on the absurdity of declaring all the asexually reproducing organisms as being single species individuals.
Basically, a species is a group of animals that has enough of the following characteristics that biologists can agree they're sufficiently different things:
Interesting - thanks.
I hadn't really considered the definition of asexually reproducing species - it seems that things are much more clear cut for ones that sexually reproduce since then we can use the more clear cut "point of no return" definition.
I suppose in cases like beefalos and mules, or these ring species, this "point of no return" comes down to is there any path for to the DNA of these divergent animals to recombine, so a fertile female beefalo (or the occasional fertile female mule) still provides that chance.
It seems that in general it's rare for widely divergent animals like zebras and horses to interbreed in the wild, but apparently western wolf-coyote hybrids are not that uncommon, so it's more than just a theoretical possibility. Who knows, maybe global warming will force polar bears to adapt to warmer climates and increasingly interbreed with grizzlies.
Even in the cases of species that roughly meet the naive high school definition, it is at least sometimes the case that they can still interbreed, they just don't. Usually, this is because of geographic isolation. Take African and Indian elephants, for instance. They diverged long enough ago that they're morphologically distinct, not just genetically distinct, but they can still interbreed. They don't because they live on different continents, and they probably wouldn't if they were put together outside of captivity because they're intelligent, social animals with culture and learned histories who rely on not entirely biological cues regarding who to breed with, just as much as humans do.
In reality, we first categorized life into species because they either looked different or we found them exclusively in different places, and only centuries later did we attempt to figure out exactly why and how this was the case and reverse engineer some sensible definition onto the pre-existing categories, but it turns out there is no single definition that works universally and has zero exceptions. It's frustrating if you're a language pedant who likes clarity, but a lot of categories and definitions are like this.