[IAmBroom]

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.

[HarHarVeryFunny]

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 ?!

[OkayPhysicist]

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.