I remember learning this vaguely as well, it's just a relatively useful rule with lots of exceptions, but it is not a law--it happens even after extended isolat

I remember learning this vaguely as well, it's just a relatively useful rule with lots of exceptions, but it is not a law--it happens even after extended isolation: https://en.wikipedia.org/wiki/Hybrid_(biology) https://en.wikipedia.org/wiki/Hybrid_(biology) https://en.wikipedia.org/wiki/Interbreeding_between_archaic_and_modern_humans https://en.wikipedia.org/wiki/Interbreeding_between_archaic_... Some examples of hybrids without human influence (well that's arguable): https://en.wikipedia.org/wiki/Lonicera_fly https://en.wikipedia.org/wiki/Lonicera_fly https://en.wikipedia.org/wiki/Red_wolf https://en.wikipedia.org/wiki/Red_wolf The grand daddy of "hybrids" (twelve species): https://en.wikipedia.org/wiki/Celosia_argentea https://en.wikipedia.org/wiki/Celosia_argentea from here: https://en.wikipedia.org/wiki/Hybrid_(biology)#Speciation https://en.wikipedia.org/wiki/Hybrid_(biology)#Speciation Also you can go beyond hybrids: https://en.wikipedia.org/wiki/Polyploidy#Types https://en.wikipedia.org/wiki/Polyploidy#Types As an amateur I'd hypothesize if the species diverges and then relocates, but is subject to similar environmental pressure, the morphological changes won't be great enough to prevent the organisms from interfacing and then reproducing. We've determined this through string similarity algorithms, so really I'd say we can't be completely certain here either.