Why Species Can't Interbreed With Each Other, Except When They Can 

Most people learn that a species is a group of living things that can interbreed. If two organisms don't normally mate, or have sterile offspring if they do, they're from different species. Except that like so many other things you learned in school, the reality is somewhat more... complicated.

In this short video from Quanta Magazine's In Theory series, Johns Hopkins University physicist David Kaplan describes one exception to the "no breeding" rule, which seems to rely on stretches of DNA that keep the species separate even as the rest of their genes flow between occasional bi-species love-children.

[Quanta Magazine]