system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are certain species (like Tigers & Lions) able to interbreed but others are not?

Just curious about the genetics of interbreeding.
gpt
Basically, the term species is a convenient kludge.  It is not as based on solid biological differences as people tend to think, especially because we often assign organisms to different species before we have fully studied their ability to breed.

Pretty much all of the big cats have some degree of inter fertility with each other, which one could argue makes them a single species. But lions and tigers and leopards all look so different and behave differently enough that of course the earliest scientists just assume that they were different enough to call different species.

It gets even more complicated when you're dealing with things like bacteria. Two different strains of e coli, which we refer to as the same species, can be as genetically distinct from each other as you are from a catfish.