In the past the rules for 'species' was not whether or not they will interbreed, but whether or not their offspring would be fertile. A horse and a donkey can

In the past the rules for 'species' was not whether or not they will interbreed, but whether or not their offspring would be fertile. A horse and a donkey can breed just fine, but a mule cannot. This makes sense because we know that species eventually differentiate and whether or not they can recombine is a good indication of how far apart that separation is. But nowadays all that stuff is thrown out of the window. I don't really understand why it is no longer considered a useful distinction.