I'm starting a world where humans have traveled to many planets and exhibit as much diversity as dog breeds would. Some changes are merely cosmetic like coloration while others are more drastic like changes in biology. In other worlds the universe is populated my many human subspecies. Now, before I go into detail about each race I would like to know exactly how far I can alter their biology before the inhabitants stop being human.
The common definition of species is a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. However this doesn't really help since in my world interbreeding is made possible by the human genes each species shares. For storytelling purposes two very different species can make a normal human, because the human genes are still present.
So with this in mind, what's the limit? How drastic can the changes be until on the species can no longer be labeled "human"?