By Terry McNamee © 2013
There is nothing unusual about separating domestic animals into breeds. After all, everything in nature is classified according to how closely they are related, and the same is true for purebred animals, too. The only difference is that breeds are the most specific within the world's classification of nature. It all starts with the most basic classifications and works down as the definition of “things that are alike” become ever narrower. For the purpose of explaining domestic breeds, I will use the dog as an example.
A dog is categorized as an animal, not a plant, fungus and so on. Animals like dogs that have spinal cords are called Vertebrates. Vertebrates include six groups, with dogs belonging to the mammals, which are the only animals which produce milk for their young. Of the three categories of mammals, dogs are among the Placental Mammals that incubate their young inside their bodies. Placental mammals are divided yet again into numerous families, such as rodents, hoofed animals, carnivores (including dogs) and so on. Carnivores are divided yet again into several groups, among them canines, cats, bears, hyenas, and so on. Dogs are canines.
None of these groups can successfully interbreed with another. You can't cross a horse with a pig, or a wolf with a bear.