A mixed-breed dog (also called a mutt, crossbreed, mongrel, a bitsa, tyke, cur, or random-bred dog) is a dog that is a mixture of two or more breeds, or a descendant of feral or pariah dog populations. Dogs interbreed freely, except where extreme variations in size exist, so mixed-breed dogs vary in size, shape, and color, making them hard to classify physically. Historically, all purebred dogs have been selected from a mixed-breed population.