Dogs have been selectively bred for thousands of years, sometimes by inbreeding dogs from the same ancestral lines, sometimes by mixing dogs from very different lines. The process continues today, resulting in a wide variety of breeds, hybrids, and types of dog. Centuries of selective breeding by humans has resulted in dogs being considerable more genetically diverse then most other mammals, and as such dogs are the only animal with such a wide variation in appearance without speciation, "from the Chihuahua to the Great Dane".
Breeds are usually categorized by the functional type from which the breed was developed. The basic types are companion dogs, guard dogs, hunting dogs, herding dogs, and working dogs, although there are many other types and subtypes. Breeds listed here may be traditional breeds with long histories as registered breeds, rare breeds with their own registries, or new breeds that may still be under development.
