Most
breeds of dog are at most a few hundred years old, having been artificially
selected for particular morphologies and behaviors by people for specific
functional roles. Through this selective breeding, the dog has developed into
hundreds of varied breeds, and shows more behavioral and morphological
variation than any other land mammal. For example, height measured to the
withers ranges from 15.2 centimetres (6.0 in) in the Chihuahua to about 76 cm
(30 in) in the Irish Wolfhound; color varies from white through grays (usually
called "blue") to black, and browns from light (tan) to dark
("red" or "chocolate") in a wide variation of patterns;
coats can be short or long, coarse-haired to wool-like, straight, curly, or
smooth. It is common for most breeds to shed this coat.
While
all dogs are genetically very similar, natural selection and selective breeding
have reinforced certain characteristics in certain populations of dogs, giving
rise to dog types and dog breeds. Dog types are broad categories based on
function, genetics, or characteristics. Dog breeds are groups of animals that
possess a set of inherited characteristics that distinguishes them from other
animals within the same species. Modern dog breeds are non-scientific
classifications of dogs kept by modern kennel clubs.
Purebred
dogs of one breed are genetically distinguishable from purebred dogs of other
breeds, but the means by which kennel clubs classify dogs is unsystematic. DNA
microsatellite analyses of 85 dog breeds showed they fell into four major types
of dogs that were statistically distinct. These include the "old world
dogs" (e.g., Malamute and Shar Pei), "Mastiff"-type (e.g.,
English Mastiff), "herding"-type (e.g., Border Collie), and "all
others" (also called "modern"- or "hunting"-type).
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