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 widening in appearance without speciation, "from the Chihuahua to the Great Dane."
Dogs with short coats require virtually no grooming, just a brush and wipe down every so often. Pet dogs with long coats require either regular clipping every eight weeks or so (where their coats are clipped short) or there long coats need to be brushed practically every day. Show dogs, or dogs with long coats, require daily brushing or else there coat will become matted and tangled. Not only can it hurt your dog if he has tangled and matted hair, but it will also become dirty and he will look shaggy and ugly if you do not regularly brush his coat.
Though groups like the American Kennel Club make very specific designations about breeds and breed hybrids, there continue to be new breeds and breed variants developed all the time, as well as "up and coming breeds" not officially recognized by kennel clubs and experts. (Many dogs not purchased directly from breeders are also mixed-breeds of some kind, or have less than purebred ancestry.)
Dogs with short coats also have another advantage. It is easier to find ticks and fleas on their body. You probably spend eight times as long searching for fleas and ticks on a dog with a long coat than you do on a dog with a short coat.
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