In layman's terms, today's domestic dogs can get away with having several hundreds of different breeds because of human care, protection and managed breeding. Many dogs live very protected lives, and we get to choose which breeds are appealing enough to continue on.
Most owners feed their dogs specially prepared food from a can or bag on a daily basis, so the animals have little need to perfect their hunting skills to ensure their survival.
Owners also often take their dogs to the vet for regular check-ups and shots, and administer monthly meds to prevent their pets from getting infections or parasites.
As a result, domestic dog breeds don't have to be any particular size or shape. They don't even need fully functional limbs.
Left alone in the wild, nose and mouth structures would have to evolve or today's domestic dogs would become extinct, the research shows. Consider the breathing problems Pugs and English Bulldogs have, or the tiny size of a Pomeranian's mouth.
Today, the American Kennel Club officially recognizes 150 dog breeds. With humans continuing to take dogs in as our pets, who knows how many more breeds will make the list in the future?