No species on earth is more diverse than the dog.
Originally intended to be reasonably fast hunting animals, many breeds now struggle to work up much of a pace and prefer a gentle walk around the block rather than a vigorous run. Human intervention rather than natural selection has worked to produce the huge range of shapes and sizes that exist today. Indeed, many breeds would die out extremely quickly if left to themselves; the respiratory problems and whelping difficulties among bulldogs provide just one example.
Human tinkering, however, is unable to replace the basic genetic characteristics that form the building blocks of canine physiology and psychology, and all dogs, regardless of their breed share a number of fundamental characteristics.
They have efficient cardiovascular systems designed for running, with deep rib cages to protect their vital organs. They have highly developed sensory organs to underline their role as natural hunters and defenders of territory.
With efficient, sensitive hearing, and a highly refined sense of smell, even the smallest dog will bark as a warning when strangers approach. All dogs, like their vulpine ancestors, are pack animals, genetically programmed to respect the hierarchy of their pack or family.
They feel secure knowing their place in the pack, which explains why dogs readily accept the dominance of their human owners.