The domestic dog (Canis familiaris or according to others: Canis lupus familiaris) who believe that the dog is a domesticated descendant of the wolf. However the original classifications of species put the dog in a separated species from all of the other canines because of having an upward turning tail, which is unlike all other of the species in the genus canis.

The modern wolf is the dog′s nearest living relative and believed to be the first species to be domesticated by hunter–gatherers thousands of years ago, long before the development of agriculture.

Due to their long association with humans, dogs have expanded to a large number of domestic individuals and gained the ability to thrive on a starch rich diet that would be inadequate for other canids.

The dog has been selectively bred for over a thousand years in order to achieve various behaviors, sensory capabilities, and physical attributes and dog breeds vary widely in shape, size, color and atributes.

They perform many roles for humans, such as hunting, herding, pulling loads, protection, assisting the police and military, companionship, therapy, and aiding disabled people.

Over time, dogs became uniquely adapted to human behavior, and the human-canine bond has been a topic of frequent study, so much so, that this influence on human society has given them the sobriquet, or nickname of man′s best friend.

A species is often defined as the largest group of organisms in which any two individual of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. Normally, different species will not or can not mate with one another, of if they do, the resulting offspring are often sterile, unviable or suffer some other sort of reduced fitness and-or ability contained in either or both of its parents.

A well know example is when a horse and donkey mate, the offspring is a mule, and that animal is always sterile, incapable of producing offspring.

Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined or determined.

Thus, when it comes to domestic dogs, despite how many thousands of breeds that exist, there is only only one species, the Canis familaris. All breeds could be listed as such: German Shepard is a Canis familaris shepherd; Chihushus is a Canis familaris chihuahua; and etc.

First, let it be known that breeds are not separate species, nor are they even subspecies. All breeds can mate between one another and form a viable offspring, in fact, that is what most mutts are.

Dogs are considered to be the most variable mammal on earth with around 450 globally recognized dog breeds. In the Victorian era, directed human selection developed the modern dog breeds, which resulted in a vast range of phenotypes, a set of observable characteristics or traits of an organism. This term, phenotype, covers the organism′s physical form and structure, its developmental process, its biochemical and physiological properties, its behavior, and the products of that behavior. If two or more phenotypes exist in the same species, the species is called polymorphic, which is when two or more clearly different forms or morphs of a species occur that occupy the same habitat at the same time and belong to a population with random inter mating. Even though there has been a lot of dispute as to this fact, still, I believe that a good example of polymorphic species if the great blue heron and the great white heron. Now, back to the dogs.

Most dog breeds were derived from small numbers of founders within the last two more hundred years, and since then dogs have undergone rapid phenotypic change and were formed into the modern breeds we have today due to artificial selection imposed by humans. The skull, body, and limb proportions vary significantly between breeds, with dogs displaying more phenotypic diversity than can be found within the entire order of carnivores.

These many different breeds possess distinct traits related to morphology, which include body size, skull shape, tail phenotype, fur type and color. Their behavioral traits include guarding, herding, hunting, retrieving, and scent detection. Their personality traits include hyper social behavior, boldness, and aggression, which demonstrates the extreme functional and behavioral diversity of dogs.

As a result, present day dogs are the most abundant carnivore species and are dispersed around the world. The most striking example of this dispersal is that of the numerous modern breeds of European lineage during the Victorian era.