The best evidence indicates that all modern dogs are descended from an ancestral wolf species.
Darwin originally thought dogs were so diverse that they must have been descended from more than one form.
And Konrad Lorenz, the expert on animal behavior, popularized the idea that dogs like huskies and German shepherds were descended from wolves, and dogs like terriers and hounds were descended from jackals.
However, this idea doesn’t hold water.
In terms of behavior and genetic affinities, there is no evidence for more than one ancestor, probably an Asian wolf.
As for the dog’s huge variety, genetic variability is built into a species so it can adapt if conditions change. In wild forms, the coyote and wolf, there are what are called buffering systems, so that all the genes they have are not necessarily expressed.
In domestication, however, the system of genes that buffer differences from the norm are bred out.
Dog breeders have selected away from the buffering system, so all possibilities can appear, and dogs can freely show all their genetic variability, from dachshund to wolfhound.
There is a tremendous choice in variables like coat, leg proportions and nose length, and without the buffering genes, breeders can select desired characteristics and, over time, develop a particular breed.
Domesticated cats, on the other hand, still have their buffering system, and so are more of a natural species than dogs.
Human beings also have a buffering system, so that while humans, like felines, are free to choose their own mates, the mismatches tend to average out.
All humans, from Pygmy to Zulu, and all cats, from Persian to Siamese, tend to have the same basic body plan and are recognizable as members of their own species.