All domestic dogs are descended from the wolf, Canis lupus, and are treated as a subspecies by Wilson & Reeder (see below), although they admit that this "may stretch the subspecies concept". Intensive selection has created an astonishing variety of behavioural and morphological variants, some distinctly freakish (see boxer and chinese crested below).
Whether this is due to high genetic variability, which would be surprising considering the probably limited initial gene pool, or more to the effect of intensive, diverse selection on a responsive genome is unclear.
The dog may have been the first domesticated animal, with archaeological evidence from more than 20 thousand years ago, most older examples assigned to the so-called European Palaeolithic dog, found in various sizes suggesting the existence of different breeds even at that time.
Dogs seem to have inherited the inflated frontal sinuses of wolves, a useful way of distinguishing their skulls from those of coyotes and foxes. (see red fox on this website for further details). The resulting high forehead may be connected to the importance of facial expression in social interactions between wolves, a possible reason for dogs’ ability to bond with humans and no doubt linked to the emphasis of these bones in child-substitute toy dogs.