Domesticated organisms evolve in artificial environments under artificial selection, and opportunistic or enforced hybridisation often occurs between species that would not normally interbreed. Natural selection cannot be eliminated and continues to operate. At least two different forms of dwarfism are common in domesticated livestock and humans, but only the rarer midget type of dwarfism occurs in wild lineages. Domesticated mammals and birds have distinctive patterns of skin pigmentation that resemble each other and differ from those found in wild animals. Genetic analysis reveals that the principal ancestors of dogs are grey wolves, but hybridisation with jackals and coyotes has also occurred. Modern dogs are genetically very similar in spite of their large phenotypic differences that include behavioural as well as structural and physiological traits. Intense artificial selection and inbreeding have produced extensive anatomical change, especially in the structure of the skull and jaws and in the shape and relative size of the ears and tail and in coat characteristics. Many features and habits of domesticated dogs appeared in red foxes that were selectively bred over many generations for tameness. This Russian experiment suggests that epigenetic processes linked to the reduction of fear alter the structure and properties in the brain and several other features, particularly skin pigmentation.
Outline the mechanisms by which the pigment-forming cells in the skin that determine the colour of epidermal structures including feathers and hair are modulated during normal development, by hormones and by minor genetic changes.
In wild birds, the same feather follicle can produce feathers of different colours as the bird matures, and under the influence of sex hormones. The plumage difference in bananaquits concerned a point mutation in the gene for the black pigment melanin in the pigment cells that were incorporated into the feathers. Several species of birds have lost all or part of their feather pigmentation under domestication (Figure 10), although this character was not the focus of artificial selection. Similarly, fur pigmentation of most domestic mammals (Figures 1, 7, 8 and 9) is very different from that of their wild ancestors (Figure 6).
All these features are present in at least one modern breed of dog. The coats of border collie sheepdogs, Dalmatians, some setters and spaniels and many other breeds are piebald. Spaniels, foxhounds, basset hounds, Shih-tzu and many toy dogs have floppy ears. In bulldogs, boxers and pugs, the snout is short with the lower jaw longer than the upper jaw.