Dog Breeding and Evolution

Another familiar example is the sculpting of the wolf, Canis lupus, into the two hundred or so breeds of dog, Canis familiaris, that are recognized as separate by the UK Kennel Club, and the larger number of breeds that are genetically isolated from one another by the apartheid-like rules of pedigree breeding.

Incidentally, the wild ancestor of all domestic dogs really does seem to be the wolf and only the wolf...

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The main point I want to draw out of domestication is its astonishing power to change the shape and behaviour of wild animals, and the speed with which it does so. Breeders are almost like modellers with endlessly malleable clay, or like sculptors wielding chisels, carving dogs or horses, or cows or cabbages, to their whim. I shall return to this image shortly. The relevance to natural evolution is that, although the selecting agent is man and not nature, the process is otherwise exactly the same. This is why Darwin gave so much prominence to domestication at the beginning of On the Origin of Species. Anybody can understand the principle of evolution by artificial selection. Natural selection is the same, with one minor detail changed.

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Something funny happens to the gene pools of domestic dogs. Breeders of pedigree Pekineses or Dalmatians go to elaborate lengths to stop genes crossing from one gene pool to another. Stud books are kept, going back many generations, and miscegenation is the worst thing that can happen in the book of a pedigree breeder. It is as though each breed of dog were incarcerated on its own little Ascension Island, kept apart from every other breed. But the barrier to interbreeding is not blue water but human rules.

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If an animal grows at the same rate in all its parts, so that the adult is just a uniformly inflated replica of the infant, it is said to grow isometrically. Isometric growth is quite rare. In allometric growth, by contrast, different parts grow at different rates. Often, the rates of growth of different parts of an animal bear some simple mathematical relation to each other, a phenomenon that was investigated especially by Sir Julian Huxley in the 1930s. Different breeds of dog achieve their different shapes by means of genes that change the allometric growth relationships between the parts of the body. For example, bulldogs get their Churchillian scowl from a genetic tendency towards slower growth of the nasal bones. This has knock-on effects on the relative growth of the surrounding bones, and indeed all the surrounding tissues. One of these knock-on effects is that the palate is pulled up into an awkward position, so the bulldog's teeth stick out and it has a tendency to dribble. Bulldogs also have breathing difficulties, which are shared by Pekineses. Bulldogs even have difficulty being born because the head is disproportionately big. Most if not all the bulldogs you see today were born by caesarian section.

Dog breeding demonstrates how quickly animals can evolve, even if it's under artificial selection.