Evolution: artificial selection and domestication
Evolution: artificial selection and domestication

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Evolution: artificial selection and domestication

3.2 Structure and behaviour in modern dog breeds

Dogs are one of the most diverse of all living species, differing in size and shape of the skull and the proportions of the body, especially the head and legs, and in the colour and texture of the coat (Figures 3a, b and d, 8a–d and 12). People have ‘improved’ dogs for specific properties and uses by artificial selection for visible anatomical characters and for more subtle aspects of behaviour, habits and intelligence. An example is the two major kinds of hounds selectively bred for hunting wild mammals.

Foxhounds (Figure 12a) hunt almost entirely by smell and chase their quarry over long distances. They perform best in large packs, communicating with each other and their human handlers by sound – ‘giving tongue’ as the huntsmen describe it – and gestures with their prominent tails. Their sturdy, wolf-like body proportions (compare Figures 5b and 11a with Figure 12a) equip them for endurance as well as speed and enable them to gallop with the sensitive nose close to the ground. If given the opportunity, they can kill small animals with their powerful jaws and necks.

Figure 12
Figures 12a, b, f,: Copyright © Caroline Pond; Figure 12c: Copyright © Gerard Lacz/ FLPA; Figure 12d: Copyright © Mandy Dyson; Figure 12e: Neff, M. W. and Rine, J. (2006) ‘A fetching model organism’, Cell, 124(2) ©
Figures 12a, b, f,: Copyright © Caroline Pond; Figure 12c: Copyright © Gerard Lacz/ FLPA; Figure 12d: Copyright © Mandy Dyson; Figure 12e: Neff, M. W. and Rine, J. (2006) ‘A fetching model organism’, Cell, 124(2)
Figure 12 Phenotypic diversity of some modern breeds of dogs. (a) Foxhound; note the stout head and jaws, large floppy ears, sturdy, well-proportioned body and legs, multicoloured coat and prominent tail. (b) Greyhound; note the narrow jaws and head with relatively large, forward-directed eyes, small floppy ears, long, flexible back, chest much larger than abdomen, long athletic legs with disproportionately long thighs and shoulders, and tail held between the hindlegs (where it is almost invisible). (c) Mastiff. (d) Boston terrier. (e) Ibizan hound. (f) Cocker spaniel

SAQ 13

  • Which morphological features of foxhounds are characteristic of domesticated dogs but not of wolves?

Answer

Foxhounds have floppy ears. Their coat colours are very varied, mostly piebald or skewbald (see Figure 12a).

Greyhounds (Figure 12b) are also specialised hunters, first developed in Egypt and the Middle East around 4000 years ago for hunting very fast mammals such as gazelles, antelopes and hares by sight rather than by scent. They always gallop with the head pointing forward not downwards. Pairs or groups of greyhounds have long been raced for display, and are now bred mainly for such organised competitions. Greyhounds have relatively large eyes, bark and howl only rarely and do not communicate with tail gestures as readily as most other dogs. Their slender, athletic bodies and long legs make them the fastest of all dogs for short sprints but they lack foxhounds’ stamina and ability to cope with rough ground. Greyhounds are also surprisingly timid dogs that are reluctant to attack large prey with their weak jaws: actually killing the quarry was left to their human handlers.

SAQ 14

  • With reference to Figure 11a, what aspect of intraspecific communication would be impossible for foxhounds, greyhounds and other floppy-eared dogs?

Answer

Wolves (and prick-eared dogs) express fear and threat by laying back their ears (note particularly the ‘underdog’ and the bystander at the back of the picture), which would be impossible for dogs with disproportionately long floppy ears.

As well as fur colour (Section 2.3), features of the head, including the size and shape of the jaws, skull and ears are among the most variable features of most mammals including dogs and humans. For example, the snout and upper jaw of the miniature dachshund (Figure 3b) are clearly longer than its lower jaw. Figure 12 illustrates the range of sizes and shapes of ears and skulls found in modern breeds of dogs. These superficial features seem to occur in all possible combinations, but there are functional limits to phenotypic modifications that can be perpetuated by artificial selection.

SAQ 15

  • Greyhounds (Figure 12b) and Boston terriers (Figure 12d) have similar-shaped bodies and legs, but very different heads and necks. Which of these breeds would be best able to reproduce successfully without human assistance?

Answer

Greyhounds. Giving birth through narrow hips to puppies with flat, long, narrow heads would be very much easier than to those with wide, round skulls.

Some modern breeds including bulldogs and Boston terriers have great difficulty giving birth naturally and are usually delivered by Caesarean section. Clearly such procedures were impossible for long-established breeds. The greyhound's narrow skull was probably not specifically selected as a desirable trait but appeared in association with more important qualities such as running speed.

Question 6

Using your own general experience and comparing all the dogs in Figures 3a, b and d, 8 and 12, answer (a) and (b) below.

  • (a) Are floppy or prick ears associated with any particular sort of snout shape or temperament?

  • (b) Are dogs with disproportionately long, thick coats (e.g. sheep dog, Figure 8b; Shih-tzu, Figure 8c) as capable of prolonged exercise in hot weather as those with sleek coats (e.g. Figures 3a and b, 8a and d, and 12)?

Answer

  • (a) No. Foxhounds and mastiffs (Figures 12a and c) that are bred to bite with powerful jaws have floppy ears, as do docile breeds like spaniels (Figure 12f), basset hounds (Figure 3d) and miniature dogs (Figures 3b and 8c). Conversely, some toy dogs (Figure 3a) have prick ears and long snouts.

  • (b) Dogs with very thick coats become uncomfortably overheated in hot weather even when sedentary. Dogs with naturally short coats or those that have been clipped are much more willing and able to perform prolonged strenuous exercise.

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