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

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Evolution: artificial selection and domestication

2.2 Size and shape

The shape of the head is determined mainly by the relative sizes of the jaws and the nose and the back of the skull containing the brain, eyes, ears and, in artiodactyls, the horns or antlers. All these structures may differ greatly between otherwise similar species.

SAQ 7

  • How are the size and shape of antlers of Svalbard reindeer different from those of the mainland subspecies?

Answer

The antlers of Svalbard reindeer are much smaller and simpler than those of the mainland subspecies.

Figure 1
Figure 1: Copyright © Caroline Pond
Figure 1 Domesticated sheep and cattle with variable, reduced or absent horns. (a) Longhorn cattle; (b) Jacob's sheep with three horns; (c) White Park cattle; (d) a modern hornless breed of sheep

This natural and probably relatively recent evolutionary change is believed to arise from the simpler and less combative social structure of Svalbard reindeer, which live in small groups where there is less fighting. The horns and antlers of wild artiodactyls are of consistent shape for each species and nearly symmetrical, at least in mature adults. Many domesticated sheep and cattle are hornless or have horns of variable, often asymmetrical, size and shape. Anomalies such as having three (or four) horns are also common even in modern breeds (Figure 1b). Almost all adult mammals have erect ears (Figure 1b, c and d) but floppy ears appear in many domesticated livestock including sheep, cattle, pigs and rabbits (Figure 2).

Figure 2
Figure 2a: Copyright © Inga Spence/FLPA; Figure 2b, c, e: Copyright © Caroline Pond; Figure 2d: Copyright © Arco Images/Alamy
Figure 2 Drooping ears in domesticated mammals. (a) Zebu cow; (b) pig; (c) sheep from Jordan; (d) lop-eared rabbit (with prick-eared companion); (e) guinea pig. Although some of these varieties are not commonly seen in Britain, all except rabbits are ancient breeds, long established for the production of meat, milk and/or wool

Dwarfism and gigantism are also common among almost all domesticated mammals and have recently been found among laboratory mice that are bred for research purposes. Some small dogs, such as Figure 3a and b, are miniatures with proportions of the head, body and feet typical of normal puppies, while the head, feet and body of the Shetland pony (Figure 3c) and the basset hound (Figure 3d) are clearly too big relative to the legs. Miniaturisation to infantile size and body proportions similar to Figure 3 a and b is known in certain wild mammals, for example Pleistocene miniature elephants on Crete.

Figure 3
Figure 3: Copyright © Caroline Pond
Figure 3 Two forms of dwarfism in domesticated mammals: (a) and (b) miniature dogs; (c) achondroplastic dwarf Shetland pony, and (d) basset hound. Note the different proportions of the head, limbs and trunk

The most common form of dwarfism among humans is achondroplasia (Figure 4a), caused by defects in an autosomal dominant gene that encodes a receptor protein for a key growth factor. (Achondroplasia is derived from Greek words: a- ‘lack of’; chondros, ’cartilage’; plasia, 'molding’. The term is not strictly accurate because such people have cartilage, but its ability to turn into bone is impaired, greatly slowing the growth of all the long bones and the nose.) This gene seems to be unusually susceptible to mutation: more than 80 per cent of human achondroplastic dwarfs are born into families with no history of dwarfism so they must carry a new mutation. Growth of the limbs, hands, feet and nose is more severely curtailed than that of the head or trunk, distorting the body proportions (Figure 4a). Most achondroplastic dwarfs cannot run far or fast. They are very susceptible to joint disorders and many suffer from chronic backache from an early age, but they are mentally normal and some marry and have children.

Figure 4
Figure 4a: AKG-Images; Figure 4b: Prado Museum, Madrid; Figure 4c: AKG-images/Joseph Martin
Figure 4 Dwarfism in humans. These portraits were produced in Spain in the early 17th century by Diego Velázquez (1599–1660) and Rodrigo de Villandrando (1590–1630), court painters to the Spanish royal family. (a) An adult achondroplastic dwarf with the typical disproportionately large head (detail from Las Meninas, 1656–7). Note the similarities to Figure 3c and d. (b) Philip III's son Philip with the dwarf Solplillo. This midget's head, limbs and body are more or less in proportion. (c) The infant Prince Baltasar Carlos with a dwarf servant, 1631

Midgets (Figure 4b and c) are a rarer form of human dwarfism; they are very short but, in contrast to achondroplastic dwarfs, have more or less normal proportions. In the 17th century, midgets such as the Spaniard, Solplillo, were the focus of much curiosity from aristocrats and scientists. The proportions of the face and of the head relative to the body enable you to distinguish the 16 month-old prince (Figure 4c) from his older midget companion. The body proportions of miniature dogs such as those shown in Figure 3a and b and almost all dwarf wild mammals, such as the pygmy hippopotamus (maximum height 1.75 m, body mass 275 kg, compared to the common hippo which weighs up to 4000 kg), resemble those of human midgets. Dwarfism similar to achondroplasia in humans is known in dogs (e.g. basset hounds, Figure 3d), horses (Figure 3c) and among cattle, pigs and goats.

SAQ 8

  • Would achondroplasia be more readily favoured by artificial selection in domesticated livestock than by natural selection in wild animals?

Answer

Yes. As long as the animals could breed successfully, impaired mobility would not reduce fecundity.

SAQ 9

  • Would a dominant gene be more easily selected than a recessive gene in domestic livestock?

Answer

Yes. Artificial selection would be much more efficient for dominant genes because all animals that have the gene display the dwarf phenotype.

Question 2

Why would achondroplastic dwarfism be much rarer in dwarf races of wild mammals than the midget form, even though mutations of the genes that produce achondroplasia probably occur much more frequently?

Answer

The experience of human achondroplastic dwarfs suggests that wild animals with this form of dwarfism would be unable to run far or fast. They would therefore be very vulnerable to predation and have difficulty catching food. Such dwarfs would be eliminated by natural selection in the wild. Movement is much less impaired in proportionate, midget dwarfs, so they are more likely to survive and therefore to become the ancestors of reduced-size races of large mammals.

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