76 Utilisation and conservation of farm animal genetic resources
Miguel Toro and Asko Mäki-Tanila
1. Animal domestication
1.1. Domestication
Domestication of plants and animals has been one of the most important events in the history of mankind. It has increased the amount of available food and, as consequence,
has supported the growth of human populations and their capability to expand into new environments, and has heavily inluenced mankind’s cultural evolution. From
a genetic point of view, animal domestication has involved selection for reduced aggressiveness, earlier sexual maturity, tolerance for living in coninement, and a number
of morphological traits. As an example of the latter, chickens were probably selected for increased size, whereas cattle for a smaller size. In this process there were signiicant
changes in both the traits selected and in correlated traits. he most notable ones were the decreased brain size and the size reduction of teeth Diamond, 2002.
Altogether some twenty terrestrial and a few ish species have become adapted to being bred and fed in captivity, and to satisfying the diverse human needs. here are several
factors that are genetically diferentiating animal populations: separate founder populations within species;
isolation accompanied by variation in the number of breeding animals; human controlled environments, mostly relaxing natural selection;
harsher environments in new regions, oten strengthening natural selection; human selection in managed populations targeted towards mankind’s needs and
objectives.
Domestication is usually deined as a process in which populations adapt to mankind and its environment. Yet it may be also considered as a form of mutualism involving
a parallel evolution in cultures and genomes. An example of how important this co- evolution can be, is the domestication of milk-producing cattle and the use of milk.
his has resulted in selection of both animals and humans Beja-Pereira
et al., 2003. In mammals, milk usually has little nutritional value for adults as the lactase enzyme,
necessary for digesting lactose in milk into galactose and glucose, is turned of at weaning. However, about 30 of mankind is now lactose-tolerant and this condition
is common in the northern latitudes in Europe where milk provides a rich source of calcium, vitamin D and protein. Lactose-intolerance was the normal ancestral condition
but a ‘recent’ mutation in the lactase gene, maintaining the ability to digest lactose through adulthood, was quickly favoured by natural selection in human populations
that raised dairy cattle.
• •
• •
•
Utilisation and conservation of farm animal genetic resources 77
Chapter 4. Genomics reveals domestication history and facilitates breed development
1.1.1. Why were some species domesticated and others not?
Animal domestication started with dogs, sheep and goats, and continues in recent times with fur animals and ish species such as the salmon. here are 8-10 centres of origin
for plant and animal domestication and food production Table 4.1. Authors from Galton 1883 to Diamond 2002 have questioned why only a dozen of species among
the 148 species of large terrestrial mammalian herbivores and omnivores, all plausible candidates for domestication, have been domesticated. Diamond 2002 claims that
the obstacles are not human abilities but characteristics of the target species itself. He quotes six main obstacles with an example of a species between brackets:
a specialised diet not easily supplied by humans anteater; a slow growth rate and a long generation interval elephant;
a nasty disposition grizzly bear; the reluctance to a breed in captivity cheetah;
a lack of follow-the-leader dominance hierarchies antelope; a tendency for extreme panic in enclosures when facing predators gazelle.
• •
• •
• •
Table 4.1. Domestication events, time points and sites for farmed animal species, based on the reviews by Bruford et al. 2003, Mignon-Grasteau et al. 2005, Dobney and Larson 2006
and Zeder et al. 2006. here are discrepancies between the estimates based on archaeological and molecular not presented evidence.
species no. events
domestication site archaeological dating
years BC dog
many East-Asia
15,000 sheep
1-3 Near East
12,000 goat
4 Near East
8,000-10,000 pig
7 Near East, Far East, Eurasia
9,000 cattle, zebu
2-3 Near East, India, Africa
2,000-8,000 chicken
1? Central Asia
5,000-7,000 horse
many? Eastern Europe, Central Asia
6,000 donkey
2 North Africa
5,000 water bufalo
1-2 South-East Asia
6,000 llama, alpaca
2-4 South America
6,000 camel
? Near East
3,000 rabbit
? Europe
2,000
78 Utilisation and conservation of farm animal genetic resources
Miguel Toro and Asko Mäki-Tanila
For example, the European horse breeders have tried over several centuries to domesticate zebras ater settling in South Africa in the 17
th
century. Finally, they abandoned the idea, because zebras have an incurably vicious habit of biting the handler and not letting
go they injure more zoo-keepers that tigers, and they have a better peripheral vision than horses making the use of lassoes impossible.
1.1.2. Are there genes for domestication?
Darwin 1868 was the irst to note that most domesticated species have undergone similar changes during domestication: appearance of dwarf and giant varieties, piebald
coat colour, depigmentation, curly hair, rolled tail, loppy ears and early sexual maturity. Many, but not all, characteristics associated with domestication seem to be linked to
pedomorphosis: the retention of juvenile characteristics in the adult body. his suggests that domestication may be the result of changes in a relatively small number of regulatory
genes afecting development.
here has been searches for genes speciically responsible for these traits, e.g. for genes underlying diferences in coat colour pattern in pigs and horses, plumage in chickens
or muscle mass in pigs Andersson and Georges, 2004. In the 1950s, Belyaev see Trut, 1999 started a selection trial for tameness in the Silver Fox. He hypothesised
that morphological, physiological and behavioural traits were simultaneously modiied by domestication and that selection for an important behavioural trait would modify
the others. Belyaev measured tameness by the ability of young sexually mature foxes to behave in a friendly manner towards their handlers, by wagging their tails and whining.
Ater more than forty years of selection, 70-80 of the test population accepted human contact and oten licked the persons looking ater them, like dogs do. As predicted
by Belyaev, additional changes appear such as piebald coat colour, drooping ears and shorter tails and snouts. Physiological changes also occurred: in domesticated animals
the corticosteroid levels rise signiicantly later and they have much lower adrenal responses to stress and more serotonin in their blood.
1.1.3. Multiple versus single domestication
It has been a long debate, whether domestic animals are the result of a single domestication event in a restricted geographical area or of multiple, independent
domestication events in diferent geographical regions. he ample distribution of domesticated Eurasian mammals from Portugal to China supports the idea of diferent
independent domestications. his has been conirmed by molecular genetic data see reviews: Bruford
et al., 2003, Mignon-Grasteau et al., 2005, Dobney and Larson, 2006, Zeder
et al., 2006, but the answer depends on species. In some cases pigs and horses
Kategori