Domestication

Domestication is the scientific theory of the mutual relationship between plants or animals with the humans who have influence on their care and reproduction.[1] Charles Darwin recognized the small number of traits that made domestic species different from their wild ancestors. He was also the first to recognize the difference between conscious selective breeding in which humans directly select for desirable traits, and unconscious selection where traits evolve as a by-product of natural selection or from selection on other traits.[2][3][4] There is a genetic difference between domestic and wild populations. There is also such a difference between the domestication traits that researchers believe to have been essential at the early stages of domestication, and the improvement traits that have appeared since the split between wild and domestic populations.[5][6][7] Domestication traits are generally fixed within all domesticates, and were selected during the initial episode of domestication of that animal or plant, whereas improvement traits are present only in a proportion of domesticates, though they may be fixed in individual breeds or regional populations.[6][7][8]

The dog was the first domesticant and was established across Eurasia before the end of the Late Pleistocene era, well before cultivation and before the domestication of other animals.[9] The archaeological and genetic data suggest that long-term bidirectional gene flow between wild and domestic stocks – including donkeys, horses, New and Old World camelids, goats, sheep, and pigs – was common.[7][10] Given its importance to humans and its value as a model of evolutionary and demographic change, domestication has attracted scientists from archaeology, palaeontology, anthropology, botany, zoology, genetics, and the environmental sciences.[11]

Dogs and sheep were among the first animals to be domesticated.

Etymology

Domestication (from the Latin domesticus) means 'belonging to the house'.[12]

Related terminology

Plants domesticated primarily for aesthetic enjoyment in and around the home are usually called house plants or ornamentals, while those domesticated for large-scale food production are generally called crops. A distinction can be made between those domesticated plants that have been deliberately altered or selected for special desirable characteristics, termed cultigens, and those plants that are used for human benefit, but are essentially no different from the wild populations of the species. Animals domesticated for home companionship are usually called pets, while those domesticated for food or work are called livestock or farm animals.

Definitions

Domestication

Domestication has been defined as "a sustained multi-generational, mutualistic relationship in which one organism assumes a significant degree of influence over the reproduction and care of another organism in order to secure a more predictable supply of a resource of interest, and through which the partner organism gains advantage over individuals that remain outside this relationship, thereby benefitting and often increasing the fitness of both the domesticator and the target domesticate."[1] This definition recognizes both the biological and the cultural components of the domestication process and the impacts on both humans and the domesticated animals and plants. All past definitions of domestication have included a relationship between humans with plants and animals, but their differences lay in who was considered as the lead partner in the relationship. This new definition recognizes a mutualistic relationship in which both partners gain benefits. Domestication has vastly enhanced the reproductive output of crop plants, livestock, and pets far beyond that of their wild progenitors. Domesticates have provided humans with resources that they could more predictably and securely control, move, and redistribute, which has been the advantage that had fueled a population explosion of the agro-pastoralists and their spread to all corners of the planet.[13]

This biological mutualism is not restricted to humans with domestic crops and livestock but is well-documented in nonhuman species, especially among a number of social insect domesticators and their plant and animal domesticates, for example the ant–fungus mutualism that exists between leafcutter ants and certain fungii.[1]

Domestication syndrome

Domestication syndrome is a term often used to describe the suite of phenotypic traits arising during domestication that distinguish crops from their wild ancestors.[5][14] The term is also applied to animals and includes increased docility and tameness, coat color changes, reductions in tooth size, changes in craniofacial morphology, alterations in ear and tail form (e.g., floppy ears), more frequent and nonseasonal estrus cycles, alterations in adrenocorticotropic hormone levels, changed concentrations of several neurotransmitters, prolongations in juvenile behavior, and reductions in both total brain size and of particular brain regions.[15]

Domestication of animals

The zoomass of wild vertebrates is now decreasingly small compared to the biomass of domestic animals, with the calculated biomass of domestic cattle alone being greater than that of all wild mammals.[16] Because the evolution of domestic animals is ongoing, the process of domestication has a beginning but not an end. Various criteria have been established to provide a definition of domestic animals, but all decisions about exactly when an animal can be labelled "domesticated" in the zoological sense are arbitrary, although potentially useful.[11]

Sheep with an ear tag that is part of a national livestock identification system

Domestication versus taming

Domestication should not be confused with taming. Taming is the conditioned behavioral modification of a wild-born animal when its natural avoidance of humans is reduced and it accepts the presence of humans, but domestication is the permanent genetic modification of a bred lineage that leads to an inherited predisposition toward humans.[17][18][19] Human selection included tameness, but without a suitable evolutionary response then domestication was not achieved.[7] Domestic animals need not be tame in the behavioral sense, such as the Spanish fighting bull. Wild animals can be tame, such as a hand-raised cheetah. A domestic animal's breeding is controlled by humans and its tameness and tolerance of humans is genetically determined. However, an animal merely bred in captivity is not necessarily domesticated. Tigers, gorillas, and polar bears breed readily in captivity but are not domesticated.[18] Asian elephants are wild animals that with taming manifest outward signs of domestication, yet their breeding is not human controlled and thus they are not true domesticates.[18][20]

Universal features of domesticated animals

Domestication is a fluid and nonlinear process that may start, stop, reverse, or go on unexpected paths with no clear or universal threshold that separates the wild from the domestic. However, there are universal features held in common by all domesticated animals.[13]

Behavioral preadaption

Certain animal species, and certain individuals within those species, make better candidates for domestication than others because they exhibit certain behavioral characteristics: (1) the size and organization of their social structure; (2) the degree of selectivity in their choice of mates; (3) the ease and speed with which the parent bonds with young, and the maturity and mobility of the young at birth; (4) the degree of flexibility in diet and habitat tolerance; and (5) responses to humans and new environments, including flight responses and reactivity to external stimuli.[13]:Fig 1[21][22][23] Reduced wariness to humans and low reactivity to both humans and other external stimuli are a key pre-adaptation for domestication, and these behaviors are also the primary target of the selective pressures experienced by the animal undergoing domestication.[7][13] This implies that not all animals can be domesticated, e.g. a wild member of the horse family, the Zebra.[7][24]

One researcher has enquired as to why among the worlds 148 big wild terrestrial herbivorous mammals only 14 were to become domesticated, and proposed that their wild ancestors must have possessed six characteristics before they could have be considered for domestication:[3]:p168-174

Hereford cattle, domesticated for beef production.
  1. Efficient diet – Animals that can efficiently process what they eat and live off plants are less expensive to keep in captivity. Carnivores feed on flesh, which would require the domesticators to raise additional animals to feed the carnivores and therefore increase the consumption of plants further.
  2. Quick growth rate – Fast maturity rate compared to the human life span allows breeding intervention and makes the animal useful within an acceptable duration of caretaking. Some large animals require many years before they reach a useful size.
  3. Ability to breed in captivity – Animals that will not breed in captivity are limited to acquisition through capture in the wild.
  4. Pleasant disposition – Animals with nasty dispositions are dangerous to keep around humans.
  5. Tendency not to panic – Some species are nervous, fast, and prone to flight when they perceive a threat.
  6. Social structure – All species of domesticated large animals had wild ancestors that lived in herds, had a dominance hierarchy amongst the herd members, and the herds had overlapping home territories rather than their own home territory. This arrangement allows humans to take control of the dominance hierarchy.

Brain size and function

Reduction in skull size with neoteny - grey wolf and chihuahua skulls.

The sustained selection for lowered reactivity among animal domesticates has resulted in profound changes in brain form and function. The larger the size of the brain to begin with and the greater its degree of folding, the greater the degree of brain-size reduction under domestication.[13][25] Foxes that had been selectively bred for tameness over 40 years had experienced a significant reduction in cranial height and width and by inference in brain size,[13][26] which supports the hypothesis that brain-size reduction is an early response to the selective pressure for tameness and lowered reactivity that is the universal feature of animal domestication.[13] The most affected portion of the brain in domestic mammals is the limbic system, which in domestic dogs, pigs, and sheep show a 40% reduction in size compared with their wild species. This portion of the brain regulates endocrine function that influences behaviors such as aggression, wariness, and responses to environmentally induced stress - the key attributes of domestic animals.[13][25]

Pleiotropy

Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. These linked traits include the animal passing through fewer development stages so that as an adult it resembles a juvenile form of its ancestor.[13][27] Pedomorphosis in development can result in a neotenization of skull morphology, making the animal a more attractive and tractable domestic companion. Lop ears may be caused by neotonization that essentially freezes the formation of cartilage in the ears at more juvenile stage.[13][26] Piebald or spotted coat coloration may be caused by a linkage in the biochemical pathways of melanins (involved in coat coloration) and neurotransmitters (dopamine) that help shape behavior and cognition.[13][28] These linked traits may arise from mutations in a few key regulatory genes.[13][29] Mutations in only a few regulatory genes can have a major impact on a network of linked genes, resulting in major phenotypic changes. A similar mechanism controls some of the responses of plants to domestication.[13][30]

Limited reversion

Feral dogs, cats, goats, donkeys, pigs, and ferrets that have lived apart from humans for generations show no sign of regaining the brain mass of their wild progenitors.[13][31] Dingos have lived apart from humans for thousands of years but still have the same brain size as that of a domestic dog.[13][32] Feral dogs that actively avoid human contact are still dependent on the human niche for survival and have not reverted to the self-sustaining behaviors of their wolf ancestors.[13][33]

Domestication categories

Domestication can be considered as the final phase of intensification in the relationship between animal or plant sub-populations and human societies, but it is divided into several grades of intensification.[34] For studies in animal domestication, researchers have proposed five distinct categories: wild, captive wild, domestic, cross-breeds and feral.[23][35][36]

Wild animals are subject to natural selection, although the action of past demographic events and artificial selection induced by game management or habitat destruction cannot be excluded.[36]

Captive wild animals are directly affected by a relaxation of natural selection associated with feeding, breeding and protection/confinement by humans, and an intensification of artificial selection through passive selection for animals that are more suited to captivity.[36]

Domestic animals are subject to intensified artificial selection through husbandry practices with relaxation of natural selection associated with captivity and management.[36]

Cross-breed animals are genetic hybrids of wild and domestic parents. They may be forms intermediate between both parents, forms more similar to one parent than the other, or unique forms distinct from both parents. Hybrids can be intentionally bred for specific characteristics or unintentionally the result of contact with wild individuals.[36]

Feral animals are domesticates that have returned to a wild state. As such, they experience relaxed artificial and selection induced by the captive environment paired with intensified natural selection induced by the wild habitat.[36]

In 2015, a study compared the diversity of dental size, shape and allometry across the proposed domestication categories of modern pigs (Genus Sus). The study showed clear differences between the dental phenotypes of wild, captive wild, domestic, and hybrid pig populations, which supported the proposed categories through physical evidence. The study did not cover feral pig populations but called for further research to be undertaken on them, and on the genetic differences with hybrid pigs.[36]

Domestication pathways

Since 2012, a multi-stage model of animal domestication has been accepted by two groups. The first group proposed that animal domestication proceeded along a continuum of stages from anthropophily, commensalism, control in the wild, control of captive animals, extensive breeding, intensive breeding, and finally to pets in a slow, gradually intensifying relationship between humans and animals.[11][34]

The second group proposed that there were three major pathways that most animal domesticates followed into domestication: (1) commensals, adapted to a human niche (e.g., dogs, cats, fowl, possibly pigs); (2) prey animals sought for food (e.g., sheep, goats, cattle, water buffalo, yak, pig, reindeer, llama and alpaca); and (3) targeted animals for draft and nonfood resources (e.g., horse, donkey, camel).[7][10][13] The beginnings of animal domestication involved a protracted coevolutionary process with multiple stages along different pathways. Humans did not intend to domesticate animals from, or at least they did not envision a domesticated animal resulting from, either the commensal or prey pathways. In both of these cases, humans became entangled with these species as the relationship between them, and the human role in their survival and reproduction, intensified.[7] Although the directed pathway proceeded from capture to taming, the other two pathways are not as goal-oriented and archaeological records suggest that they take place over much longer time frames.[11]

Commensal pathway

The commensal pathway was traveled by animals that fed on refuse around human habitats or by animals that preyed on other animals drawn to human camps. Those animals established a commensal relationship with humans in which the animals benefited but the humans received no harm but little benefit. Those animals that were most capable of taking advantage of the resources associated with human camps would have been the tamer, less aggressive individuals with shorter fight or flight distances. Later, these animals developed closer social or economic bonds with humans that lead to a domestic relationship.[7][13][37] The leap from a synanthropic population to a domestic one could only have taken place after the animals had progressed from anthropophily to habituation, to commensalism and partnership, when the relationship between animal and human would have laid the foundation for domestication, including captivity and human-controlled breeding. From this perspective, animal domestication is a coevolutionary process in which a population responds to selective pressure while adapting to a novel niche that included another species with evolving behaviors.[7]

Commensal pathway animals include dogs, cats, fowl, and possibly pigs. The dog is a classic example of a domestic animal that likely traveled a commensal pathway into domestication. The dog was the first domesticant, and was domesticated and widely established across Eurasia before the end of the Pleistocene, well before cultivation or the domestication of other animals.[9] Ancient DNA supports the hypothesis that dog domestication preceded the emergence of agriculture[38][39] and was initiated close to the Last Glacial Maximum 27,000 YBP when hunter-gatherers preyed on megafauna, and when proto-dogs might have taken advantage of carcasses left on site by early hunters, assisted in the capture of prey, or provided defense from large competing predators at kill-sites.[39] The wolves most likely drawn to human camps were the less-aggressive, subdominant pack members with lowered flight response, higher stress thresholds, less wary around humans, and therefore better candidates for domestication.[13] The earliest sign of domestication in dogs was the neotonization of skull morphology[13][26][40] and the shortening of snout length that results in tooth crowding, reduction in tooth size, and a reduction in the number of teeth,[13][41] which has been attributed to the strong selection for reduced aggression.[13][26] This process may have begun during the initial commensal stage of dog domestication, even before humans began to be active partners in the process.[7][13]

A maternal mitochondrial, paternal Y chromosome, and microsatellite assessment of two wolf populations in North America and combined with satellite telemetry data revealed significant genetic and morphological differences between one population that migrated with and preyed upon caribou, and another territorial ecotype population that remained in a boreal coniferous forest. Though these two populations spend a period of the year in the same place, and though there was evidence of gene flow between them, the difference in prey–habitat specialization has been sufficient to maintain genetic and even coloration divergence.[7][42] A study has identified the remains of a population of extinct Pleistocene Beringian wolves with unique mitochondrial signatures. The skull shape, tooth wear, and isotopic signatures suggested these were specialist megafauna hunters and scavengers that went extinct while less specialized wolf ecotypes survived.[7][43] Analogous to the modern wolf ecotype that has evolved to track and prey upon caribou, a Pleistocene wolf population could have begun following mobile hunter-gatherers, thus slowly acquiring genetic and phenotypic differences that would have allowed them to more successfully adapt to the human habitat.[7][44]

The chicken is one of the most widespread domesticated species and one of the human world's largest sources of protein. Although the chicken was domesticated in South-East Asia, archaeological evidence suggests that it did not become kept as a livestock species until 400 BCE in the Levant.[45] Prior to this, chickens had been associated with humans for thousands of years and kept for cock-fighting, rituals, and royal zoos, therefore they were not originally a prey species.[45][46]

Prey pathway

Cow domestication in North India for milk production.

The prey pathway was the way in which most major livestock species entered into domestication as these were once hunted by humans for their meat. Domestication was likely initiated when humans began to experiment with hunting strategies designed to increase the availability of these prey, perhaps as a response to localized pressure on the supply of the animal. Over time and with the more responsive species, these game-management strategies developed into herd-management strategies that included the sustained multi-generational control over the animals’ movement, feeding, and reproduction. As human interference in the life-cycles of prey animals intensified, the evolutionary pressures for a lack of aggression would have led to an acquisition of the same domestication syndrome traits found in the commensal domesticates.[7][13][37]

Prey pathway animals include sheep, goats, cattle, water buffalo, yak, pig, reindeer, llama and alpaca. The right conditions for the domestication for some of them appear to have been in place in the central and eastern Fertile Crescent at the end of the Younger Dryas climatic downturn and the beginning of the Early Holocene about 11,700 YBP, and by 10,000 YBP people were preferentially killing young males of a variety of species and allowed the females to live in order to produce more offspring.[7][13] By measuring the size, sex ratios, and mortality profiles of zooarchaeological specimens, archeologists have been able to document changes in the management strategies of hunted sheep, goats, pigs, and cows in the Fertile Crescent starting 11,700 YBP. A recent demographic and metrical study of cow and pig remains at Sha’ar Hagolan, Israel, demonstrated that both species were severely overhunted before domestication, suggesting that the intensive exploitation led to management strategies adopted throughout the region that ultimately led to the domestication of these populations following the prey pathway. This pattern of overhunting before domestication suggests that the prey pathway was as accidental and unintentional as the commensal pathway.[7][37]

Directed pathway

Kazakh shepherd with horse and dogs. Their job is to guard the sheep from predators.

The directed pathway was a more deliberate and directed process initiated by humans with the goal of domesticating a free-living animal. It probably only came into being once people were familiar with either commensal or prey-pathway domesticated animals. These animals were likely not to possess many of the behavioral preadaptions some species show before domestication. Therefore, the domestication of these animals requires more deliberate effort by humans to work around behaviors that do not assist domestication, with increased technological assistance needed.[7][13][37]

Humans were already reliant on domestic plants and animals when they imagined the domestic versions of wild animals. Although horses, donkeys, and Old World camels were sometimes hunted as prey species, they were each deliberately brought into the human niche for sources of transport. Domestication was still a multi-generational adaptation to human selection pressures, including tameness, but without a suitable evolutionary response then domestication was not achieved.[7] For example, despite the fact that hunters of the Near Eastern gazelle in the Epipaleolithic avoided culling reproductive females to promote population balance, neither gazelles[7][24] nor zebras[7][47] possessed the necessary prerequisites and were never domesticated. There is no clear evidence for the domestication of any herded prey animal originating in Africa.[7]

Multiple pathways

The pathways that animals may have followed are not mutually exclusive. Pigs, for example, may have been domesticated as their populations became accustomed to the human niche, which would suggest a commensal pathway, or they may have been hunted and followed a prey pathway, or both.[7][13][37]

Post-domestication gene flow

As agricultural societies migrated away from the domestication centers taking their domestic partners with them, they encountered populations of wild animals of the same or sister species. Because domestics often shared a recent common ancestor with the wild populations, they were capable of producing fertile offspring. Domestic populations were small relative to the surrounding wild populations, and repeated hybridizations between the two eventually led to the domestic population becoming more genetically divergent from its original domestic source population.[11][48]

Advances in DNA sequencing technology allow the nuclear genome to be accessed and analyzed in a population genetics framework. The increased resolution of nuclear sequences has demonstrated that gene flow is common, not only between geographically diverse domestic populations of the same species but also between domestic populations and wild species that never gave rise to a domestic population.[7] The yellow leg trait possessed by numerous modern commercial chicken breeds was acquired via introgression from the grey junglefowl indigenous to South Asia.[7][49] African cattle are hybrids that possess both a European Taurine cattle maternal mitochondrial signal and an Asian Indicine cattle paternal Y-chromosome signature.[7][50] Numerous other bovid species, including bison, yak, banteng, and gaur also hybridize with ease.[7][51] Cats[7][52] and horses[7][53] have been shown to hybridize with many closely related species, and domestic honey bees have mated with so many different species they now possess genomes more variable than their original wild progenitors.[7][54] The archaeological and genetic data suggests that long-term bidirectional gene flow between wild and domestic stocks - including donkeys, horses, New and Old World camelids, goats, sheep, and pigs - was common.[7][10] Bidirectional gene flow between domestic and wild reindeer continues today.[7]

The consequence of this introgression is that modern domestic populations can often appear to have much greater genomic affinity to wild populations that were never involved in the original domestication process. Therefore, it is proposed that the term "domestication" should be reserved solely for the initial process of domestication of a discrete population in time and space. Subsequent admixture between introduced domestic populations and local wild populations that were never domesticated should be referred to as "introgressive capture". Conflating these two processes muddles our understanding of the original process and can lead to an artificial inflation of the number of times domestication took place.[7][11]

The sustained admixture between different dog and wolf populations across the Old and New Worlds over at least the last 10,000 years has blurred the genetic signatures and confounded efforts of researchers at pinpointing the origins of dogs.[9] None of the modern wolf populations are related to the Pleistocene wolves that were first domesticated,[7][55] and the extinction of the wolves that were the direct ancestors of dogs has muddied efforts to pinpoint the time and place of dog domestication.[7]

Positive selection

Charles Darwin recognized the small number of traits that made domestic species different from their wild ancestors. He was also the first to recognize the difference between conscious selective breeding in which humans directly select for desirable traits, and unconscious selection where traits evolve as a by-product of natural selection or from selection on other traits.[2][3][4] Domestic animals have variations in coat color as well as texture, dwarf and giant varieties, and changes in their reproductive cycle, and many others have tooth crowding and floppy ears.

Although it is easy to assume that each of these traits was uniquely selected for by hunter-gatherers and early farmers, beginning in 1959 Dmitri K. Belyaev tested the reactions of silver foxes to a hand placed in their cage and selected the tamest, least aggressive individuals to breed. His hypothesis was that, by selecting a behavioral trait, he could also influence the phenotype of subsequent generations, making them more domestic in appearance. Over the next 40 years, he succeeded in producing foxes with traits that were never directly selected for, including piebald coats floppy ears, upturned tails, shortened snouts, and shifts in developmental timing.[11][26][56] In the 1980s, a researcher used a set of behavioral, cognitive, and visible phenotypic markers, such as coat colour, to produce domesticated fallow deer within a few generations.[11][57] Similar results for tameness and fear have been found for mink[58] and Japanese quail.[59] In addition to demonstrating that domestic phenotypic traits could arise through selection for a behavioral trait, and domestic behavioral traits could arise through the selection for a phenotypic trait, these experiments provided a mechanism to explain how the animal domestication process could have begun without deliberate human forethought and action.[11]

Pig herding in fog, Armenia. Human selection for domestic traits is not impacted by later gene flow from wild boar.[60][61]

The genetic difference between domestic and wild populations can be framed within two considerations. The first distinguishes between domestication traits that are presumed to have been essential at the early stages of domestication, and improvement traits that have appeared since the split between wild and domestic populations.[5][6][7] Domestication traits are generally fixed within all domesticates and were selected during the initial episode of domestication, whereas improvement traits are present only in a proportion of domesticates, though they may be fixed in individual breeds or regional populations.[6][7][8] A second issue is whether traits associated with the domestication syndrome resulted from a relaxation of selection as animals exited the wild environment or from positive selection resulting from intentional and unintentional human preference. Some recent genomic studies on the genetic basis of traits associated with the domestication syndrome have shed light on both of these issues.[7]

Geneticists have identified more than 300 genetic loci and 150 genes associated with coat color variability.[11][62] Knowing the mutations associated with different colors has allowed the correlation between the timing of the appearance of variable coat colors in horses with the timing of their domestication.[11][63] Other studies have shown how human-induced selection is responsible for the allelic variation in pigs.[11][64] Together, these insights suggest that, although natural selection has kept variation to a minimum before domestication, humans have actively selected for novel coat colors as soon as they appeared in managed populations.[11][28]

In 2015, a study looked at over 100 pig genome sequences to ascertain their process of domestication. The process of domestication was assumed to have been initiated by humans, involved few individuals and relied on reproductive isolation between wild and domestic forms, but the study found that the assumption of reproductive isolation with population bottlenecks was not supported. The study indicated that pigs were domesticated separately in Western Asia and China, with Western Asian pigs introduced into Europe where they crossed with wild boar. A model that fitted the data included admixture with a now extinct ghost population of wild pigs during the Pleistocene. The study also found that despite back-crossing with wild pigs, the genomes of domestic pigs have strong signatures of selection at genetic loci that affect behavior and morphology. The study concluded that human selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars and created domestication islands in the genome. The same process may also apply to other domesticated animals.[60][61]

Unlike other domestic species which were primarily selected for production-related traits, dogs were initially selected for their behaviors.[65][66] In 2016, a study found that there were only 11 fixed genes that showed variation between wolves and dogs. These gene variations were unlikely to have been the result of natural evolution, and indicate selection on both morphology and behavior during dog domestication. These genes have been shown to have an impact on the catecholamine synthesis pathway, with the majority of the genes affecting the fight-or-flight response[67][66] (i.e. selection for tameness), and emotional processing.[66] Dogs generally show reduced fear and aggression compared to wolves. [68][66] Some of these genes have been associated with aggression in some dog breeds, indicating their importance in both the initial domestication and then later in breed formation.[66]

Domestication of plants

The initial domestication of animals impacted most on the genes that controlled their behavior, but the initial domestication of plants impacted most on the genes that controlled their morphology (seed size, plant architecture, dispersal mechanisms) and their physiology (timing of germination or ripening).[13][24]

Farmers with wheat and cattle - Ancient Egyptian art 1,422 YBP
Industrialized wheat harvest - North America today

The domestication of wheat provides an example. Wild wheat falls to the ground to reseed itself when ripe, but domesticated wheat stays on the stem for easier harvesting. There is evidence that this change was possible because of a random mutation that happened in the wild populations at the beginning of wheat's cultivation. Wheat with this mutation was harvested more frequently and became the seed for the next crop. Therefore, without realizing, early farmers selected for this mutation, which may otherwise have died out. The result is domesticated wheat, which relies on farmers for its own reproduction and dissemination.[69]

The earliest human attempts at plant domestication occurred in South-Western Asia. There is early evidence for conscious cultivation and trait selection of plants by pre-Neolithic groups in Syria: grains of rye with domestic traits have been recovered from Epi-Palaeolithic (c. 11,050 BCE) contexts at Abu Hureyra in Syria,[70] but this appears to be a localised phenomenon resulting from cultivation of stands of wild rye, rather than a definitive step towards domestication.[70]

By 10,000 BCE the bottle gourd (Lagenaria siceraria) plant, used as a container before the advent of ceramic technology, appears to have been domesticated. The domesticated bottle gourd reached the Americas from Asia by 8000 BCE, most likely due to the migration of peoples from Asia to America.[71]

Cereal crops were first domesticated around 9000 BCE in the Fertile Crescent in the Middle East. The first domesticated crops were generally annuals with large seeds or fruits. These included pulses such as peas and grains such as wheat. The Middle East was especially suited to these species; the dry-summer climate was conducive to the evolution of large-seeded annual plants, and the variety of elevations led to a great variety of species. As domestication took place humans began to move from a hunter-gatherer society to a settled agricultural society. This change would eventually lead, some 4000 to 5000 years later, to the first city states and eventually the rise of civilization itself.

Continued domestication was gradual, a process of trial and error that occurred intermittently. Over time perennials and small trees began to be domesticated including apples and olives. Some plants were not domesticated until recently such as the macadamia nut and the pecan.

In other parts of the world very different species were domesticated. In the Americas squash, maize, beans, and perhaps manioc (also known as cassava) formed the core of the diet. In East Asia millet, rice, and soy were the most important crops. Some areas of the world such as Southern Africa, Australia, California and southern South America never saw local species domesticated.

Domesticated plants often differ from their wild relatives in the way they spread to a more diverse environment and have a wider geographic range;[72] they may also have a different ecological preference; flower and fruit simultaneously; may lack shattering or scattering of seeds, and may have lost their dispersal mechanisms completely; have larger fruits and seeds, and so lower efficiency of dispersal; may have been converted from a perennial to annual; have lost seed dormancy and photoperiodic controls; lack normal pollinating organs; may have a different breeding system; may lack defensive adaptations such as hairs, spines and thorns, protective coverings and sturdiness; may have better palatability and chemical composition, rendering them more likely to be eaten by animals; may be more susceptible to diseases and pests; may develop seedless parthenocarpic fruits; may have undergone selection for double flowers, which may involve conversion of stamens into petals; may have become sexually sterile and therefore only reproduce vegetatively.

Negative aspects

Jared Diamond in his book Guns, Germs, and Steel discusses the universal tendency for populations that have acquired agriculture and domestic animals to first develop a large population and then to move. He recounts migrations of people armed with a suite of domestic crops overtaking, displacing or killing indigenous hunter-gatherers,[3]:p112 until after millions of years of following the hunter-gatherer lifestyle there will shortly be none remaining.[3]:p86

Selection of animals for visible "desirable" traits may make them unfit in other, unseen, ways. The consequences for the captive and domesticated animals were reduction in size, piebald color, shorter faces with smaller and fewer teeth, diminished horns, weak muscle ridges, and less genetic variability. Poor joint definition, late fusion of the limb bone epiphyses with the diaphyses, hair changes, greater fat accumulation, smaller brains, simplified behavior patterns, extended immaturity, and more pathology are a few of the defects of domestic animals. All of these changes have been documented in direct observations of the rat in the 19th century, by archaeological evidence, and confirmed by animal breeders in the 20th century.[73] A 2014, a study proposed the theory that under selection, docility in mammals and birds results partly from a slowed pace of neural crest development, that would in turn cause a reduced fear–startle response due to mild neurocristopathy that causes domestication syndrome. The theory was unable to explain curly tails nor domestication syndrome exhibited by plants.[15]

One side effect of domestication has been zoonotic diseases. For example, cattle have given humanity various viral poxes, measles, and tuberculosis; pigs and ducks have given influenza; and horses have given the rhinoviruses. Humans share over sixty diseases with dogs. Many parasites also have their origins in domestic animals.[3] The advent of domestication resulted in denser human populations which provided ripe conditions for pathogens to reproduce, mutate, spread, and eventually find a new host in humans.

Other negative aspects of domestication have been explored. For example, Paul Shepherd writes "Man substitutes controlled breeding for natural selection; animals are selected for special traits like milk production of passivity, at the expense of overall fitness and naturewide relationships...Though domestication broadens the diversity of forms – that is, increases visible polymorphism – it undermines the crisp demarcations that separate wild species and cripples our recognition of the species as a group. Knowing only domestic animals dulls our understanding of the way in which unity and discontinuity occur as patterns in nature, and substitutes an attention to individuals and breeds. The wide variety of size, color, shape, and form of domestic horses, for example, blurs the distinction among different species of Equus that once were constant and meaningful."[74]

Going further, some anarcho-primitivist authors describe domestication as the process by which previously nomadic human populations shifted towards a sedentary or settled existence through agriculture and animal husbandry. They claim that this kind of domestication demands a totalitarian relationship with both the land and the plants and animals being domesticated. They say that whereas, in a state of wildness, all life shares and competes for resources, domestication destroys this balance. Domesticated landscape (e.g. pastoral lands/agricultural fields and, to a lesser degree, horticulture and gardening) ends the open sharing of resources; where "this was everyone's," it is now "mine." Anarcho-primitivists state that this notion of ownership laid the foundation for social hierarchy as property and power emerged. It also involved the destruction, enslavement, or assimilation of other groups of early people who did not make such a transition.[75]

Genetic pollution

Main article: Genetic pollution

Genetic pollution is a controversial term for uncontrolled gene flow from domestic into wild populations.[76][77] This gene flow is undesirable according to some environmentalists and conservationists, including groups such as Greenpeace, TRAFFIC, and GeneWatch UK.

Lists

Early domestication: cow being milked in ancient Egypt.

List of domesticated animals

Domesticated animals include, or have included, a variety of mammals and birds. Some common domesticated animals includes the Cat, Cattle, Chicken, Dog, Donkey, Duck, Goat, Goose, Horse, Pig and Sheep.

For more details on this topic, see List of domesticated animals.

List of hybrid domestic animals

Hybrid domestic animals include those domestic animals that have been crossed with their wild relatives, and include the Beefalo, Bengal cat, Cama (animal), Chausie, Coydog, Dzo, Domesticated hedgehog, Sheep-goat hybrid, Hinny, Huarizo, Iron Age pig, Mule, Savannah (cat), Wolfdog, Yakalo, Zeedonk, Zorse, Zony and Zubron.

List of domesticated plants

See also

References

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Bibliography

Further reading

External links

Look up domestication or taming in Wiktionary, the free dictionary.
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