Domestic Animals And Wild Animals Essay

The domestication of animals is the mutual relationship between animals with the humans who have influence on their care and reproduction.[1]Charles Darwin recognized the small number of traits that made domesticated 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]

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.[9][10][11] 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 availability and the degree of selectivity in their choice of mates; (3) the ease and speed with which the parents bond with their 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.[12]:Fig 1[13][14][15]

It is 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, alpaca, and turkey); and (3) targeted animals for draft and nonfood resources (e.g., horse, donkey, camel).[7][12][16][17][18][19][20][21][22] The dog was the first to be domesticated,[23][24] and was established across Eurasia before the end of the Late Pleistocene era, well before cultivation and before the domestication of other animals.[23] Unlike other domestic species which were primarily selected for production-related traits, dogs were initially selected for their behaviors.[25][26] 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][17] One study has concluded that human selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars into pigs and created domestication islands in the genome. The same process may also apply to other domesticated animals.[27][28]

Definitions[edit]

Domestication[edit]

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][12][29][30][31] This definition recognizes both the biological and the cultural components of the domestication process and the effects 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.[12]

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[edit]

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][32] 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.[33]

Difference from taming[edit]

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.[9][10][11] 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.[10] 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.[10][34]

Impact on humans[edit]

Further information: History of agriculture

The domestication of animals began with the wolf (Canis lupus) at least 15,000 years before present (YBP), which then led to a rapid shift in the evolution, ecology, and demography of both humans and numerous species of animals and plants.[35][7] The sudden appearance of the domestic dog (Canis lupus familiaris) in the archaeological record was followed by livestock and crop domestication, as well as the transition of humans from foraging to farming in different places and times across the planet.[35][36][37]

Around 10,000 YBP, a new way of life emerged for humans through the management and exploitation of plant and animal species, leading to higher-density populations in the centers of domestication,[35][38] the expansion of agricultural economies, and the development of urban communities.[35][39]

Universal features[edit]

The biomass 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.[40] 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.[41] 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.[12]

Behavioral preadaption[edit]

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 availability and the degree of selectivity in their choice of mates; (3) the ease and speed with which the parents bond with their 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.[12]:Fig 1[13][14][15] 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][12] This implies that not all animals can be domesticated, e.g. a wild member of the horse family, the Zebra.[7][37]

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

  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 mammals had wild ancestors that lived in herds with a dominance hierarchy amongst the herd members, and the herds had overlapping home territories rather than mutually exclusive home territories. This arrangement allows humans to take control of the dominance hierarchy.

Brain size and function[edit]

The sustained selection for lowered reactivity among mammal 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.[12][42]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,[12][43] 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.[12] 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, all attributes which are dramatically affected by domestication.[12][42]

Pleiotropy[edit]

A putative cause for the broad changes seen in domestication syndrome is pleiotropy. Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Certain physiological changes characterize domestic animals of many species. These changes include extensive white markings (particularly on the head) floppy ears, and curly tails. These arise even when tameness is the trait under selective pressure.[44] The genes involved in tameness are largely unknown, so it is not known how or to what extent pleiotropy contributes to domestication syndrome. Tameness may be caused by the down regulation of fear and stress responses via reduction of the adrenal glands.[44] Based on this, the pleiotropy hypotheses can be separated into two theories. The Neural Crest Hypothesis relates adrenal gland function to deficits in neural crest cells during development. The Single Genetic Regulatory Network Hypothesis claims that genetic changes in upstream regulators affect downstream systems.[45][46]

Neural crest cells (NCC) are vertebrate embryonic stem cells that function directly and indirectly during early embryogenesis to produce many tissue types.[45] Because the traits commonly affected by domestication syndrome are all derived from NCC in development, the neural crest hypothesis suggests that deficits in these cells cause the domain of phenotypes seen in domestication syndrome.[46] These deficits could cause changes we see to many domestic mammals, such as lopped ears (seen in rabbit, dog, fox, pig, sheep, goat, cattle, and donkeys) as well as curly tails (pigs, foxes, and dogs). Although they do not affect the development of the adrenal cortex directly, the neural crest cells may be involved in relevant upstream embryological interactions.[45] Furthermore, artificial selection targeting tameness may affect genes that control the concentration or movement of NCCs in the embryo, leading to a variety of phenotypes.[46]

The single genetic regulatory network hypothesis proposes that domestication syndrome results from mutations in genes that regulate the expression pattern of more downstream genes.[44] For example piebald, or spotted coat coloration, may be caused by a linkage in the biochemical pathways of melanins involved in coat coloration and neurotransmitters such as dopamine that help shape behavior and cognition.[12][47] These linked traits may arise from mutations in a few key regulatory genes.[12] A problem with this hypothesis is that it proposes that there are mutations in gene networks that cause dramatic effects that are not lethal, however no currently known genetic regulatory networks cause such dramatic change in so many different traits.[45]

Limited reversion[edit]

Feral mammals such as 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.[12][48]Dingos have lived apart from humans for thousands of years but still have the same brain size as that of a domestic dog.[12][49] 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.[12][50]

Categories[edit]

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.[51] For studies in animal domestication, researchers have proposed five distinct categories: wild, captive wild, domestic, cross-breeds and feral.[15][52][53]

Wild animals 
Subject to natural selection, although the action of past demographic events and artificial selection induced by game management or habitat destruction cannot be excluded.[53]
Captive wild animals 
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.[53]
Domestic animals 
Subject to intensified artificial selection through husbandry practices with relaxation of natural selection associated with captivity and management.[53]
Cross-breed animals 
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 can arise unintentionally as the result of contact with wild individuals.[53]
Feral animals 
Domesticates that have returned to a wild state. As such, they experience relaxed artificial selection induced by the captive environment paired with intensified natural selection induced by the wild habitat.[53]

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.[53]

Pathways[edit]

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.[41][51]

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][12][16][17][18][19][20][21][22] 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.[41]

Commensal pathway[edit]

The commensal pathway was traveled by vertebrates 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.[54][55][56] Later, these animals developed closer social or economic bonds with humans that led to a domestic relationship.[7][12][16] 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[23][24] and was domesticated and widely established across Eurasia before the end of the Pleistocene, well before cultivation or the domestication of other animals.[23]Ancient DNA supports the hypothesis that dog domestication preceded the emergence of agriculture[57][58] 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.[58] The wolves most likely drawn to human camps were the less aggressive, subdominant pack members with lowered flight response, higher stress thresholds, and less wariness around humans, and therefore they were better candidates for domestication.[12] The earliest sign of domestication in dogs was the neotenization of skull morphology[12][43][59] and the shortening of snout length that results in tooth crowding, reduction in tooth size, and a reduction in the number of teeth,[12][60] which has been attributed to the strong selection for reduced aggression.[12][43] 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][12]

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][61] A study has identified the remains of a population of extinct PleistoceneBeringian wolves with unique mitochondrial signatures. The skull shape, tooth wear, and isotopic signatures suggested these were specialist megafauna hunters and scavengers that became extinct while less specialized wolf ecotypes survived.[7][62] 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][63]

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 was not kept as a livestock species until 400 BCE in the Levant.[64] Prior to this, chickens had been associated with humans for thousands of years and kept for cock-fighting, rituals, and royal zoos, so they were not originally a prey species.[64][65] The chicken was not a popular food in Europe until only one thousand years ago.[66]

Prey pathway[edit]

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][12][16]

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][12] 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][16]

Directed pathway[edit]

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][12][16]

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][37] nor zebras[7][67] 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[edit]

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][12][16]

Post-domestication gene flow[edit]

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.[41][68]

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][69] African cattle are hybrids that possess both a European Taurine cattle maternal mitochondrial signal and an Asian Indicine cattle paternal Y-chromosome signature.[7][70] Numerous other bovid species, including bison, yak, banteng, and gaur also hybridize with ease.[7][71] Cats[7][72] and horses[7][73] 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][74] 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][17] 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][41]

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.[23] None of the modern wolf populations are related to the Pleistocene wolves that were first domesticated,[7][75] 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[edit]

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 Dmitry Belyayev 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.[41][43][76] 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.[41][77] Similar results for tameness and fear have been found for mink[78] and Japanese quail.[79] 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.[41]

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.[41][80] Knowing the mutations associated with different colors has allowed some correlation between the timing of the appearance of variable coat colors in horses with the timing of their domestication.[41][81] Other studies have shown how human-induced selection is responsible for the allelic variation in pigs.[41][82] 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.[41][47]

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.[27][28]

Unlike other domestic species which were primarily selected for production-related traits, dogs were initially selected for their behaviors.[25][26] 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 affect the catecholamine synthesis pathway, with the majority of the genes affecting the fight-or-flight response[26][83] (i.e. selection for tameness), and emotional processing.[26] Dogs generally show reduced fear and aggression compared to wolves.[26][84] 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.[26]

See also[edit]

References[edit]

  1. ^ abcZeder, M. A. (2015). "Core questions in domestication Research". Proceedings of the National Academy of Sciences of the United States of America. 112 (11): 3191–3198. doi:10.1073/pnas.1501711112. PMC 4371924. PMID 25713127. 
  2. ^ abDarwin, Charles (1868). The Variation of Animals and Plants under Domestication. London: John Murray. OCLC 156100686. 
  3. ^ abcDiamond, Jared (1997). Guns, Germs, and Steel. London: Chatto and Windus. ISBN 978-0-09-930278-0. 
  4. ^ abLarson, G.; Piperno, D. R.; Allaby, R. G.; Purugganan, M. D.; Andersson, L.; Arroyo-Kalin, M.; Barton, L.; Climer Vigueira, C.; Denham, T.; Dobney, K.; Doust, A. N.; Gepts, Paul; Gilbert, M. T. P.; Gremillion, K. J.; Lucas, L.; Lukens, L.; Marshall, F. B.; Olsen, K. M.; Pires, J. C.; Richerson, P. J.; Rubio De Casas, R.; Sanjur, O. I.; Thomas, M. G.; Fuller, D. Q. (2014). "Current perspectives and the future of domestication studies". Proceedings of the National Academy of Sciences. 111 (17): 6139–6146. doi:10.1073/pnas.1323964111. PMC 4035915. PMID 24757054. 
  5. ^ abcOlsen, K. M.; Wendel, J. F. (2013). "A bountiful harvest: genomic insights into crop domestication phenotypes". Annuaul Review of Plant Biology. 64: 47–70. doi:10.1146/annurev-arplant-050312-120048. PMID 23451788. 
  6. ^ abcdDoust, A. N.; Lukens, L.; Olsen, K. M.; Mauro-Herrera, M.; Meyer, A.; Rogers, K. (2014). "Beyond the single gene: How epistasis and gene-by-environment effects influence crop domestication". Proceedings of the National Academy of Sciences. 111 (17): 6178–6183. doi:10.1073/pnas.1308940110. PMC 4035984. PMID 24753598. 
  7. ^ abcdefghijklmnopqrstuvwxyzaaabacadaeafagahaiajakalamanLarson, G. (2014). "The Evolution of Animal Domestication"(PDF). Annual Review of Ecology, Evolution, and Systematics. 45: 115–36. doi:10.1146/annurev-ecolsys-110512-135813. 
  8. ^ abMeyer, Rachel S.; Purugganan, Michael D. (2013). "Evolution of crop species: Genetics of domestication and diversification". Nature Reviews Genetics. 14 (12): 840–52. doi:10.1038/nrg3605. PMID 24240513. 
  9. ^ abPrice, Edward O. (2008). Principles and Applications of Domestic Animal Behavior: An Introductory Text. Cambridge University Press. ISBN 9781780640556. Retrieved January 21, 2016. 
  10. ^ abcd
Dogs and sheep were among the first animals to be domesticated.
Hereford cattle, domesticated for beef production.
Reduction in skull size with neoteny - grey wolf and chihuahua skulls
Domesticated dairy cows in North India
Kazakh shepherd with horse and dogs. Their job is to guard the sheep from predators.
Pig herding in fog, Armenia. Human selection for domestic traits is not affected by later gene flow from wild boar.[27][28]

One day in London in 1855, during an unusually cold winter, Charles Darwin went for a walk. As he strolled along the banks of the ice-bound Thames, he noticed some pigeons foraging for food. He began to wonder about their relationship to so-called ‘fancy pigeons’, the more exotic varieties favoured by fanciers and breeders. Was there an ancestor common to the nondescript blue-grey creatures on the riverbank, and those featured on the front page of Darwin’s newspaper that day, all puffed-up chests and improbable neck-ruffs?

Darwin was a pigeon-fancier himself and raised birds at his home in Kent. Observing these creatures closely, he became convinced that the various pigeon breeds descended from a single ancestor: the rock dove, or Columba livia. His fancy pigeons all interbred freely, and it seemed unlikely that a different species corresponded to each particular characteristic, none of which lived on in the wild. If man could breed a multiplicity of forms from a single thing, Darwin thought, perhaps nature could too. Domestication, for Darwin, was a laboratory for the study of evolution.

But what, exactly, is domestication? Darwin noticed that, when it came to mammals, virtually all domesticated species shared a bundle of characteristics that their wild ancestors lacked. These included traits you might expect, such as tameness and increased sociability, but also a number of more surprising ones, such as smaller teeth, floppy ears, variable colour, shortened faces and limbs, curly tails, smaller brains, and extended juvenile behaviour. Darwin thought these features might have something to do with the hybridisation of different breeds or the better diet and gentler ‘conditions of living’ for tame animals – but he couldn’t explain how these processes would produce such a broad spectrum of attributes across so many different species.

We’re still puzzling over the hows and whys of domestication. Advances in animal genetics, both ancient and modern, coupled with new techniques in archaeology, have illuminated at least some of the mechanisms behind this previously hidden transition. It’s deeply bound up with the origins of the so-called ‘Neolithic revolution’, when humans first turned to farming around 12,000 years ago. But the history of the human relationship to animals and agriculture is now being rewritten. Domestication, it appears, wasn’t a one-way street: new research suggests that species moved from wild to tame multiple times over their history, and that human agency played a far smaller role than previously believed. It’s also becoming clearer that, in the millennia we’ve spent changing animal genetics, they’ve been changing us in turn.

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Melinda Zeder, an archaeologist at the Smithsonian Institution in Washington, DC, has identified three main routes to domestication. The ‘directed’ pathway is the most straightforward. It happens when humans deliberately set out to amplify some desired trait in a species, for example, breeding donkeys to be good for transport, or minks to have luxuriant fur – or, for that matter, fancy pigeons to look fancy. (Darwin marvelled at the ‘astonishing’ diversity of fancy pigeons in On the Origin of Species, from the hooded Jacobin to the immensely heavy Runt, the short-faced Tumbler to the magnificently berumped Fantail. ‘I have kept every breed which I could purchase or obtain, and have been most kindly favoured with skins from several quarters of the world,’ he wrote in 1859, adding that ‘not one man in a thousand has accuracy of eye and judgment sufficient to become an eminent breeder.’)

The ‘prey’ pathway, meanwhile, happens when humans take animals they previously hunted in the wild, and begin managing them in herds, either by creating environments that suit them, or capturing and confining them. Goats, sheep and cattle all fit into this paradigm, as do, somewhat more unexpectedly, horses, which were raised for meat well before anyone figured out you could ride them as well.

Aurochs, the wild progenitors of today’s cattle, were particularly difficult to domesticate. Picture a cow the size and weight of a Volkswagen bus, with a set of wickedly curved horns extending three feet either side of its skull. Farmers started trying to shepherd these immense creatures some 10,500 years ago, probably somewhere near the border of Turkey and Syria. Most modern-day cattle breeds descend from just 81 founding female auroch lineages. And, unlike their contemporaries the mammoth and woolly rhino, aurochs almost made it into the present. ‘Their strength and speed are extraordinary,’ Caesar wrote after he saw aurochs during his conquest of Gaul, in the first century BC. ‘They spare neither man nor wild beast which they have espied.’ But aurochs gradually vanished from most of northern Europe, making a last stand in a few pockets of forest in Poland, where hunting them was the exclusive privilege of the king. The last auroch died in 1627.

Finally there is the ‘commensal’ pathway, in which animals are drawn to humans by attractive food sources – especially trash, but also crops, mice and other pests. Through long interaction, they essentially domesticate themselves. Cats seem to have come into our lives this way, lured by the grain-eating rodents that accompanied the earliest farmers in the Middle East some 12,000 years ago. The descendants of these Middle Eastern cats then spread to Egypt and across the Mediterranean, before expanding their range to northern Europe and the rest of the world by jumping on ships. One cat with Egyptian roots made it to Germany, apparently by stowing away on a Viking vessel – unless, that is, it was invited on board to hunt rats.

Domestication was a gradual affair, full of pitfalls and false starts, during which the border between wild and tame remained fluid

Dogs also evolved along a commensal route from wolves, thousands of years before other species, somewhere between 14,000 and 32,000 years ago. Because they were domesticated so long ago, and have crossbred with wolves many times in the intervening millennia, pinning down precisely when and where dogs first emerged is one of the most daunting quests in the study of domestication. Researchers have tried searching for early dog burial sites, working backwards from living breeds, and studying the DNA of so-called ‘pariah dogs’, the feral dogs who live on trash in villages across Europe and Asia. So far, the answers have been mixed. The research into the village dogs indicates that the species has its origin in Central Asia, somewhere between India and Nepal, while other studies point variously to East Asia, Central Asia, the Middle East or Europe. DNA extracted from the bodies of dogs at an ancient Irish burial mound suggest that there were two separate instances of domestication, one in Asia and one in Europe, according to a study published in 2016.

Other animals might have followed a few different paths to human cohabitation. Studies of pig genomes, both ancient and modern, have revealed that boars were domesticated on at least two separate occasions, in China and in the Middle East, around 8,500 BC. The first farmers in Europe arrived from Anatolia with Middle Eastern swine in tow, which then interbred with local boars over thousands of years, to the point that today’s pigs look as if they’re of native origin. A similar thing happened with chickens: they gained their yellow legs relatively recently, within the past 2,000 years, by interbreeding with wild junglefowl native to South Asia. Tame llamas mixed with wild guanacos and vicuñas (types of South American camelid) so many times that their family tree is impossible to pull apart.

The overall picture is that domestication was a gradual affair, full of pitfalls and false starts. It took thousands of years of tinkering before agriculture as we know it came into being, and for much of that time, the border between wild and tame remained fluid. At the outset, this probably didn’t matter much. Early sea-faring pioneers who travelled from the Middle East to Cyprus brought wheat, barley and pigs, according to archaeological investigations of village sites dating back 10,000 years. But they also took with them species that weren’t domesticated, such as fallow deer and foxes. They didn’t distinguish between wild and tame. Instead of transporting just a few valuable species, they took with them a whole ecological niche. As Zeder writes: ‘They simply took with them the world that they knew.’

In 1959, the Russian biologist Dmitry Belyaev moved to Siberia to see if he could simulate the evolution of dogs. He had fallen out with the Soviet scientific establishment, who didn’t look kindly on his adherence to Gregor Mendel’s theory of genetic inheritance. Belyaev hoped to be able to continue his research away from prying eyes.

Belyaev started with the fox, Vulpes vulpes, a distant cousin of the dog. He took a pack of 100 females and 30 males, specially chosen from thousands kept in captivity on an Estonian fur farm. He selected his foxes based on a single trait: tameness, which he measured by their capacity to tolerate human proximity without fear or aggression. Only 5 per cent of the tamest males and 20 per cent of the tamest females were allowed to breed.

Within a few generations, Belyaev started noticing some odd things. After six generations, the foxes began to wag their tails for their caretakers. After 10, they would lick their faces. They were starting to act like puppies. Their appearance was also changing. Their ears grew more floppy. Their tails became curly. Their fur went from silver to mottled brown. Some foxes developed a white blaze. Their snouts got shorter and their faces became broader. Their breeding season lengthened. These pet foxes could also read human intentions, through gestures and glances.

In the wild, it paid to be skittish. But under human management, animals who could handle stress with equanimity did best

The Belayaev farm experiment has now been running for nearly 60 years. The foxes display virtually all the features of ‘domestication syndrome’ – the constellation of disparate characteristics that Darwin first spotted in mammals. All this came about surprisingly quickly, and by selecting for just a single trait: tameness.

What the Belayaev results suggest is that the manifold aspects of domestication might have a common cause in a gene or set of genes, which occur naturally in different species but tend to be selected out by adaptive and environmental pressures. However, the precise mechanism underlying the genetics of domestication syndrome remains unclear. One recent hypothesis suggests that the genetic key might lie in what are called neural crest cells. These cells appear very early in the development of the embryo and then migrate through the body, turning into all kinds of important tissues, including tail cartilage, ear cartilage, pigment cells, and tissues in the adrenal glands, teeth, jaws and much of the skull. A change in any one of the many genes regulating the migration and development of neural crest cells could be the key responsible for the domestication syndrome in animals.

Domestication does come at a certain cost to the animals involved. Most tame mammals have brains that are noticeably punier than those of their wild relatives. Brains of domestic pigs are 35 per cent smaller than those of boars, for example, while dogs’ brains are around 30 per cent smaller than those of wolves. However, it isn’t clear whether this shrinkage translates into lower intelligence. Much of the reduction appears to occur in parts of the brain related to motor control and sensory processes, such as vision and smell. And it was probably advantageous for domestic animals to have reduced sensory acuity. In the wild it paid to be skittish, while under human management, those individuals who could handle stress with equanimity did best.

The process appears to be very hard to reverse, however. Dingoes have been wild in Australia for more than 3,000 years and retain the smaller brains of their domesticated ancestors. The same holds true for feral pigs, even those that have lived away from human control for hundreds of years.

In all this time that we’ve had animal companions, what have they been doing to us? The ability to digest milk into adulthood is one of the most striking examples of how we’ve been affected by domestication. Known as ‘lactase persistence’, a term that refers to the enzyme that breaks down lactose in milk, it’s one of the greatest evolutionary adaptations in any species of the past few thousand years. Tolerance developed in humans at least five times, once in Europe and four times in areas of sub-Saharan Africa. It came about only after people dedicated themselves to pastoralism, and received year-round supplies of milk from domesticated cows, horses or sheep –  although studies of ancient human DNA suggest that humans might have been consuming milk (uncomfortably) for thousands of years before our bodies finally learned to metabolise it.

Agriculture also brought humans into contact with thousands of new pathogens. Living in proximity to mammals and birds exposed humans to new species of flu and other viruses. Clearing land for crops in the tropics created standing pools of water, which served as perfect habitats for the mosquito species that carried yellow fever and malaria. Possession of the sickle cell variation and other genetic traits associated with haemoglobin irregularities in the blood appear to have arisen as a defence against these insect-borne illnesses.

Domestication means replacing a relationship founded on trust with one ‘based on domination’

Perhaps the most profound changes aren’t to our genes, but to our morals. On one hand, keeping pets has created new forms of cross-species intimacy. Dogs have been used for hunting since before the end of the last Ice Age 12,000 years ago. Excavations on Cyprus revealed that cats were being kept as pets as far back as 9,500 years ago, while archaeologists investigating one of the world’s first villages, Çatalhöyük in Turkey, found a man buried alongside a lamb in a pose that suggested kinship.

Keeping pets meant inviting animals into the family. It also created new relationships of inequality. The anthropologist Tim Ingold at the University of Aberdeen in Scotland, who has spent years studying the reindeer herders of Lapland, argues that it is a mistake to regard domestication as a form of progress, from living in opposition to nature to harnessing it for our benefit. In The Perception of the Environment (2000), he notes that foraging peoples generally regard animals as their equals. Hunting is not a form of violence so much as a willing sacrifice on the part of the animal. Pastoralists, on the other hand, tend to regard animals as servants, to be mastered and controlled. Domestication doesn’t entail making wild animals tame, Ingold says. Instead, it means replacing a relationship founded on trust with one ‘based on domination’.

When humans start treating animals as subordinates, it becomes easier to do the same thing to one another. The first city-states in Mesopotamia were built on this principle of transferring methods of control from creatures to human beings, according to the archaeologist Guillermo Algaze at the University of California in San Diego. Scribes used the same categories to describe captives and temple workers as they used for state-owned cattle.

Top-down domination no longer defines the horizon of what we can do to animals. The AquAdvantage Salmon, for example, is a genetic hybrid cooked up by scientists using two different salmon species and an eelpout fish. It matures twice as fast as regular salmon, and was approved for public consumption by the US Food and Drug Administration just over a year ago. Even newer is CRISPR gene-editing technology, which allows humans to directly modify genomes without the need for mediating external genes – holding out the promise of hypoallergenic eggs, disease-resistant livestock, infertile malarial mosquitoes, and pet micro-pigs with customisable coats.

However we choose to use this technology, it’s likely that anything we do to animals will reflect back on us too. CRISPR has just been approved for testing on humans. If Algaze and Ingold are right, and subjugating animals paved the way to slavery and the state, what will today’s factory farms do to societies of the future? How will opening up the genome of our companion species affect how we think about humans’ genetic code? ‘Judging from the past, we may safely infer that not one living species will transmit its unaltered likeness to a distant futurity,’ Darwin wrote towards the end of On the Origin of Species. That sentence is true of all creatures, but truest, perhaps, of humans.

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Jacob Mikanowski

writes about science, history and art, and his work has appeared in Prospect, The Awl and The Los Angeles Review of Books, among others. He lives in Berkeley, California.

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