An adverse drug reaction (ADR) is a harmful, unintended result caused by taking medication. ADRs may occur following a single dose or prolonged administration of a drug or may result from the combination of two or more drugs. The meaning of this term differs from the term "side effect" because side effects can be beneficial as well as detrimental. The study of ADRs is the concern of the field known as pharmacovigilance. An adverse event
(AE) refers to any unexpected and inappropriate occurrence at the time a
drug is used, whether or not the event is associated with the
administration of the drug. An ADR is a special type of AE in which a causative relationship can be shown.
ADRs are only one type of medication-related harm. Another type of
medication-related harm type includes not taking prescribed medications,
which is also known as non-adherence. Non-adherence to medications can lead to death and other negative outcomes. Adverse drug reactions require the use of a medication.
Classification
Traditional Classification
Type A: augmented pharmacological effects, which are dose-dependent and predictable
Type A reactions, which constitute approximately 80% of adverse
drug reactions, are usually a consequence of the drug's primary
pharmacological effect (e.g., bleeding when using the anticoagulant warfarin) or a low therapeutic index of the drug (e.g., nausea from digoxin), and they are therefore predictable. They are dose-related and usually mild, although they may be serious or even fatal (e.g. intracranial bleeding from warfarin). Such reactions are usually due to inappropriate dosage, especially when drug elimination is impaired. The term side effects may be applied to minor type A reactions.
Type B: Type B reactions are not dose-dependent and are not predictable, and so may be called idiosyncratic. These reactions can be due to particular elements within the person or the environment.
Types A and B were proposed in the 1970s, and the other types were proposed subsequently when the first two proved insufficient to classify ADRs.
Other types of adverse drug reactions are Type C, Type D, Type E, and Type F.
Type C was categorized for chronic adverse drug reactions, Type D for
delayed adverse drug reactions, Type E for withdrawal adverse drug
reactions, and Type F for failure of therapy as an adverse drug
reaction. Adverse drug reactions can also be categorized using
time-relatedness, dose-relatedness, and susceptibility, which
collectively are called the DoTS classification.
Disability - significant, persistent, or permanent change,
impairment, damage or disruption in the patient's body
function/structure, physical activities or quality of life.
Congenital abnormality
Requires intervention to prevent permanent impairment or damage
Severity is a measure of the intensity of the adverse event in question. The terms "severe" and "serious", when applied to adverse events, are technically very different. They are easily confused but can not be used interchangeably, requiring care in usage. Seriousness usually indicates patient outcome (such as negative outcomes including disability, long-term effects, and death).
A headache is severe if it causes intense pain. There are scales
like "visual analog scale" that help clinicians assess the severity. On
the other hand, a headache is not usually serious (but may be in case of
subarachnoid hemorrhage, subdural bleed, even a migraine may temporally fit criteria), unless it also satisfies the criteria for seriousness listed above.
In adverse drug reactions, the seriousness of the reaction is important for reporting.
Location
Adverse effects may be local, i.e. limited to a certain location, or systemic, where medication has caused adverse effects throughout the systemic circulation.
For instance, some ocular antihypertensives cause systemic effects, although they are administered locally as eye drops, since a fraction escapes to the systemic circulation.
Mechanisms
Adverse drug reaction leading to hepatitis (drug-induced hepatitis) with granulomata. Other causes were excluded with extensive investigations. Liverbiopsy. H&E stain.
As research better explains the biochemistry of drug use, fewer ADRs are Type B and more are Type A. Common mechanisms are:
Various diseases, especially those that cause renal or hepatic
insufficiency, may alter drug metabolism. Resources are available that
report changes in a drug's metabolism due to disease states.
The Medication Appropriateness Tool for Comorbid Health Conditions in Dementia (MATCH-D) criteria warns that people with dementia are more likely to experience adverse effects, and that they are less likely to be able to reliably report symptoms.
Genetic factors
Pharmacogenomics includes how genes can predict potential adverse drug reactions.
However, pharmacogenomics is not limited to adverse events (of any
type), but also looks at how genes may impact other responses to
medications, such as low/no effect or expected/normal responses
(especially based on drug metabolism).
Abnormal drug metabolism may be due to inherited factors of either Phase I oxidation or Phase II conjugation.
Phase I reactions
Phase I reactions include metabolism by cytochrome P450. Patients have abnormal metabolism by cytochrome P450 due to either inheriting abnormal alleles or due to drug interactions. Tables are available to check for drug interactions due to P450 interactions.
Protein binding interactions are usually transient and mild until a new steady state is achieved. These are mainly for drugs without much first-pass liver metabolism. The principal plasma proteins for drug binding are:
Some medications can either inhibit or induce key drug metabolizingenzymes or drug transporters,
which when combined with other medications that utilize the same
proteins can lead to either toxic or sub-therapeutic adverse effects.
One example of this is a patient taking a cytochrome P450 3A4 (CYP3A4) inhibitor such as the antibioticclarithromycin, as well as another medication metabolized by CYP3A4 such as the anticoagulantapixaban, which results in elevated blood concentrations of apixaban and greater risk of serious bleeds. Additionally, Clarithromycin is a permeability glycoprotein
(P-gp) efflux pump inhibitor, which when given with apixaban (a
substrate for P-gp) will lead to increased absorption of apixaban,
resulting in the same adverse effects as with CYP3A4 inhibition.
Assessing causality
Causality assessment is used to determine the likelihood that a drug caused a suspected ADR. There are a number of different methods used to judge causation, including the Naranjo algorithm,
the Venulet algorithm and the WHO causality term assessment criteria.
Each have pros and cons associated with their use and most require some
level of expert judgement to apply.
An ADR should not be labeled as 'certain' unless the ADR abates with a challenge-dechallenge-rechallenge protocol (stopping and starting the agent in question). The chronology
of the onset of the suspected ADR is important, as another substance or
factor may be implicated as a cause; co-prescribed medications and
underlying psychiatric conditions may be factors in the ADR.
Assigning causality
to a specific agent often proves difficult, unless the event is found
during a clinical study or large databases are used. Both methods have
difficulties and can be fraught with error. Even in clinical studies,
some ADRs may be missed as large numbers of test individuals are
required to find a specific adverse drug reaction, especially for rare
ADRs. Psychiatric ADRs are often missed as they are grouped together in
the questionnaires used to assess the population.
Monitoring bodies
Many countries have official bodies that monitor drug safety and reactions. On an international level, the WHO runs the Uppsala Monitoring Centre. The European Union runs the European Medicines Agency (EMA). In the United States, the Food and Drug Administration (FDA) is responsible for monitoring post-marketing studies. The FDA has a reporting system called the FDA Adverse Event Reporting System, where individuals can report adverse drug events. Healthcare professionals, consumers, and the pharmaceutical industry can all submit information to this system. For health products marketed in Canada, a branch of Health Canada called The Canada Vigilance Program is responsible for surveillance. Both healthcare professionals and consumers can report to this program. In Australia, the Therapeutic Goods Administration (TGA) conducts postmarket monitoring of therapeutic products. In the UK, a monitoring system called the Yellow Card Scheme was established in 1964. The Yellow Card Scheme was set up to surveil medications and other health products.
Epidemiology
A study by the Agency for Healthcare Research and Quality (AHRQ) found that in 2011, sedatives and hypnotics were a leading source for adverse drug events seen in the hospital
setting. Approximately 2.8% of all ADEs present on admission and 4.4%
of ADEs that originated during a hospital stay were caused by a sedative
or hypnotic drug.
A second study by AHRQ found that in 2011, the most common
specifically identified causes of adverse drug events that originated
during hospital stays in the U.S. were steroids, antibiotics, opiates/narcotics, and anticoagulants. Patients treated in urban teaching hospitals had higher rates of ADEs involving antibiotics
and opiates/narcotics compared to those treated in urban nonteaching
hospitals. Those treated in private, nonprofit hospitals had higher
rates of most ADE causes compared to patients treated in public or
private, for-profit hospitals.
Medication related harm (MRH) is common after hospital discharge
in older adults, but methodological inconsistencies between studies and a
paucity of data on risk factors limits clear understanding of the
epidemiology. There was a wide range in incidence, from 0.4% to 51.2% of
participants, and 35% to 59% of harm was preventable. Medication
related harm incidence within 30 days after discharge ranged from 167 to
500 events per 1,000 individuals discharged (17–51% of individuals).
In the U.S., females had a higher rate of ADEs involving opiates and narcotics than males in 2011, while male patients had a higher rate of anticoagulant
ADEs. Nearly 8 in 1,000 adults aged 65 years or older experienced one
of the four most common ADEs (steroids, antibiotics, opiates/narcotics,
and anticoagulants) during hospitalization.
A study showed that 48% of patients had an adverse drug reaction to at
least one drug, and pharmacist involvement helps to pick up adverse drug
reactions.
In 2012, McKinsey & Company concluded that the cost of the 50-100 million preventable error-related adverse drug events would be between US$18–115 billion.
An article published in The Journal of the American Medical Association (JAMA) in 2016 reported adverse drug event statistics from emergency departments around the United States in 2013-2014.
From this article, an estimated prevalence of adverse drug events that
were presented to the emergency department (ED) was 4 events out of
every 1000 people. This article reported that 57.1% of these adverse drug events presented to the ED were in females.
As well, out of all of the adverse drug events presented to the
emergency department documented in this article, 17.6% were from anticoagulants, 16.1% were from antibiotics, and 13.3% from diabetic agents.
In classical Freudianpsychoanalytic theory, the death drive (German: Todestrieb) is the drive toward death and destruction, often expressed through behaviors such as aggression, repetition compulsion, and self-destructiveness. It was originally proposed by Sabina Spielrein in her paper "Destruction as the Cause of Coming Into Being" (Die Destruktion als Ursache des Werdens) in 1912, which was then taken up by Sigmund Freud in 1920 in Beyond the Pleasure Principle. This concept has been translated as "opposition between the ego or death instincts and the sexual or life instincts". In Beyond thePleasure Principle, Freud used the plural "death drives" (Todestriebe) much more frequently than the singular.
The death drive opposes Eros,
the tendency toward survival, propagation, sex, and other creative,
life-producing drives. The death drive is sometimes referred to as "Thanatos"
in post-Freudian thought, complementing "Eros", although this term was
not used in Freud's own work, being rather introduced by Wilhelm Stekel in 1909 and then by Paul Federn in the present context. Subsequent psychoanalysts such as Jacques Lacan and Melanie Klein have defended the concept.
Terminology
The standard edition of Freud's works in English confuses two terms that are different in German, Instinkt (instinct) and Trieb (drive), often translating both as instinct; for example, "the hypothesis of a death instinct, the task of which is to lead organic life back into the inanimate state". "This equating of Instinkt and Trieb has created serious misunderstandings". Freud actually refers to the term "Instinkt" in explicit use elsewhere,
and so while the concept of "instinct" can loosely be referred to as a
"drive," any essentialist or naturalist connotations of the term should
be put in abeyance. In a sense, the death drive is a force that is not
essential to the life of an organism (unlike an "instinct") and tends to
denature it or make it behave in ways that are sometimes
counter-intuitive. In other words, the term death "instinct" is simply a
false representation of death drive. The term is almost universally
known in scholarly literature on Freud as the "death drive", and
Lacanian psychoanalysts often shorten it to simply "drive" (although
Freud posited the existence of other drives as well, and Lacan explicitly states in Seminar XI that all drives are partial to the death drive). The contemporary Penguin translations of Freud translate Trieb and Instinkt as "drive" and "instinct" respectively.
Origin of the theory: Beyond the Pleasure Principle
It was a basic premise of
Freud's that "the course taken by mental events is automatically
regulated by the pleasure principle...[associated] with an avoidance of
unpleasure or a production of pleasure".
Three main types of conflictual evidence, difficult to explain
satisfactorily in such terms, led Freud late in his career to look for
another principle in mental life beyond the pleasure principle—a search that would ultimately lead him to the concept of the death drive.
The first problem Freud encountered was the phenomenon of repetition in (war) trauma. When Freud worked with people with trauma (particularly the trauma experienced by soldiers returning from World War I),
he observed that subjects often tended to repeat or re-enact these
traumatic experiences: "dreams occurring in traumatic patients have the
characteristic of repeatedly bringing the patient back into the
situation of his accident", contrary to the expectations of the pleasure principle.
A second problematic area was found by Freud in children's play (such as the Fort/Da
Forth/here game played by Freud's grandson, who would stage and
re-stage the disappearance of his mother and even himself). "How then
does his repetition of this distressing experience as a game fit in with
the pleasure principle?"
The third problem came from clinical practice. Freud found his
patients, dealing with painful experiences that had been repressed,
regularly "obliged to repeat the repressed material as a contemporary experience instead of ... remembering it as something belonging to the past".
Combined with what he called "the compulsion of destiny ... come across
[in] people all of whose human relationships have the same outcome",
such evidence led Freud "to justify the hypothesis of a compulsion to
repeat—something that would seem more primitive, more elementary, more
instinctual than the pleasure principle which it over-rides".
He then set out to find an explanation of such a compulsion, an
explanation that some scholars have labeled as "metaphysical biology".
In Freud's own words, "What follows is speculation, often far-fetched
speculation, which the reader will consider or dismiss according to his
individual predilection". Seeking a new instinctual paradigm for such problematic repetition, he found it ultimately in "an urge in organic life to restore an earlier state of things"—the
inorganic state from which life originally emerged. From the
conservative, restorative character of instinctual life, Freud derived
his death drive, with its "pressure towards death", and the resulting
"separation of the death instincts from the life instincts"
seen in Eros. The death drive then manifested itself in the individual
creature as a force "whose function is to assure that the organism shall
follow its own path to death".
Seeking further potential clinical support for the existence of
such a self-destructive force, Freud found it through a reconsideration
of his views of masochism—previously "regarded as sadism that has been
turned round upon the subject's own ego"—so as to allow that "there might be such a thing as primary masochism—a possibility which I had contested"
before. Even with such support, however, he remained very tentative to
the book's close about the provisional nature of his theoretical
construct: what he called "the whole of our artificial structure of
hypotheses".
Although Spielrein's paper was published in 1912, Freud initially
resisted the concept as he considered it to be too Jungian.
Nevertheless, Freud eventually adopted the concept, and in later years
would build extensively upon the tentative foundations he had set out in
Beyond the Pleasure Principle. In The Ego and the Id
(1923) he would develop his argument to state that "the death instinct
would thus seem to express itself—though probably only in part—as an instinct of destruction directed against the external world".
The following year he would spell out more clearly that the "libido has
the task of making the destroying instinct innocuous, and it fulfils
the task by diverting that instinct to a great extent outwards .... The
instinct is then called the destructive instinct, the instinct for
mastery, or the will to power", a perhaps much more recognisable set of manifestations.
At the close of the decade, in Civilization and Its Discontents
(1930), Freud acknowledged that "To begin with it was only tentatively
that I put forward the views I have developed here, but in the course of
time they have gained such a hold upon me that I can no longer think in
any other way".
From a philosophical perspective, the death drive may be viewed in relation to the work of the German philosopher Arthur Schopenhauer. His philosophy, expounded in The World as Will and Representation (1818) postulates that all exists by a metaphysical "will" (more clearly, a will to live), and that pleasure affirms this will. Schopenhauer's pessimism
led him to believe that the affirmation of the "will" was a negative
and immoral thing, due to his belief of life producing more suffering
than happiness. The death drive would seem to manifest as a natural and
psychological negation of the "will".
Freud was well aware of such possible linkages. In a letter of
1919, he wrote that regarding "the theme of death, [that I] have
stumbled onto an odd idea via the drives and must now read all sorts of
things that belong to it, for instance Schopenhauer". Ernest Jones
(who like many analysts was not convinced of the need for the death
drive, over and above an instinct of aggression) considered that "Freud
seemed to have landed in the position of Schopenhauer, who taught that
'death is the goal of life'".
However, as Freud put it to the imagined auditors of his New Introductory Lectures
(1932), "You may perhaps shrug your shoulders and say: "That isn't
natural science, it's Schopenhauer's philosophy!" But, ladies and
gentlemen, why should not a bold thinker have guessed something that is
afterwards confirmed by sober and painstaking detailed research?"
He then went on to add that "what we are saying is not even genuine
Schopenhauer....we are not overlooking the fact that there is life as
well as death. We recognise two basic instincts and give each of them
its own aim".
Cultural application: Civilization and Its Discontents
Freud applied his new theoretical construct in Civilization and Its Discontents (1930) to the difficulties inherent in Western civilization—indeed,
in civilization and in social life as a whole. In particular, given
that "a portion of the [death] instinct is diverted towards the external
world and comes to light as an instinct of aggressiveness', he saw 'the
inclination to aggression ... [as] the greatest impediment to
civilization".
The need to overcome such aggression entailed the formation of the
[cultural] superego: "We have even been guilty of the heresy of
attributing the origin of conscience to this diversion inwards of
aggressiveness".
The presence thereafter in the individual of the superego and a related
sense of guilt—"Civilization, therefore, obtains mastery over the
individual's dangerous desire for aggression by ... setting up an agency
within him to watch over it"—leaves
an abiding sense of uneasiness inherent in civilized life, thereby
providing a structural explanation for 'the suffering of civilized man'.
Freud made a further connection between group
life and innate aggression, where the former comes together more
closely by directing aggression to other groups, an idea later picked up
by group analysts like Wilfred Bion.
Continuing development of Freud's views
In
the closing decade of Freud's life, it has been suggested, his view of
the death drive changed somewhat, with "the stress much more upon the
death instinct's manifestations outwards".
Given "the ubiquity of non-erotic aggressivity and destructiveness", he
wrote in 1930, "I adopt the standpoint, therefore, that the inclination
to aggression is an original, self-subsisting instinctual disposition
in man".
In 1933, he conceived of his original formulation of the death
drive 'the improbability of our speculations. A queer instinct, indeed,
directed to the destruction of its own organic home!'.
He wrote moreover that "Our hypothesis is that there are two
essentially different classes of instincts: the sexual instincts,
understood in the widest sense—Eros, if you prefer that name—and the
aggressive instincts, whose aim is destruction".
In 1937, he went so far as to suggest privately that 'We should have a
neat schematic picture if we supposed that originally, at the beginning
of life, all libido was directed to the inside and all aggressiveness to
the outside'. In his last writings, it was the contrast of "two basic instincts, Eros and the destructive instinct ... our two primal instincts, Eros and destructiveness",
on which he laid stress. Nevertheless, his belief in "the death
instinct ... [as] a return to an earlier state ... into an inorganic
state" continued to the end.
Mortido and Destrudo
The terms mortido and destrudo, formed analogously to libido, refer to the energy of the death instinct. In the early 21st century, their use amongst Freudian psychoanalysts has been waning, but still designate destructive energy. The importance of integrating mortido into an individual's life, as opposed to splitting it off and disowning it, has been taken up by figures like Robert Bly in the men's movement.
Paul Federn used the term mortido for the new energy source, and has generally been followed in that by other analytic writers. His disciple and collaborator Weiss, however, chose destrudo, which was later taken up by Charles Brenner.
Mortido has also been applied in contemporary expositions of the Cabbala.
Whereas Freud himself never named the aggressive and destructive
energy of the death drive (as he had done with the life drive,
"libido"), the next generation of psychoanalysts vied to find suitable names for it.
Literary criticism has been almost more prepared than psychoanalysis to make at least metaphorical use of the term 'Destrudo'. Artistic images were seen by Joseph Campbell in terms of "incestuous 'libido' and patricidal 'destrudo'"; while literary descriptions of the conflict between destrudo and libido are still fairly widespread in the 21st century.
Destrudo as an evocative name also appears in rock music and video games.
Paul Federn
Mortido was introduced by Freud's pupil Paul Federn to cover the psychic energy of the death instinct, something left open by Freud himself:
Providing what he saw as clinical proof of the reality of the death
instinct in 1930, Federn reported on the self-destructive tendencies of
severely melancholic patients as evidence of what he would later call
inwardly-directed mortido.
However, Freud himself favoured neither term – mortido or destrudo. This worked against either of them gaining widespread popularity in the psychoanalytic literature.
Edoardo Weiss
Destrudo is a term introduced by Italian psychoanalyst Edoardo Weiss in 1935 to denote the energy of the death instinct, on the analogy of libido—and thus to cover the energy of the destructive impulse in Freudian psychology.
Destrudo is the opposite of libido—the urge to create, an energy
that arises from the Eros (or "life") drive—and is the urge to destroy
arising from Thanatos (death), and thus an aspect of what Sigmund Freud termed "the aggressive instincts, whose aim is destruction".
Weiss related aggression/destrudo to secondary narcissism, something generally only described in terms of the libido turning towards the self.
Eric Berne, who was a pupil of Federn's, made extensive use of the term mortido in his pre-transactional analysis study, The Mind in Action
(1947). As he wrote in the foreword to the third edition of 1967, "the
historical events of the last thirty years...become much clearer by
introducing Paul Federn's concept of mortido".
Berne saw mortido as activating such forces as hate and cruelty, blinding anger and social hostilities; and considered that inwardly directed mortido underlay the phenomena of guilt and self-punishment, as well as their clinical exacerbations in the form of depression or melancholia.
Berne saw sexual acts as gratifying mortido at the same time as
libido; and recognised that on occasion the former becomes more
important sexually than the latter, as in sadomasochism and destructive emotional relationships.
Berne's concern with the role of mortido in individuals and
groups, social formations and nations, arguably continued throughout all
his later writings.
Jean Laplanche
Jean Laplanche has explored repeatedly the question of mortido, and of how far a distinctive instinct of destruction can be identified in parallel to the forces of libido.
Analytic reception
As
Freud wryly commented in 1930, "The assumption of the existence of an
instinct of death or destruction has met with resistance even in
analytic circles". Indeed, Ernest Jones would comment of Beyond the Pleasure Principle
that the book not only "displayed a boldness of speculation that was
unique in all his writings" but was "further noteworthy in being the
only one of Freud's which has received little acceptance on the part of
his followers".
Otto Fenichel in his compendious survey of the first Freudian
half-century concluded that "the facts on which Freud based his concept
of a death instinct in no way necessitate the assumption ... of a
genuine self-destructive instinct". Heinz Hartmann set the tone for ego psychology
when he "chose to ... do without 'Freud's other, mainly biologically
oriented set of hypotheses of the "life" and "death instincts"'". In the object relations theory, among the independent group 'the most common repudiation was the loathsome notion of the death instinct'.
Indeed, "for most analysts Freud's idea of a primitive urge towards
death, of a primary masochism, was ... bedevilled by problems".
Nevertheless, the concept has been defended, extended, and
carried forward by some analysts, generally those tangential to the
psychoanalytic mainstream; while among the more orthodox, arguably of
"those who, in contrast to most other analysts, take Freud's doctrine of
the death drive seriously, K. R. Eissler has been the most
persuasive—or least unpersuasive".
Melanie Klein
and her immediate followers considered that "the infant is exposed from
birth to the anxiety stirred up by the inborn polarity of instincts—the
immediate conflict between the life instinct and the death instinct";
and Kleinians indeed built much of their theory of early childhood
around the outward deflection of the latter. "This deflection of the
death instinct, described by Freud, in Melanie Klein's view consists
partly of a projection, partly of the conversion of the death instinct
into aggression".
French psychoanalyst Jacques Lacan,
for his part, castigated the "refusal to accept this culminating point
of Freud's doctrine ... by those who conduct their analysis on the basis
of a conception of the ego ... that death instinct whose enigma Freud propounded for us at the height of his experience".
Characteristically, he stressed the linguistic aspects of the death
drive: "the symbol is substituted for death in order to take possession
of the first swelling of life .... There is therefore no further need to
have recourse to the outworn notion of primordial masochism in order to
understand the reason for the repetitive games in ... his Fort! and in his Da!."
Eric Berne
too would proudly proclaim that he, "besides having repeated and
confirmed the conventional observations of Freud, also believes right
down the line with him concerning the death instinct, and the
pervasiveness of the repetition compulsion".
For the twenty-first century, "the death drive today ... remains a
highly controversial theory for many psychoanalysts ... [almost] as
many opinions as there are psychoanalysts".
Freud's conceptual opposition of death and eros drives in the human psyche was applied by Walter A. Davis in Deracination: Historicity, Hiroshima, and the Tragic Imperative and Death's Dream Kingdom: The American Psyche since 9/11.
Davis described social reactions to both Hiroshima and 9/11 from the
Freudian viewpoint of the death force. Unless they consciously take
responsibility for the damage of those reactions, Davis claims that
Americans will repeat them.
Dogs and sheep were among the first animals to be domesticated.
The domestication of vertebrates is the mutual relationship between vertebrate animals including birds and mammals, and the humans who have influence on their care and reproduction.
Charles Darwin recognized a 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
(i.e. artificial selection) 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. There is a genetic difference between domestic and wild populations.
There is also a genetic 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.
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 portion of
domesticates, though they may be fixed in individual breeds or regional populations.
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.
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.
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) animals sought for food and other
byproducts (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). The dog was the first to be domesticated, and was established across Eurasia before the end of the Late Pleistocene era, well before cultivation and before the domestication of other animals.
Unlike other domestic species which were primarily selected for
production-related traits, dogs were initially selected for their
behaviors. 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.
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. Some of the most commonly domesticated animals are cats and
dogs.
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."
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.
Domestication syndrome
Traits used to define the animal 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.
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. The set of traits used to define the animal domestication syndrome is inconsistent.
Difference from 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. Human selection included tameness, but without a suitable evolutionary response then domestication was not achieved.
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.
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.
Evolution of temperatures in the postglacial period, after the Last Glacial Maximum,
showing very low temperatures for the most part of the Younger Dryas,
rapidly rising afterwards to reach the level of the warm Holocene, based on Greenland ice cores.
The domestication of animals and plants was triggered by the climatic
and environmental changes that occurred after the peak of the Last Glacial Maximum
around 21,000 years ago and which continue to this present day. These
changes made obtaining food difficult. The first domesticate was the domestic dog (Canis lupus familiaris) from a wolf ancestor (Canis lupus) at least 15,000 years ago. The Younger Dryas
that occurred 12,900 years ago was a period of intense cold and aridity
that put pressure on humans to intensify their foraging strategies. By
the beginning of the Holocene
from 11,700 years ago, favorable climatic conditions and increasing
human populations led to small-scale animal and plant domestication,
which allowed humans to augment the food that they were obtaining
through hunter-gathering.
The increased use of agriculture and continued domestication of species during the Neolithic transition marked the beginning of a rapid shift in the evolution, ecology, and demography of both humans and numerous species of animals and plants. Areas with increasing agriculture, underwent urbanization, developing higher-density populations, expanded economies, and became centers of livestock and crop domestication. Such agricultural societies emerged across Eurasia, North Africa, and South and Central America.
In the Fertile Crescent 10,000-11,000 years ago, zooarchaeology indicates that goats, pigs, sheep, and taurine cattle
were the first livestock to be domesticated. Archaeologists working in
Cyprus found an older burial ground, approximately 9500 years old, of an
adult human with a feline skeleton. Two thousand years later, humped zebu cattle were domesticated in what is today Baluchistan in Pakistan. In East Asia
8,000 years ago, pigs were domesticated from wild boar that were
genetically different from those found in the Fertile Crescent. The
horse was domesticated on the Central Asian steppe 5,500 years ago. The
chicken in Southeast Asia was domesticated 4,000 years ago.
Universal features
The
biomass of wild vertebrates is now increasingly small compared to the
biomass of domestic animals, with the calculated biomass of domestic
cattle alone being greater than that of all wild mammals.
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.
Domestication is a fluid and nonlinear process that may start, stop,
reverse, or go down 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.
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 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.
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. This implies that not all animals can be domesticated, e.g. a wild member of the horse family, the zebra.
Jared Diamond in his book Guns, Germs, and Steel
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:
Hereford cattle, domesticated for beef production.
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.
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.
Ability to breed in captivity – Animals that will not breed in captivity are limited to acquisition through capture in the wild.
Pleasant disposition – Animals with nasty dispositions are dangerous to keep around humans.
Tendency not to panic – Some species are nervous, fast, and prone to flight when they perceive a threat.
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
Reduction in skull size with neoteny - grey wolf and chihuahua skulls
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.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,
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.
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.
Pleiotropy
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 only trait under selective pressure.
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.
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.
Neural crest cells (NCC) are vertebrate embryonic stem cells that function directly and indirectly during early embryogenesis to produce many tissue types.
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.
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.
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.
The single genetic regulatory network hypothesis proposes that
domestication syndrome results from mutations in genes that regulate the
expression pattern of more downstream genes.
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. These linked traits may arise from mutations in a few key regulatory genes.
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.
Limited reversion
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. Dingos have lived apart from humans for thousands of years but still have the same brain size as that of a domestic dog.
Feral dogs that actively avoid human contact are still dependent on
human waste for survival and have not reverted to the self-sustaining
behaviors of their wolf ancestors.
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.
For studies in animal domestication, researchers have proposed five
distinct categories: wild, captive wild, domestic, cross-breeds and
feral.
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.
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.
Domestic animals
Subject to intensified artificial selection through husbandry
practices with relaxation of natural selection associated with captivity
and management.
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.
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.
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.
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.
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). 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.
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.
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.
Commensal
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. Later, these animals developed closer social or economic bonds with humans that led to a domestic relationship. 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. Commensal pathway animals include dogs, cats, fowl, and possibly pigs.
The domestication of animals commenced over 15,000 years before present (YBP), beginning with the grey wolf (Canis lupus) by nomadic hunter-gatherers. It was not until 11,000 YBP that people living in the Near East entered into relationships with wild populations of aurochs,
boar, sheep, and goats. A domestication process then began to develop.
The grey wolf most likely followed the commensal pathway to
domestication. When, where, and how many times wolves may have been
domesticated remains debated because only a small number of ancient
specimens have been found, and both archaeology and genetics continue to
provide conflicting evidence. The most widely accepted, earliest dog
remains date back 15,000 YBP to the Bonn–Oberkassel dog. Earlier remains dating back to 30,000 YBP have been described as Paleolithic dogs, however their status as dogs or wolves remains debated. Recent studies indicate that a genetic divergence
occurred between dogs and wolves 20,000–40,000 YBP, however this is the
upper time-limit for domestication because it represents the time of
divergence and not the time of domestication.
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.
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. The chicken was not a popular food in Europe until only one thousand years ago.
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.
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. 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.
Directed
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.
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. 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 nor zebras
possessed the necessary prerequisites and were never domesticated.
There is no clear evidence for the domestication of any herded prey
animal in Africa, with the notable exception of the donkey, which was domesticated in Northeast Africa sometime in the 4th millennium BCE.
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.
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.
The yellow leg trait possessed by numerous modern commercial chicken breeds was acquired via introgression from the grey junglefowl indigenous to South Asia.
African cattle are hybrids that possess both a European Taurine cattle maternal mitochondrial signal and an Asian Indicine cattle paternal Y-chromosome signature.
Numerous other bovid species, including bison, yak, banteng, and gaur hybridize with ease.
Cats and horses have been shown to hybridize with many closely related species.
The archaeological and genetic data suggests that long-term
bidirectional gene flow between wild and domestic stocks – including
canids, donkeys, horses, New and Old World camelids, goats, sheep, and
pigs – was common. Bidirectional gene flow between domestic and wild reindeer continues today.
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
understanding of the original process and can lead to an artificial
inflation of the number of times domestication took place.
This introgression can, in some cases, be regarded as adaptive
introgression, as observed in domestic sheep due to gene flow with the
wild European Mouflon.
The sustained admixture between 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 domestic dogs. None of the modern wolf populations are related to the Pleistocene wolves that were first domesticated,
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.
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.
Domestic animals vary in coat color, craniofacial morphology,
reduced brain size, floppy ears, and changes in the endocrine system and
reproductive cycle. The domesticated silver fox
experiment demonstrated that selection for tameness within a few
generations can result in modified behavioral, morphological, and
physiological traits.
The experiment demonstrated that domestic phenotypic traits could arise
through selection for a behavioral trait, and that domestic behavioral
traits could arise through the selection for a phenotypic trait. In
addition, the experiment provided a mechanism for the start of the
animal domestication process that did not depend on deliberate human
forethought and action.
In the 1980s, a researcher used a set of behavioral, cognitive, and
visible phenotypic markers, such as coat color, to produce domesticated
fallow deer within a few generations. Similar results for tameness and fear have been found for mink and Japanese quail.
Pig herding in fog, Armenia. Human selection for domestic traits is not affected by later gene flow from wild boar.
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.
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.
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.
Geneticists have identified more than 300 genetic loci and 150 genes associated with coat color variability.
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. Other studies have shown how human-induced selection is responsible for the allelic variation in pigs.
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.
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. Human selection for domestic
traits likely counteracted the homogenizing effect of gene flow from
wild boars and created domestication islands in the genome.
Unlike other domestic species which were primarily selected for
production-related traits, dogs were initially selected for their
behaviors.
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 (i.e. selection for tameness), and emotional processing. Dogs generally show reduced fear and aggression compared to wolves.
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.
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