FEATURE ARTICLE
Early Canid Domestication: The Farm-Fox Experiment
Foxes bred for tamability in a 40-year experiment exhibit remarkable transformations that suggest an interplay between behavioral genetics and development
Lyudmila Trut
When scientists ponder how animals came to be domesticated, they
almost inevitably wind up thinking about dogs. The dog was probably
the first domestic animal, and it is the one in which domestication
has progressed the furthest—far enough to turn Canis
lupus into Canis familiaris. Evolutionary theorists
have long speculated about exactly how dogs' association with human
beings may have been linked to their divergence from their wild wolf
forebears, a topic that anthropologist Darcy Morey has discussed in
some detail in the pages of this magazine (July–August 1994).
As Morey pointed out, debates about the origins of animal
domestication tend to focus on "the issue of
intentionality"—the extent to which domestication was the
result of deliberate human choice. Was domestication actually
"self-domestication," the colonization of new ecological
niches by animals such as wolves? Or did it result from intentional
decisions by human beings? How you answer those questions will
determine how you understand the morphological and physiological
changes that domestication has brought about—whether as the
results of the pressure of natural selection in a new niche, or as
deliberately cultivated advantageous traits.
In many ways, though, the question of intentionality is beside the
point. Domestication was not a single event but rather a long,
complex process. Natural selection and artificial selection may both
have operated at different times or even at the same time. For
example, even if prehistoric people deliberately set out to
domesticate wolves, natural selection would still have been at work.
The selective regime may have changed drastically when wolves
started living with people, but selective pressure continued
regardless of anything Homo sapiens chose to do.
Another problem with the debate over intentionality is that it can
overshadow other important questions. For example, in becoming
domesticated, animals have undergone a host of changes in
morphology, physiology and behavior. What do those changes have in
common? Do they stem from a single cause, and if so, what is it? In
the case of the dog, Morey identifies one common factor as
pedomorphosis, the retention of juvenile traits by adults.
Those traits include both morphological ones, such as skulls that
are unusually broad for their length, and behavioral ones, such as
whining, barking and submissiveness—all characteristics that
wolves outgrow but that dogs do not. Morey considers pedomorphosis
in dogs a by-product of natural selection for earlier sexual
maturity and smaller body size, features that, according to
evolutionary theory, ought to increase the fitness of animals
engaged in colonizing a new ecological niche.
The common patterns are not confined to a single species. In a wide
range of mammals—herbivores and predators, large and
small— domestication seems to have brought with it strikingly
similar changes in appearance and behavior: changes in size, changes
in coat color, even changes in the animals' reproductive cycles. Our
research group at the Institute of Cytology and Genetics in
Novosibirsk, Siberia, has spent decades investigating such patterns
and other questions of the early evolution of domestic animals. Our
work grew out of the interests and ideas of the late director of our
institute, the geneticist Dmitry K. Belyaev.
Like Morey, Belyaev believed
that the patterns of changes observed in domesticated animals
resulted from genetic changes that occurred in the course of
selection. Belyaev, however, believed that the key factor selected
for was not size or reproduction, but behavior—specifically
amenability to domestication, or tamability. More than any
other quality, Belyaev believed, tamability must have determined how
well an animal would adapt to life among human beings. Because
behavior is rooted in biology, selecting for tameness and against
aggression means selecting for physiological changes in the systems
that govern the body's hormones and neurochemicals. Those changes,
in turn, could have had far-reaching effects on the development of
the animals themselves, effects that might well explain why
different animals would respond in similar ways when subjected to
the same kinds of selective pressures.
To test his hypothesis, Belyaev decided to turn back the clock to
the point at which animals received the first challenge of
domestication. By replaying the process, he would be able to see how
changes in behavior, physiology and morphology first came about. Of
course, reproducing the ways and means of those ancient
transformations, even in the roughest outlines, would be a
formidable task. To keep things as clear and simple as possible,
Belyaev designed a selective-breeding program to reproduce a single
major factor, strong selection pressure for tamability. He chose as
his experimental model a species taxonomically close to the dog but
never before domesticated: Vulpes vulpes, the silver fox.
Belyaev's fox-breeding experiment occupied the last 26 years of his
life. Today, 14 years after his death, it is still in progress.
Through genetic selection alone, our research group has created a
population of tame foxes fundamentally different in temperament and
behavior from their wild forebears. In the process we have observed
some striking changes in physiology, morphology and behavior, which
mirror the changes known in other domestic animals and bear out many
of Belyaev's ideas.
Belyaev's Hypothesis
Belyaev began his experiment in 1959, a time when Soviet genetics
was starting to recover from the anti-Darwinian ideology of Trofim
Lysenko. Belyaev's own career had suffered. In 1948 his commitment
to orthodox genetics had cost him his job as head of the Department
of Fur Animal Breeding at the Central Research Laboratory of Fur
Breeding in Moscow. During the 1950s he continued to conduct genetic
research under the guise of studying animal physiology. He moved to
Novosibirsk, where he helped found the Siberian Department of the
Soviet (now Russian) Academy of Sciences and became the director of
the Department's Institute of Cytology and Genetics, a post he held
from 1959 until his death in 1985. Under his leadership the
institute became a center of basic and applied research in both
classical genetics and modern molecular genetics. His own work
included ground-breaking investigations of evolutionary change in
animals under extreme conditions (including domestication) and of
the evolutionary roles of factors such as stress, selection for
behavioral traits and the environmental photoperiod, or duration of
natural daylight. Animal domestication was his lifelong project, and
fur bearers were his favorite subjects.
Early in the process of
domestication, Belyaev noted, most domestic animals had undergone
the same basic morphological and physiological changes. Their bodies
changed in size and proportions, leading to the appearance of dwarf
and giant breeds. The normal pattern of coat color that had evolved
as camouflage in the wild altered as well. Many domesticated animals
are piebald, completely lacking pigmentation in specific body areas.
Hair turned wavy or curly, as it has done in Astrakhan sheep,
poodles, domestic donkeys, horses, pigs, goats and even laboratory
mice and guinea pigs. Some animals' hair also became longer (Angora
type) or shorter (rex type).
Tails changed, too. Many breeds of dogs and pigs carry their tails
curled up in a circle or semicircle. Some dogs, cats and sheep have
short tails resulting from a decrease in the number of tail
vertebrae. Ears became floppy. As Darwin noted in chapter 1 of
On the Origin of Species, "not a single domestic
animal can be named which has not in some country drooping
ears"—a feature not found in any wild animal except the
elephant. Another major evolutionary consequence of domestication is
loss of the seasonal rhythm of reproduction. Most wild animals in
middle latitudes are genetically programmed to mate once a year,
during mating seasons cued by changes in daylight. Domestic animals
at the same latitudes, however, now can mate and bear young more
than once a year and in any season.
Belyaev believed that similarity in the patterns of these traits was
the result of selection for amenability to domestication. Behavioral
responses, he reasoned, are regulated by a fine balance between
neurotransmitters and hormones at the level of the whole organism.
The genes that control that balance occupy a high level in the
hierarchical system of the genome. Even slight alterations in those
regulatory genes can give rise to a wide network of changes in the
developmental processes they govern. Thus, selecting animals for
behavior may lead to other, far-reaching changes in the animals'
development. Because mammals from widely different taxonomic groups
share similar regulatory mechanisms for hormones and neurochemistry,
it is reasonable to believe that selecting them for similar
behavior—tameness—should alter those mechanisms, and the
developmental pathways they govern, in similar ways.
For Belyaev's hypothesis to make evolutionary sense, two more things
must be true. Variations in tamability must be determined at least
partly by an animal's genes, and domestication must place that
animal under strong selective pressure. We have looked into both
questions. In the early 1960s our team studied the patterns and
nature of tamability in populations of farm foxes. We cross-bred
foxes of different behavior, cross-fostered newborns and even
transplanted embryos between donor and host mothers known to react
differently to human beings. Our studies showed that about 35
percent of the variations in the foxes' defense response to the
experimenter are genetically determined. To get some idea of how
powerful the selective pressures on those genes might have been, our
group has domesticated other animals, including river otters
(Lutra lutra) and gray rats (Rattus
norvegicus) caught in the wild. Out of 50 otters caught during
recent years, only eight of them (16 percent) showing weak defensive
behavior made a genetic contribution to the next generation. Among
the gray rats, only 14 percent of the wild-caught yielded offspring
living to adulthood. If our numbers are typical, it is clear that
domestication must place wild animals under extreme stress and
severe selective pressure.
The Experiment
In setting up our breeding experiment, Belyaev bypassed that initial
trauma. He began with 30 male foxes and 100 vixens, most of them
from a commercial fur farm in Estonia. The founding foxes were
already tamer than their wild relatives. Foxes had been farmed since
the beginning of this century, so the earliest steps of
domestication—capture, caging and isolation from other wild
foxes—had already left their marks on our foxes' genes and behavior.
From the outset, Belyaev selected foxes for tameness and tameness
alone, a criterion we have scrupulously followed. Selection is
strict; in recent years, typically not more than 4 or 5 percent of
male offspring and about 20 percent of female offspring have been
allowed to breed. To ensure that their tameness results from genetic
selection, we do not train the foxes. Most of them spend their lives
in cages and are allowed only brief "time dosed" contacts
with human beings. Pups are caged with their mothers until they are
11/2 to 2 months old. Then they are caged with their litter
mates but without their mothers. At three months, each pup is moved
to its own cage.
To evaluate the foxes for
tameness, we give them a series of tests. When a pup is one month
old, an experimenter offers it food from his hand while trying to
stroke and handle the pup. The pups are tested twice, once in a cage
and once while moving freely with other pups in an enclosure, where
they can choose to make contact either with the human experimenter
or with another pup. The test is repeated monthly until the pups are
six or seven months old.
At seven or eight months, when the foxes reach sexual maturity, they
are scored for tameness and assigned to one of three classes. The
least domesticated foxes, those that flee from experimenters or bite
when stroked or handled, are assigned to Class III. (Even Class III
foxes are tamer than the calmest farm-bred foxes. Among other
things, they allow themselves to be hand fed.) Foxes in Class II let
themselves be petted and handled but show no emotionally friendly
response to experimenters. Foxes in Class I are friendly toward
experimenters, wagging their tails and whining. In the sixth
generation bred for tameness we had to add an even higher-scoring
category. Members of Class IE, the "domesticated elite,"
are eager to establish human contact, whimpering to attract
attention and sniffing and licking experimenters like dogs. They
start displaying this kind of behavior before they are one month
old. By the tenth generation, 18 percent of fox pups were elite; by
the 20th, the figure had reached 35 percent. Today elite foxes make
up 70 to 80 percent of our experimentally selected population.
Now, 40 years and 45,000 foxes after Belyaev began, our experiment
has achieved an array of concrete results. The most obvious of them
is a unique population of 100 foxes (at latest count), each of them
the product of between 30 and 35 generations of selection. They are
unusual animals, docile, eager to please and unmistakably
domesticated. When tested in groups in an enclosure, pups compete
for attention, snarling fiercely at one another as they seek the
favor of their human handler. Over the years several of our
domesticated foxes have escaped from the fur farm for days. All of
them eventually returned. Probably they would have been unable to
survive in the wild.
Physical Changes
Physically, the foxes differ markedly from their wild relatives.
Some of the differences have obvious links to the changes in their
social behavior. In dogs, for example, it is well known that the
first weeks of life are crucial for forming primary social bonds
with human beings. The "window" of bonding opens when a
puppy becomes able to sense and explore its surroundings, and it
closes when the pup starts to fear unknown stimuli. According to our
studies, nondomesticated fox pups start responding to auditory
stimuli on day 16 after birth, and their eyes are completely open by
day 18 or 19. On average, our domesticated fox pups respond to
sounds two days earlier and open their eyes a day earlier than their
nondomesticated cousins. Nondomesticated foxes first show the fear
response at 6 weeks of age; domesticated ones show it after 9 weeks
or even later. (Dogs show it at 8 to 12 weeks, depending on the
breed.) As a result, domesticated pups have more time to become
incorporated into a human social environment.
Moreover, we have found that
the delayed development of the fear response is linked to changes in
plasma levels of corticosteroids, hormones concerned with an
animal's adaptation to stress. In foxes, the level of
corticosteroids rises sharply between the ages of 2 to 4 months and
reach adult levels by the age of 8 months. One of our studies found
that the more advanced an animal's selection for domesticated
behavior was, the later it showed the fear response and the later
came the surge in its plasma corticosteroids. Thus, selection for
domestication gives rises to changes in the timing of the postnatal
development of certain physiological and hormonal mechanisms
underlying the formation of social behavior.
Other physical changes mirror those in dogs and other domesticated
animals. In our foxes, novel traits began to appear in the eighth to
tenth selected generations. The first ones we noted were changes in
the foxes' coat color, chiefly a loss of pigment in certain areas of
the body, leading in some cases to a star-shaped pattern on the face
similar to that seen in some breeds of dog. Next came traits such as
floppy ears and rolled tails similar to those in some breeds of dog.
After 15 to 20 generations we noted the appearance of foxes with
shorter tails and legs and with underbites or overbites. The novel
traits are still fairly rare. Most of them show up in no more than a
few animals per 100 to a few per 10,000. Some have been seen in
commercial populations, though at levels at least a magnitude lower
than we recorded in our domesticated foxes.
Alternative Explanations
What might have caused these changes in the fox population? Before
discussing Belyaev's explanation, we should consider other
possibilities. Might rates and patterns of changes observed in foxes
be due, for example, to inbreeding? That could be true if enough
foxes in Belyaev's founding population carried a recessive mutant
gene from the trait along with a dominant normal gene that masked
its effects. Such mixed-gene, or heterozygous, foxes would
have been hidden carriers, unaffected by the mutation themselves but
capable of passing it on to later generations.
As Morey pointed out, inbreeding might well have been rampant during
the early steps of dog domestication. But it certainly cannot
explain the novel traits we have observed in our foxes, for two
reasons. First, we designed the mating system for our experimental
fox population to prevent it. Through outbreeding with foxes from
commercial fox farms and other standard methods, we have kept the
inbreeding coefficients for our fox population between 0.02 and
0.07. That means that whenever a fox pup with a novel trait has been
born into the herd, the probability that it acquired the trait
through inbreeding (that is, by inheriting both of its mutant genes
from the same ancestor) has varied between only 2 and 7 percent.
Second, some of the new traits are not recessive: They are
controlled by dominant or incompletely dominant genes. Any fox with
one of those genes would have shown its effects; there could have
been no "hidden carriers" in the original population.
Another, subtler possibility
is that the novelties in our domesticated population are classic
by-products of strong selection for a quantitative trait. In
genetics, quantitative traits are characteristics that can vary over
a range of possibilities; unlike Gregor Mendel's peas, which were
either smooth or wrinkly with no middle ground, quantitative traits
such as an animal's size, the amount of milk it produces or its
overall friendliness toward human beings can be high, low or
anywhere in between. What makes selecting for quantitative traits so
perilous is that they (or at least the part of them that is genetic)
tend to be controlled not by single genes but by complex systems of
genes, known as polygenes. Because polygenes are so
intricate, anything that tampers with them runs the risk of
upsetting other parts of an organism's genetic machinery. In the
case of our foxes, a breeding program that alters a polygene might
upset the genetic balance in some animals, causing them to show
unusual new traits, most of them harmful to the fox. Note that in
this argument, it does not matter whether the trait being selected
for is tameness or some other quantitative trait. Any breeding
program that affects a polygene might have similar effects.
The problem with that explanation is that it does not explain why we
see the particular mutations we do see. If disrupted polygenes are
responsible, then the effects of a selection experiment ought to
depend strongly on which mutations already existed in the
population. If Belyaev had started with 130 foxes from, say, North
America, then their descendants today would have ended up with a
completely different set of novelties. Domesticating a population of
wolves, or pigs, or cattle ought to produce novel traits more
different still. Yet as Belyaev pointed out, when we look at the
changes in other domesticated animals, the most striking things
about them are not how diverse they are, but how similar. Different
animals, domesticated by different people at different times in
different parts of the world, appear to have passed through the same
morphological and physiological evolutionary pathways. How can that be?
According to Belyaev, the
answer is not that domestication selects for a quantitative
trait but that it selects for a behavioral one. He
considered genetic transformations of behavior to be the key factor
entraining other genetic events. Many of the polygenes determining
behavior may be regulatory, engaged in stabilizing an organism's
early development, or ontogenesis. Ontogenesis is an
extremely delicate process. In principle, even slight shifts in the
sequence of events could throw it into chaos. Thus the genes that
orchestrate those events and keep them on track have a powerful role
to play. Which genes are they? Although numerous genes interact to
stabilize an organism's development, the lead role belongs to the
genes that control the functioning of the neural and endocrine
systems. Yet those same genes also govern the systems that control
an animal's behavior, including its friendliness or hostility toward
human beings. So, in principle, selecting animals for behavioral
traits can fundamentally alter the development of an organism.
As our breeding program has progressed, we have indeed observed
changes in some of the animals' neurochemical and neurohormonal
mechanisms. For example, we have measured a steady drop in the
hormone-producing activity of the foxes' adrenal glands. Among
several other roles in the body, the adrenal cortex comes into play
when an animal has to adapt to stress. It releases hormones such as
corticosteroids, which stimulate the body to extract energy from its
reserves of fats and proteins.
After 12 generations of selective breeding, the basal levels of
corticosteroids in the blood plasma of our domesticated foxes had
dropped to slightly more than half the level in a control group.
After 28 to 30 generations of selection, the level had halved again.
The adrenal cortex in our foxes also responds less sharply when the
foxes are subjected to emotional stress. Selection has even affected
the neurochemistry of our foxes' brains. Changes have taken place in
the serotonin system, thought to be the leading mediator inhibiting
animals' aggressive behavior. Compared with a control group, the
brains of our domesticated foxes contain higher levels of serotonin;
of its major metabolite, 5-oxyindolacetic acid; and of tryptophan
hydroxylase, the key enzyme of serotonin synthesis. Serotonin, like
other neurotransmitters, is critically involved in shaping an
animal's development from its earliest stages.
Selection and Development
Evidently, then, selecting foxes for domestication may have
triggered profound changes in the mechanisms that regulate their
development. In particular, most of the novel traits and other
changes in the foxes seem to result from shifts in the rates of
certain ontogenetic processes—in other words, from changes in
timing. This fact is clear enough for some of the novelties
mentioned above, such as the earlier eye opening and response to
noises and the delayed onset of the fear response to unknown
stimuli. But it also can explain some of the less obvious ones.
Floppy ears, for example, are characteristic of newborn fox pups but
may get carried over to adulthood.
Even novel coat colors may be attributable to changes in the timing
of embryonic development. One of the earliest novel traits we
observed in our domesticated foxes was a loss of pigment in parts of
the head and body. Belyaev determined that this piebald pattern is
governed by a gene that he named Star. Later my colleague
Lyudmila Prasolova and I discovered that the Star gene
affects the migration rate of melanoblasts, the embryonic
precursors of the pigment cells (melanocytes) that give
color to an animal's fur. Melanocytes form in the embryonic fox's
neural crest and later move to various parts of the embryo's
epidermis. Normally this migration starts around days 28 to 31 of
the embryo's development. In foxes that carry even a single copy of
the Star gene, however, melanoblasts pass into the
potentially depigmented areas of the epidermis two days later, on
average. That delay may lead to the death of the tardy melanoblasts,
thus altering the pigmentation in ways that give rise to the
distinctive Star pattern.
One developmental trend to which we have devoted particular
attention has to do with the growth of the skull. In 1990 and 1991,
after noticing abnormal developments in the skulls and jaws of some
of our foxes, we decided to study variations in the animals' cranial
traits. Of course, changes in the shape of the skull are among the
most obvious ways in which dogs differ from wolves. As I mentioned
earlier, Morey believes that they are a result of selection (either
natural or artificial) for reproductive timing and smaller body size.
In our breeding experiment, we
have selected foxes only for behavior, not size; if anything, our
foxes may be slightly longer, on average, than the ones Belyaev
started with 40 years ago. Nevertheless, we found that their skulls
have been changing. In our domesticated foxes of both sexes, cranial
height and width tended to be smaller, and snouts tended to be
shorter and wider, than those of a control group of farmed foxes.
Another interesting change is that the cranial morphology of
domesticated adult males became somewhat "feminized." In
farmed foxes, the crania of males tended to be larger in volume than
those of females, and various other proportions differed sharply
between the sexes. In the domesticated foxes the sexual dimorphism
decreased. The differences in volume remained, but in other respects
the skulls of males became more like those of females. Analysis of
cranial allometry showed that the changes in skull proportions
result either from changes in the timing of the first appearance of
particular structures or from changes in their growth rates. Because
we studied the skulls only of adult foxes, however, we cannot judge
whether any of these changes are pedomorphic, as Morey believes they
are in dogs.
The most significant changes in developmental timing in our foxes
may be the smallest ones: those that have to do with reproduction.
In the wild, foxes reach sexual maturity when they are about 8
months old. They are strict seasonal breeders, mating once a year in
response to changes in the length of the day (in Siberia the mating
season runs from late January to late March) and giving birth to
litters ranging from one to thirteen pups, with an average of four
or five. Natural selection has hard-wired these traits into foxes
with little or no genetic variation. Fur farmers have tried for
decades to breed foxes that would reproduce more often than
annually, but all their attempts have failed.
In our experimental fox population, however, some reproductive
traits have changed in a correlated manner. The domesticated foxes
reach sexual maturity about a month earlier than nondomesticated
foxes do, and they give birth to litters that are, on average, one
pup larger. The mating season has lengthened. Some females breed out
of season, in November–December or April–May, and a few
of them have mated twice a year. Only a very small number of our
vixens have shown such unusual behavior, and in 40 years, no
offspring of an extraseasonal mating has survived to adulthood.
Nevertheless, the striking fact is that, to our knowledge,
out-of-season mating has never been previously observed in foxes
experiencing a natural photoperiod.
Lessons Learned
Forty years into our unique lifelong experiment, we believe that
Dmitry Belyaev would be pleased with its progress. By intense
selective breeding, we have compressed into a few decades an ancient
process that originally unfolded over thousands of years. Before our
eyes, "the Beast" has turned into "Beauty," as
the aggressive behavior of our herd's wild progenitors entirely
disappeared. We have watched new morphological traits emerge, a
process previously known only from archaeological evidence. Now we
know that these changes can burst into a population early in
domestication, triggered by the stresses of captivity, and that many
of them result from changes in the timing of developmental
processes. In some cases the changes in timing, such as earlier
sexual maturity or retarded growth of somatic characters, resemble pedomorphosis.
Some long-standing puzzles
remain. We believed at the start that foxes could be made to
reproduce twice a year and all year round, like dogs. We would like
to understand why this has turned out not to be quite so. We are
also curious about how the vocal repertoire of foxes changes under
domestication. Some of the calls of our adult foxes resemble those
of dogs and, like those of dogs, appear to be holdovers from
puppyhood, but only further study will reveal the details.
The biggest unanswered question is just how much further our
selective-breeding experiment can go. The domestic fox is not a
domestic dog, but we believe that it has the genetic potential to
become more and more doglike. We can continue to increase that
potential through further breeding, but the foxes will realize it
fully only through close contact with human beings. Over the years,
other investigators and I have raised several fox pups in domestic
conditions, either in the laboratory or at home as pets. They have
shown themselves to be good-tempered creatures, as devoted as dogs
but as independent as cats, capable of forming deep-rooted pair
bonds with human beings—mutual bonds, as those of us who work
with them know. If our experiment should continue, and if fox pups
could be raised and trained the way dog puppies are now, there is no
telling what sort of animal they might one day become.
Whether that will happen remains
to be seen. For the first time in 40 years, the future of our
domestication experiment is in doubt, jeopardized by the continuing
crisis of the Russian economy. In 1996 the population of our
breeding herd stood at 700. Last year, with no funds to feed the
foxes or to pay the salaries of our staff, we had to cut the number
to 100. Earlier we were able to cover most of our expenses by
selling the pelts of the foxes culled from the breeding herd. Now
that source of revenue has all but dried up, leaving us increasingly
dependent on outside funding at a time when shrinking budgets and
changes in the grant-awarding system in Russia are making long-term
experiments such as ours harder and harder to sustain. Like many
other enterprises in our country, we are becoming more
entrepreneurial. Recently we have sold some of our foxes to
Scandinavian fur breeders, who have been pressured by animal-rights
groups to develop animals that do not suffer stress in captivity. We
also plan to market pups as house pets, a commercial venture that
should lead to some interesting, if informal, experiments in its own
right. Many avenues of both applied and basic research remain for us
to pursue, provided we save our unique fox population.
Acknowledgments
This article is dedicated to the memory of Dmitry K. Belyaev.
The research was supported by grants RBD000 and RBD300 from the
International Scientific Funds, grants 93-04-06936 and
96-04-49972 from the Russian Fund of Fundamental Research, and
grant 1757 of the Russian University Fund. The author expresses
her gratitude to Anna Fadeeva for translation of the manuscript
from Russian into English. She is also grateful to Irina
Plysnina for help during preparation of the manuscript and to
Yekaterina Omelchenko for technical assistance.
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