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Transcript of Jensen,_P._2006[1]
Domestication—From behaviour
to genes and back again§
Per Jensen *
IFM-Biology, Linkoping University, SE-58183 Linkoping, Sweden
Available online 18 January 2006
Abstract
During domestication, animals have adapted with respect to behaviour and an array of
other traits. This tends to give rise to a specific domestication phenotype, involving similar
changes in colour, size, physiology and behaviour among different species. Hence, domestication
offers a model for understanding the genetic mechanisms involved in the trade-off between
behaviour and other traits in response to selection. We compared the behaviour and other
phenotypic traits of junglefowl and white leghorn layers, selected for egg production (and
indirectly for growth). To examine the genetic mechanisms underlying the domestication-related
differences, we carried out a genome scan for quantitative trait loci (QTLs) affecting behaviour
and production traits in F2-birds of a junglefowl � white leghorn intercross. Several significant
or suggestive QTLs for different production traits were located and some of these coincided
with QTLs for behaviour, suggesting that QTLs with pleiotropic effects (or closely linked QTLs)
may be important for the development of domestication phenotypes. Two genes and their
causative mutations for plumage colouration have been identified, and one of these has a
strong effect on the risk of being a victim of feather pecking, a detrimental behaviour disorder. It
is likely that fast and large evolutionary changes in many traits simultaneously may be caused
by mutations in regulatory genes, causing differences in gene expression orchestration.
Modern genomics paired with analysis of behaviour may offer a route for understanding the
www.elsevier.com/locate/applanim
Applied Animal Behaviour Science 97 (2006) 3–15
§ This paper is part of the special issue entitled International Society for Applied Ethology Special Issue—A
Selection of Papers from the 38th International Congress of the ISAE, Helsinki, Finland, August 2004, Guest
Edited by Victoria Sandilands and Carol Petherick.
* Tel.: +46 13 281298.
E-mail address: [email protected].
0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.applanim.2005.11.015
relation between behaviour and production and predicting possible side-effects of breeding
programs.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Domestication; Gene; Genomics; Poultry; Chicken; QTL
1. Introduction: genes and behaviour
Let’s not mince matters: genes control behaviour, and this insight is one of the great
achievements of ethology. Of course, every biologist is well aware that a statement like
this will have to be expressed carefully to be generally true, for example, by saying that
a certain part of the phenotypic variation in behaviour is attributable to variation in
genotype (Alcock, 2001). However, it is quite clear that a particular behaviour
expression will never be possible unless there is a particular genetically determined
development of sensory organs, neurosystems and muscular systems. Hence, whether
we want to subscribe to the rather definite statement above will largely depend on what
we understand by ‘‘behaviour’’ and ‘‘genetic control’’ (Baker et al., 2001). For the
present discussion, I will use the term behaviour to include both the actual pattern of
muscle contractions forming a specific behaviour, and the level and intensity with
which it is expressed in a given situation. Genetic control will include all genetic
specifications of developmental pathways necessary for the expression of a particular
behaviour.
Embarrassingly enough, science has very little knowledge about how such control is
executed. Genes code for proteins, and modern genomics have excellent tools to
understand the genetic code on the level of proteins. Modern ethology likewise has
excellent tools for measuring and quantifying behaviour, but the link from DNA to
observable behaviour is – except for a few, rather simple cases – obscured by the seemingly
inaccessible complexity. Nevertheless, understanding the links is necessary if we want to
make real progress in understanding how behaviour is shaped by evolution, since natural
selection acts on the phenotype, but selects alleles for the next generation. The branch of
science involved in dissecting the molecular mechanisms involved in genetic control of
behaviour could be termed ‘‘behaviour genomics’’.
2. Domestication—a model for evolution
As already realised by Darwin, domestication offers a beautiful model for studying
phenomena like this. According to Price (1997), three processes are central to
domestication. Firstly, there is a relaxation of certain natural selection factors, such as
predation and starvation. Secondly, there is an intensified selection of traits preferred by
humans. Thirdly, there is natural selection under captivity, leading to adaptation. Side-
effects of selection, such as those outlined above, constitute a separate process, which also
needs attention when investigating domestication effects.
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–154
There is limited experimental research on the evolution of different traits, including
behaviour, during domestication. However, there is sufficient evidence based on
comparative studies of domestic stocks and their wild ancestors, to identify a number
of typical domestication changes, which can be summarized under the concept of ‘‘the
domesticated phenotype’’. This includes the following aspects:
(1) External morphological changes such as altered fur and plumage colours (mainly an
increased frequency of white and spotted colour morphs), changes in body size and
growth pattern, and changes in relative size of different body parts (including
brachycephaly, the shortening of skulls, and chondrodystrophy, the shortening of legs)
(Clutton-Brock, 1999).
(2) Internal morphological changes, such as an overall decrease in brain size, and modified
relative sizes of other internal organs, for example intestines (Jackson and Diamond,
1996; Kruska, 1996).
(3) Physiological changes, such as changes in endocrine responses and reproductive
cycles (Setchell, 1992; Kuenzl and Sachser, 1999).
(4) Developmental changes, such as earlier sexual maturity and changes in the length of
sensitive periods for socialisation (Belyaev et al., 1984).
(5) Behavioural changes, such as reduced fear, increased sociability, and reduced anti-
predator responses (Hedenskog, 1995; Johnsson et al., 1996; Price, 1997).
A typical domesticated phenotype of a species could therefore grossly be summarised
as differing from its wild ancestor in having a different plumage colour (probably being
white or spotted), being brachycephalic and chondrodystrophic, having a reduced brain
size and increased reproductive capabilities, developing faster and in a more flexible
manner, and being less fearful, more sociable and more risk-prone towards predators. This
is a trait complex, which tends to reoccur in many different domesticated species, and
therefore suggests that it may represent a general adaptation pattern to captivity and
domestication.
Interestingly, this complex of changes may develop rapidly, in only few generations, and
in concert, even though only one of the traits is selected for. Belyaev and co-workers
selected farm foxes only for reduced fearfulness towards humans, and found that the
frequency of animals showing this complex of adaptations, including morphological and
physiological changes, increased dramatically within 10–20 generations (Belyaev et al.,
1984; Vasilyeva, 1995). Observations such as this has led some researchers to suggest that
domestication phenotypes may be under control of few genes, perhaps regulating large
complexes of other genes affecting developmental and other traits (Stricklin, 2001).
Identifying such genes would be of prime importance for understanding the genetic
mechanisms involved in evolutionary change.
3. The genomic strategy
Genomics generally proceeds along a specific pathway of investigations in order to
identify genes involved in specific traits (Andersson, 2001), and determining its
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–15 5
mechanisms. First, we need animals which differ on the traits we are interested in—for
example growth rates, if we are interested in growth-related genes, or aggression if we are
interested in genes controlling aggressive behaviour. In such animals we may use
molecular methods to search for allelic differences which may help explaining the
phenotypic differences. Often, this may be facilitated by crossing parental animals of the
extreme phenotypes and looking at the inheritance patterns of the traits.
Once we have access to a suitable pedigree of animals with relevant variation in
interesting traits, unless we already have strong candidate genes, the genomic strategy
is to map the location of the genes we are looking for. Since behaviour is normally
inherited in a polygenic, additive manner, we are actually looking for many genes,
and we wish to estimate the contribution of each of the genes to the phenotypic
variation.
When the location of the genes is known, finding the actual gene and the mutations
causing the phenotypic variation can be done by utilising genome sequences and
bioinformatics tools. Most mapping methods will give a chromosome location which limits
the number of possible genes to perhaps a couple of hundred, so the actual gene
identification may be a rather time-consuming task.
Only when the gene is known with some certainty, we can start examining how the
mutations may cause the phenotypic variation we started out to examine. This process will
probably lead into proteomics and developmental biology. Applied to the case of
behavioural variation, the strategy of behaviour genomics will therefore lead us from
behaviour to genes and back again.
4. Chickens as model species
The first part of the strategy is to find a suitable animal material and pedigree. The
chicken has proved to be an excellent model animal for a number of reasons.
All poultry breeds are domesticated genotypes of the red junglefowl, Gallus gallus,
which live wild in south-east Asia, and it appears that domestication commenced at least
8000 years ago (Siegel et al., 1992; Yamashita et al., 1994). Junglefowl are readily
available, since they are kept in zoos throughout the world, and chickens exhibit among the
largest breed variability of all domestic animals, along with species such as dogs and
rabbits. For example, breeds are selected for appearance (show and hobby breeds),
aggression (fighting cocks), egg production (laying breeds), or rapid growth (broilers). This
makes the chicken an excellent model for genetic research, since crossings which produce
fertile offspring are possible between all breeds and their ancestors.
Birds have a definitive advantage over mammals such as rats and mice as behaviour
genetics models: their environments can be controlled from the point of egg laying (shortly
after fertilisation). By using artificial incubation and controlled rearing, environmental
variation can be controlled and reduced to a minimum, which means that genetic variation
will account for a larger proportion of the phenotypic variation in behaviour. Hence,
components of behaviour affected by genetic factors will be easier to detect in birds than in
mammals, where maternal effects during pregnancy and maternal care will add a large
portion of environmental variation which is different to control.
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–156
In addition, selection for production has been more intense in chickens than in any other
domestic species, and the production levels of modern poultry have therefore increased
dramatically. Production birds of today fall into two categories, the specialised laying hen,
selected for egg production, and the specialised broiler, selected for rapid growth.
However, even if the laying hen is not specifically selected for growth, it grows to about
double the size of junglefowl, and lays about ten times more eggs per year, which each is
more than double the size of those of junglefowls (Schutz, 2002). Some correlated side-
effects, both in health and behaviour, have been reported as a result of this (Braastad and
Katle, 1989; Rauw et al., 1998).
Last, but not least, the chicken genome recently became the first bird genome to be
sequenced (Consortium, 2004), which makes the species even more attractive as a model in
genomic studies.
5. Behavioural differences between laying hens and red junglefowl
In order to characterise the behavioural differences between the ancestor and a
selected model strain of laying hens, we compared their undisturbed behaviour in semi-
natural enclosures and in different controlled behaviour test situations (Schutz and
Jensen, 2001; Schutz et al., 2001). We found that mainly four aspects of behaviour
differed. Firstly, layers were generally less active, showing a reduced foraging and
exploratory behaviour. Secondly, they showed a less intense social behaviour, expressed
as a lower frequency of social interactions. Thirdly, they had a modified and less intense
antipredatory behaviour in tests where they were exposed to predator models, and
fourthly, there was a modified foraging strategy, where layers were less inclined to
explore unknown food sources. This is generally in line with the expectations from the
domestication phenotype theory, and would indicate a behavioural adaptation to
domestication in layers.
Phenotypic characterisations of these behavioural differences were then performed in a
number of different experiments involving junglefowl and laying hens which were
incubated, hatched and reared under identical conditions. In these experiments, we found
that junglefowl were generally more exploratory and appeared able to use the information
obtained by this exploration to adapt better to a sudden change in environmental conditions
(Lindqvist et al., 2002).
The social behaviour of the strains has also been further characterised. We found that the
type of behaviour patterns used by white leghorns in social interactions were very similar,
so no signal has been lost during domestication; however, junglefowl tend to display more
of the sexual and aggressive signals under identical conditions (Vaisanen et al., 2004).
Furthermore, we found indications that layers may have greater difficulty forming and
remembering social relationships (dominance–subordinance) than junglefowl, since the
aggression level after regrouping was generally higher in layers and persisted for a longer
time (Vaisanen et al., 2004). Again, the results indicate an adaptation to the domestication
environment on the part of laying hens, and signs of a negative effect on social adaptability.
The main phenotypical differences in behaviour and other traits between the laying hen
and its wild ancestor is summarised in Table 1.
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–15 7
6. QTL-analysis
The next step in the behaviour genomic strategy is to map the phenotypic differences to
specific loci. As already mentioned, behaviour as well as the other typical components of
the domestication phenotype (growth, physiology, etc.), are most likely polygenic and
show a quantitative inheritance pattern. Such traits have historically been very difficult to
map to specific loci, since mapping used to depend on analysing co-segregation of linked
loci with Mendelian inheritance, i.e. the offspring should fall into clear phenotypic
categories as a consequence of dominance patterns at single loci. However, the discovery of
molecular markers and the possibility of relatively simple analysis of these opened the
possibility for mapping so called quantitative loci, by means of analysing the inheritance of
neutral markers and measuring quantitative phenotypic traits in the same pedigree
(Andersson, 2001). This is referred to as quantitative trait locus analysis (QTL-analysis),
and a QTL is defined as a locus which contains alleles that differentially affect the
expression of a continuously distributed phenotypic trait. Finding a QTL for a trait is
therefore the first step towards identifying a gene affecting a phenotypic trait.
To start locating and identifying genes controlling the phenotypic differences between
junglefowl and layers, we performed a large scale QTL-analysis of different traits, including
morphological, physiological and behavioural ones. A QTL-analysis is commonly
performed by breeding a segregating population, for example an F2-intercross between
divergent lines, and then analysing the segregation of DNA-markers in this population. By
analysing the statistical association between DNA-markers and phenotypic traits, the control
of polygenic traits can be linked to specific chromosomal areas (Weller, 2001). The
junglefowl we used stemmed from zoo populations, and were obviously affected genetically
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–158
Table 1
Some important phenotypic differences in morphology, production traits, and behaviour between red junglefowl
and white leghorn layers; average values (data adopted from Schutz and Jensen, 1999; Lindqvist et al., 2002;
Schutz et al., 2002)
Phenotypic character Red junglefowl White leghorn
Adult body weight (g) Males: 1100, females: 800 Males: 2100, females: 1600
Age at start of
egglaying (week)
25 20
Egg size (g) 23 57
Egg mass produced
per week (g)
97 367
Plumage colour Wild-type White
Feeding behaviour Extensive, wide-ranging Intensive, local feeding
Explorative behaviour Frequent, wide-ranging
exploration
Less frequent, less wide-ranging
Anti-predator behaviour Vigilant, intensive reactions Less vigilant, less intensive reactions
General fearfulness Fearful to novel
stimuli and humans
Less fearful to novel stimuli and humans
Social behaviour Forms dominance relations fast,
frequent interactions
in stable groups
Forms dominance relations slower,
less frequent interactions in stable groups
For precise figures of behavioural differences, see the original publications.
and behaviourally by captivity (Hakansson and Jensen, 2005). However, they were
sufficiently different from domesticated breeds to suffice for our analysis.
We crossed one junglefowl male with four white leghorn females and intercrossed 36
F1-birds to obtain more than 1000 F2-animals. The parental male had a distinct genotype
on the DNA-markers, which allowed a powerful QTL-analysis. From 751 F2-
individuals, we obtained a full data matrix containing 101 DNA-markers (mostly
microsatellites), data on growth, egg production and feed consumption, and behavioural
data from an array of different tests, designed mainly to quantify aspects, which had been
characterised as major differences between the parental strains (as summarised in
Table 1).
Fig. 1 shows the general layout of the F2-intercross, together with a schematic layout of
how the QTL-analysis was performed.
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–15 9
Fig. 1. Schematic representation of the layout of the F2-intercross and the associated QTL-analysis. In the
parental chromosomes, indicated as vertical bars, two markers are denoted by M and N, with alleles 1 and 2,
respectively. The position of the QTL is indicated by Q, and in this example, the parentals are assumed to be fixated
for alternative alleles of Q, denoted 1 and 2. F1-animals in the middle row were all heterozygous on both marker
loci and QTLs. During gamete formation in the F1s, recombination gives rise to mosaic chromosomes in the F2-
generation, where the associations between marker alleles and QTL-alleles will depend on the distance between
them. In the bottom panel, some possible recombinations are shown. In F2, the parental origin of each marker can
be ascertained, and the regression between the probability of a specific marker genotype and the trait value
associated with the QTL can be calculated. When markers are evenly spaced on all chromosomes, a probability for
a particular locus to be associated with a particular trait (QTL) can therefore be obtained for all loci in the genome.
7. Coinciding QTLs for production and behaviour
We analysed for genome-wise significance and used Monte Carlo simulations to
ascertaing the critical p-values for the different traits (Carlborg et al., 2003). A number of
QTLs associated with growth and egg production were located. A surprising finding was
that a limited number of QTLs explained a large proportion of the difference in growth rate
between the junglefowl and the white leghorn—four QTLs explained 50% of the difference
in adult body weight of females and 80% of that of males (Kerje et al., 2003a). A QTL
analysis testing for epistasis revealed that epistatic interaction between genes played a
significant role for early growth but not for late growth (Carlborg et al., 2003). The two
most important growth QTLs were located on chromosome 1 (tentatively named Growth1
and Growth2). Growth1 was also found to be significantly related to egg production (mean
egg size), even after allometric corrections.
Both Growth1 and Growth2 were also related to different aspects of behaviour. In a
genome-wide scan, QTLs for tonic immobility duration and induction were located in the
same region as Growth1 and Growth2. Regression analysis of these two QTLs on various
behavioural variables showed significant effects on other fear-related behaviour as well,
such as latency to approach a novel object, activity in an open field and corticosterone
reaction in an open field (Schutz, 2002; Schutz et al., 2004) (Table 2).
It is possible that pleiotropic QTLs like these may have played a major role during
domestication, although a similar effect can be caused by closely linked QTLs. In
particular Growth1 is a potentially interesting locus to analyse further. Interestingly, no
other QTL analyses of growth performed on chickens have revealed this locus despite the
fact that it had the most prominent effect in our intercross with the red junglefowl (Kerje
et al., 2003a). Since all other studies have involved intercrosses between domestic breeds
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–1510
Table 2
Position of genome-wide QTLs (chromosome number and position in centimorgans, cM) with behavioural effects,
and some of their pleiotropic effects, as mapped in an F2-intercross between red junglefowl and white leghorn
layers
Chromosome Position (cM) Behavioural trait affected Observed pleiotropic trait effects
1 67 Tonic immobility** Growth**, egg mass**
1 480 Tonic immobility* Growth**
1 467 Novel object reaction** Growth**
1 394 Headflick frequency* –
1 176 Restraint inactivity* –
3 272 Restraint defecation* –
6 51 Restraint activity* –
7 174 Foraging behaviour** –
11 0 Restraint reaction** –
13 14 Open field activity* –
26 32 Victim of feather-pecking** Plumage melanisation**
27 0 Sociality** –
Data adopted from Schutz et al. (2002, 2004), Carlborg et al. (2003), Kerje et al. (2003a,b, 2004) and Keeling et al.
(2004). For precise estimates of precision in positions and significance levels, see the original publications.* p < 0.2.
** p < 0.05.
(for example, crosses between broilers and layers), this suggests that the domestic alleles at
the Growth1 locus are fixed in domestic breeds, which in turn may be an indication that this
allele was selected early during domestication.
8. Gene localisation and animal welfare
Of course, localisation of a QTL is only the first step towards finding the actual genes
and mutations causing a phenotypic effect. Using homologies between other sequenced
genomes (for example mouse, rat and human) and the chicken genome, it has been possible
to identify and characterise some of the genes and their causative mutations in our animals.
This has enabled us to identify genes involved in plumage colouration variation in fowl. We
have identified a mutation in the melanocortin 1-receptor (MC1R) gene, which has a
significant effect on the phenotypic expression of black pigment (Kerje et al., 2003b).
Furthermore, we identified a causative mutation in the PMEL17 gene, which we could
show to be responsible for the well-known dominant white-phenotype: birds with the
mutation, which involves a nine base-pair insertion in one of the exons, causing a
dysfunction of the eumelanosomes, do not express any black pigment at all (although they
may express other pigments, such as red) (Kerje et al., 2004). Since plumage colour is a
significant element of the domestication phenotype, these results are potentially interesting
in their own right in understanding domestication biology.
One of the colour mutation genes was also found to have a profound effect on an
important welfare-related behaviour, namely feather pecking. This is one of the most
important welfare-related behavioural problems in modern egg production, where birds
peck at and pull out the feathers of other individuals in the same group (Fig. 2). We found
that feather pecking was common in junglefowl, and more common in females than in
males (Jensen et al., 2005). Examining both the performance of feather pecking and the
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–15 11
Fig. 2. The average feather pecking damage score of birds with pigmented plumage, compared to white birds
(where the white colour is caused by a mutation in the gene PMEL17). Data adopted from Keeling et al. (2004).
resulting plumage condition in all F2-birds, we found a significant QTL for plumage
condition, indicating the risk of being the victim of the behaviour (Keeling et al., 2004).
This QTL coincided perfectly with the PMEL17-locus, and homozygote for the wild
genotype were significantly more vulnerable to being victims, whereas heterozygote were
almost as protected from the behaviour as the homozygous mutant (both heterozygote and
homozygote mutants were largely white). Furthermore, the risk of being victimised
apparently increased when the wild-types were more common in a cohort.
Even though the full significance of these findings remains speculative, it is clear that
victim traits may influence the development of this detrimental behaviour, which have
many parallel cases in other domesticated species (for example, tail biting in fattening pigs
and wool eating in sheep). It may also suggest that lack of plumage pigmentation could be
an adaptive evolutionary response, which reduces the risk of being victim of feather
pecking in chickens, and thus help explaining the development of domesticated white
phenotypes in this species.
9. Beyond allelic variation: gene expression patterns
Traditionally, evolutionary biologists have thought in terms of Mendelian genetics,
where phenotypic variation is ascribed to mutations in specific alleles, and where the
inheritance patterns of these mutations hold the keys to evolution of a population. However,
it has become increasingly clear that allelic variations cannot explain the vast phenotypic
variation between organisms with rather similar genomes. For example, humans and
chimpanzees have DNA-sequences which are on average 98.8% similar, and it has so far
been very difficult to pinpoint specific mutations explaining the main differences between
the species (Paabo, 2003). Recently, scientists have therefore started to look deeper into
how and when genes are actually expressed in mRNA and proteins, and found striking
effects on behaviour (Hofmann, 2003). For example, in voles, experimentally changing the
expression level of one single gene (V1aR, encoding a vasopressin receptor) in the ventral
forebrain changed the behaviour of a normally promiscuous species into a pair-forming
animal (Lim et al., 2004). Hence, large phenotypical differences can be achieved without
large allelic differences.
The orchestration of gene expression during development may be an important part of
developmental biology and domestication (Saetre et al., 2004), and such patterns of
expression differences may be affected by mutations in regulatory genes (Andersson and
Georges, 2004). In such a scenario, a single nucleotide mutation may have huge effects on a
variety of phenotypic traits, and such mutations may therefore underlie the rapid and
complex phenotypic changes observed during domestication. Our findings, where we have
found one specific genomic region involved in many apparently unrelated phenotypes, may
lend support to this possibility. To analyse effects caused by changes in gene expression, we
have developed a cDNA chip containing over 13 000 expressed sequence tags (EST;
roughly corresponding to genes). In future research, we will therefore be able to analyse not
only allelic differences between domesticated and wild birds, but also the relative
expression of thousands of different genes in different tissues, such as the brain, at different
times.
P. Jensen / Applied Animal Behaviour Science 97 (2006) 3–1512
10. Implications: breeding and animal welfare
Over the last decades, breeding for increased production has been the dominating goal
for animal agriculture. It has been estimated that average production levels have increased
by more than 85% since 1960, and parallel to that, many production-related diseases and
disorders have increased; for example, leg problems in fattening pigs, mastitis and
lameness in dairy cattle, and locomotory and circulatory problems in fast-growing broilers
(Rauw et al., 1998). Hence, breeding animals with a large emphasis on increasing
production may be associated with risks for animal welfare. To be able to maintain, and
even increase, production levels in farm animals in the future, without jeopardising
welfare, there is a need for increased biological knowledge about the mechanisms behind
side-effects on traits, which are not explicitly selected for. For example, increasing the
frequency of alleles which cause faster growth may at the same time cause a modification
in developmental, behavioural, physiological or immunological traits under the influence
of the same genes.
Behaviour is a central part of the mechanisms allowing animals to adapt to their social
and physical environments (for example, through learning and through forming social
systems). Therefore, selection side-effects on behaviour may have serious effects on the
welfare of animals. If genes that are under selection pressure during breeding for increased
production simultaneously affect behaviour, the adaptive capacity of the selected animals
may be affected.
11. Conclusions
Domestication involves a rapid and complex change of many different phenotypic
changes, which act in concert in a similar manner in many different species. We have
shown that, in chickens, an array of these changes are affected by few loci, and I suggest
that this may indicate that domestication changes can be caused by only few genes,
possibly with regulatory functions. In addition to increasing the understanding of genetic
control of behaviour, this may help us understand how animals adapt to selection pressures
induced by man during domestication.
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