Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 42

However, in this chapter we will not consider the experiential effects, since
these are shared with any animal in captivity (for example, habituation to
humans and other aspects of the captive environment), and since they are not
transmitted to the next generation, they should not be included in a formal defi-
nition of domestication. However, epigenetic effects may partly bridge the gap
between genetics and experience; this will be considered more closely later.
Hence, we suggest that a more fruitful definition should focus on the effects,
which transfer across generations. Domestication is the process whereby
populations of animals change genetically and phenotypically in response to
the selection pressure associated with a life under human supervision. Hence,
domestication is an evolutionary process, and it acts through three different
pathways, in addition to natural selection, which of course continues also in
captivity. Firstly, there is a relaxation of specific natural selection pressures,
such as predation and starvation; secondly, there is an intensified human
selection for preferred traits, for example, growth, appearance, and reproduc-
tion; thirdly, there is a development of traits which are genetically or function-
ally correlated to the ones selected, for example, increased relative gut length in
fast-growing broilers ( Jackson and Diamond, 1995, 1996 ).
These three processes have also been discussed at length by ( Price, 2002 ).
Our definition leads to the view that domestication should be viewed as a
genetic process. Alleles, which ultimately increase the reproductive output—the
fitness—of an individual, are favored and will therefore be over-represented
in coming generations. But due to the third process, certain alleles may also
“hitch-hike” by being genetically linked to the selected locus.
In this chapter, we will therefore focus on behaviorally relevant genes and
mutations which may have been selected during domestication, and discuss
their functions and the mechanisms whereby they affect behavior. But we will
also discuss genetic architecture, i.e. the way in which genes are physically
organized in the genome, as this will be an important piece of information
for understanding correlated selection and how the different aspects of the
domestication phenotype are controlled genetically.
THE DOMESTICATED PHENOTYPE
A striking aspect of domestication is the emergence in various, unrelated
species, of very similar phenotypic changes. The complex of convergent traits
is often referred to as “the domestic phenotype” ( Price, 2002 ).
The first part of this complex relates to the appearance of the domesticated
animals in relation to their ancestors. For example, domesticates are generally
smaller and have modified and often reduced pigmentation. The size reduc-
tion can be seen both as an overall smaller body mass. This can however be
reversed by selective breeding, and many domestic breeds are in fact consid-
erably larger than their wild progenitors. For example, some dog breeds
exceed the weight and wither height of wolves ( Mosher et al., 2007 ), and