Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 44

It is therefore reasonable to claim that domestication is an evolutionary
process whereby populations overall adapt to the prevailing selection pres-
sures. In that sense, the domesticated phenotype may largely be caused by an
adaptive response to the human protection. For example, body size of the wild
ancestors has been optimally adjusted to the natural selection pressures in their
wild habitats, such as food availability and predation risk. When predation
pressure decreases and food availability goes up, it may be an adaptive
response to reduce the body size. This will in turn enable faster development
and increase the lifetime reproductive success, something that may also have
been appreciated and exacerbated by early farmers. It has been argued that
many of the different parts of the domestic phenotype are typical of adaptive
responses to r-selection, as opposed to k-selection ( Price, 2002 ).
In the same way, several behavioral responses appear to be adaptive.
For example, reduced rigidity in tolerance of variations of social structures can
save energy and reduces the risk of injuries in situations where conditions are
more likely to be crowded with many individuals in a limited area. It can also
be adaptive to save energy by not engaging in excessive exploratory behavior
when the net benefits from this are outweighed by the fact that food is provided
by humans.
In a systematic comparison between domesticated White Leghorn laying
hens and their ancestors, the Red Junglefowl, Sch¨tz and Jensen (2001) studied
groups of birds reared and kept under identical conditions. The domesticates
were in general less active, displayed less social behavior, and were less investi-
gative. The latter was shown, for example, by a decreased level of so-called
contrafreeloading; given a choice between hidden and freely available food, the
domesticates ate less than the Red Junglefowl from the food source requiring
some searching, digging, and scratching. The behavior of the White Leghorns
was interpreted as an adaptive response to life under human protection, where
food is abundant, predation is less of a risk, and crowding is common.
It has been argued that the reduced natural selection pressure under
domestication should lead to an increase in trait variation ( Price, 2002 ). The
logic of the argument is that, in absence of directed selection, the fitness
of individuals carrying sub-optimal traits should be relatively higher, and the
population distribution should be flattened. There is little actual evidence to
support this when considering specific domestic populations. However, when
directed selection is applied on a specific trait, between-breed variation may
be dramatic. Dog breeds, for example, vary in adult size from about 700 g
to 70 kg, and chickens come in dwarf versions of less than 500 g adult size,
as well as giant breeds, reaching 7 9 kg. However, within breeds the varia-

tion is less pronounced, and mostly does not appear to be larger than in a
population of the wild ancestors. Hence, the directed, human-controlled
selection under domestication may be as strong as the natural selection in
the wild, causing trait distributions to change in domesticated populations
without significantly changing the variation.