Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 43

many horse breeds are considerably larger than their presumed ancestor
( Clutton-Brock, 1999 ). Nevertheless, in the absence of specific selection
for the opposite, the general pattern is that domestication causes animals to
decrease in size.
There is also a disproportionate size modification of certain body parts.
For example, legs tend to be shorter in domesticated animals, referred to
as chondrodystrophy, and the skull tends to show a more or less drastic
shortening from nose to neck, so called brachycephaly ( Clutton-Brock, 1999 ).
Further examples of typical aspects of the domesticated phenotype are floppy
ears, curled tails, and modifications of fur structure. Also in birds, similar
analogous changes occur, including alterations of size, skull shape, leg length,
pigmentation, and plumage morphology.
Behavior and physiology have undergone a range of changes. Most
prominently, domesticated animals generally exhibit a greatly reduced fear
of humans ( Price, 2002 ), which seems to generalize to other aspects of fear
as well ( Campler et al., 2009 ). Furthermore, domesticates tend to have an
overall reduced activity level and perform less explorative behavior, and
they also show less rigid social structures and often less agonistic behavior
( Sch ¨ tz and Jensen, 2001 ). For example, free-range dogs are known often
to form dynamically changing packs of 30

50 individuals, while wolves
live in strictly closed kinship groups of usually fewer than ten. Domesticated
animals usually develop faster and reach sexual maturity earlier than their
wild relatives. For example, dogs usually become sexually mature before one
year of age, whereas wolves take twice that time ( Boitani and Ciucci, 1995 ).
Red Junglefowl females start laying eggs at about 25 weeks of age, whereas
modern layers are reproductive already seven or eight weeks earlier ( Sch¨ tz
et al., 2002 ).
Thus, although the variation is large, mainly due to a high variety of selec-
tion pressures for different, human-controlled purposes, a relatively uniform
set of phenotypic changes reoccur in many species as a consequence of domesti-
cation. So this begs the question: why do these changes keep occurring? Are
they part of an adaptive process or solely caused by human preferences? In any
case, which are the genetic mechanisms involved?

EVOLUTIONARY MECHANISMS IN DOMESTICATION
Although natural selection does not end when a population is domesticated—
diseases, parasites, competition, and predation still exert a strong effect on
which individuals will reproduce—a strong selection is carried out by humans.
Still, the traits, which can be selected will depend on the trait variation and
the heritability of this, just like in the natural population. Hence, variation,
heritability, and selection remain the same driving forces during domestication
as they are in the wild.