Behavioral Genetics and Animal Science - Genetics and the Behavior of Domestic Animals - page 28

OVERSELECTION FOR SPECIFIC TRAITS
There are countless examples in the medical literature of serious problems
caused by continuous selection for a single trait ( Dykman et al., 1969;
Steinberg et al., 1994 ). People experienced in animal husbandry know that
overselection for single traits can ruin animals. Good dog breeders have
known this for centuries. Some traits that appear unrelated are in fact linked.
Wright (1922, 1978) demonstrated this clearly by continuous selection for
hair color and hair patterns in inbred strains of guinea pigs. Selection for
hair color and patterns resulted in decreased reproduction in all the strains.
Furthermore, differences in temperament, body conformation, and the size
and shape of internal organs were found. Belyaev (1979) showed that contin-
uous selection for tameness in foxes reduced maternal behavior and caused
neurological problems. Graded changes occured in many traits over several
years of continuous selection for tame behavior. Physiological and behav-
ioral problems increased with each successive generation. In fact, some of
the tamest foxes developed abnormal maternal behavior and cannibalized
their pups. Belyaev et al.(1981) called this “destabilizing selection,” in contrast
to “stabilizing selection,” found in nature ( Dobzhansky, 1970; Gould, 1977 ).
There are countless examples in the veterinary medical literature of
abnormal bone structure and other physiological defects caused by over
selecting for appearance traits in dog breeds ( Ott, 1996 ). The abnormalities
range from bulldogs with breathing problems to German shepherds with hip
problems. Scott and Fuller (1965) reported the negative effects of continuous
selection for a certain head shape in cocker spaniels:
In our experiments, we began with what were considered good breeding stocks,
with a fair number of champions in their ancestry. When we bred these animals to
their close relatives for even one or two generations, we uncovered serious defects
in every breed ... Cocker spaniels are selected for a broad forehead with prominent
eyes and a pronounced “stop” or angle, between the nose and forehead. When we
examined the brains of some of these animals during autopsy, we found that they
showed a mild degree of hydroencephaly, that is, in selecting for skull shape, the
breeders accidentally selected for a brain defect in some individuals. Besides all this,
in most of our strains, only about 50% of the females were capable of rearing
normal, healthy litters, even under nearly ideal conditions of care.
Overselection in Livestock
In pigs and cattle, indiscriminate selection for production traits such as rapid
gain and leanness result in more excitable temperaments in pigs ( Grandin,
1994 ). The first author has observed thousands of modern lean hybrid pigs.
Lean hybrids have a greater startle response and are more excitable and diffi-
cult to drive through races compared to older genetic lines with more back