Genetic Influences on the Behavior of Chickens Associated with Welfare and Productivity - Genetics and the Behavior of Domestic Animals - page 336

phenotype. Therefore, to improve overall eggs per hen housed it is not only
necessary to measure production in multiple-bird cages but to select sepa-
rately on rate of lay and days' survival.
Therefore the problem of adaptation is a joint one: the environment of
selection and the traits of selection. The usual solution to eliminate, or guard
against, genotype by environment interactions in production animals, is to
select the animal in the environment in which it is to perform, i.e. to make
the basis of selection performance in multiple-bird cages rather than in
single-bird cages. However, that solution will not work in this case and could
actually make the situation worse. Selecting the highest performing individ-
ual in a group environment will select for the more dominant birds.
From a genetics perspective, the environment is not constant but is
constantly changing because the environment is defined not only as that of a
group but also of the individuals within that group. Individual genotypes in
groups are constantly changing as selection progresses, i.e. the associate
effects of other genotypes in the population are involved. Thus, theory
predicts that selection based on the individual will be antagonistic to group
performance.
Although the studies cited indicated major effects of agonistic behavior
on performance in egg-production stocks, most breeders select their elite
stocks on the basis of records obtained in single-hen cages ( Hunton, 1990 ).
The genotype by housing environment interactions found in these studies
emphasize the risk taken by breeders who select for egg production on
the basis of hens' performance in single-bird cages for stocks to be housed
commercially in social groupings.
MULTI-LEVEL SELECTION
Siegel (1989) considered adaptability to be an individual's ability to adapt
to its environment. He concluded that individuals that adapt have a higher
probability of contributing genes to subsequent generations than those that
do not. This concept emphasizes the individual. What if an individual adapts
to its environment by eating its cagemates? Survival of the individual is
maximized, but what of that of the group?
There are numerous ways that performance of one individual can influ-
ence that of another. Accommodation for such interactions presents an
insurmountable dilemma from the point of view of classical (non-interaction)
quantitative genetic methodology. Griffing (1967) recognized that with com-
petition, the usual gene model for a given genotype must be extended to
include not only the direct effects of its own genes, but also the associate
contributions from other genotypes in the group. The problem is to optimize
production of a given genotype in a competitive environment. As a conse-
quence of interacting genotypes, the same genotype can have different
expressions in populations having different population structures.