Biology Reference
In-Depth Information
Griffing (1967, 1977) extended selection theory to take into consideration
interactions of genotypes. The conceptual biological model was extended to
define the group and the usual model was extended to include not only direct
effects of its own genes, but also associate contributions from other geno-
types in the group. It is also of special note that Griffing (1967) shows that
selection only on associate components cannot guarantee a positive response
to selection, i.e. selection for reduced aggression will not ensure that produc-
tion will increase.
The modern general classification of methods that use group selection or
some combination of group and individual selection is now termed multi-
level selection. Multi-level selection is the direct analogy of the classical
index selection results based on the non-interaction model. Let Y kl be the
phenotype of the lth individual in the kth group (or family), then, the
between and within group de vi atio ns can be combined in an index,
I kl 5
are respec-
tively the between- and within-group deviations, and B 1 , B 2 are weights.
Classical group selection occurs when B 1 5
B 1 τ k 1
B 2 γ
( k ) l , where
τ k 5 ð
Y k : 2
Y
:: Þ
and
γ ð k Þ l 5 ð
Y kl 2
y k : Þ
1 and B 2 5
1, within group selec-
tion results when B 1 5
0 and B 2 5
1, and multilevel selection results when
B 1
0 and B 2
0.
Group Selection Experiments
There is limited, but ample, experimental evidence to support Griffing's
(1967) theory. The first experiment was that of Goodnight (1985) who
showed that leaf area of Arabidopsis thaliana would respond to group but
not individual selection. The first experiment to use group selection with
chickens was unsuccessful ( Craig et al., 1982 ). Craig (1994) reflected that
the reason for lack of response was because he had not provided an environ-
ment in which the hidden genetic variability could be expressed, i.e. the
beak trimmed hens, density was low, and only a part record was used.
The first successful experiment with group selection in poultry was initi-
ated in 1981 at Purdue University. After four generations of group selection
based on half-sister families housed initially in groups of nine, and later in
groups of 12, performance was compared between the selected and control
lines in six-hen cages with 387 cm 2 floor space per bird from 16 to 40 and
16 to 36 weeks of age, respectively, by Kuo et al. (1991) and Craig and
Muir (1991) . In the second study a stock derived from a competitive com-
mercial stock by two generations of relaxed selection was included also. In
both experiments, hens had their beaks left intact or beaks were trimmed to
two different lengths.
Results of the first study showed highly significant beak-treatment by
genetic stock by age interactions for hen-housed rate of lay and daily egg
mass; as mortality from cannibalism increased dramatically with age in the
control, but not in the selected stock, differences between the selected and
Search WWH ::




Custom Search