Agriculture Reference
In-Depth Information
Index selection
Index selection involves creating an equation which
includes values recorded for both variates. Selection
indices can be either additive or multiplicative. For
example, an additive selection index for the
i
th genotype
with only two variates would be represented by:
rejected
. Therefore the only variable decision is the
selec-
tion intensity
, or the proportion of the total population
that will be selected for further evaluation in relation to
the proportion that are to be rejected or discarded. This
form of selection is called
cull selection
. In such a scheme
a target value is set and all phenotypes which meet the
target are said to fulfill the selection criteria while those
that do not reach the target value are rejected or fail to
meet the selection criteria.
A successful new cultivar is rarely due to desirabil-
ity in only a single character but is rather an overall
increase over several different traits. Therefore deciding
which individuals in a population are to be retained and
which are to be discarded usually involves simultaneous
evaluation for more than a single character.
When more than a single character is to be considered
in a selection scheme a plant breeder can make selection
by either
independent culling
of a number of characters
or by using some defined
selection index
.
I
i
=
(
w
1
×
x
i
1
)
+
(
w
2
×
x
i
2
)
where
I
i
is the index value,
w
1
and
w
2
are the
weights
for each variate and
x
1
and
x
2
are the actual recorded
values for each variate of the
i
th genotype. Obviously
if
n
variates were included in the index value then the
index equation would be represented by:
I
i
=
(
w
1
×
x
i
1
)
+
(
w
2
×
x
i
2
)
+···+
(
w
n
×
x
in
)
A similar multiplicative index with only two variates
would be:
I
i
=
(
w
1
×
x
i
1
)
×
(
w
2
×
x
i
2
)
Independent cull selection
To examine this consider a simple case where there are
only two variates to be included in the selection deci-
sion. If independent culling is used then the breeder will
choose
target values
for each of the two characters inde-
pendently. In order for a genotype to be selected, then
the phenotype must exceed (or be less than, depending
on the trait of interest) the target values of both of the
characters simultaneously. Therefore each of the geno-
types from the initial base population will fall into one
of four possible categories. Which, for example if we
are selecting for greater expression of both characters,
will be:
where
I
i
,
w
1
,
w
2
,
x
i
1
and
x
i
2
are as above. Finally if
n
variates were included in a multiplicative selection index
we would have:
I
i
=
(
w
1
×
x
i
1
)
×
(
w
2
×
x
i
2
)
×···×
(
w
n
×
x
in
)
The difference in results between index selection and
independent culling are primarily related to the asso-
ciation between the two (or more) variates and the
differences in the relative weighting of them. If there is
good association between the variates (i.e. high expres-
sion in one variate is related to high expression in the
other, and
vice versa
) and both are nearly equally val-
ued, then there may be little difference between the
genotypes selected by either method (Figure 7.4). If,
however, there is poor association between variates (i.e.
high expression in one trait is not related to a similar
high expression in the other variate) or one character is
of vital importance, then there could be a large differ-
ence in the genotypes that would be selected by index
selection over independent culling (Figure 7.5).
In almost all studies carried out it has been shown that
index selection is more effective in identifying genotypes
that are 'superior' for many different traits. The diffi-
culty in all selection index schemes is how to determine
the
index weights
(i.e. the
w
i
s).
It will not be possible within the scope of this topic
to fully explore the possibilities available with selection
Greater than the target value set for both Variate 1
and Variate 2
•
Greater than the target value set for Variate 1 but less
than the target value set for Variate 2
•
Less than the target value set for Variate 1 but greater
than the target value set for Variate 2
•
•
Less than the target value set for both Variate 1 and
Variate 2
With this form of selection, only the genotypes which
fall into category 1 (i.e. greater than both target val-
ues for each variate) would be retained, while all other
categories would be discarded.
