2 Principles of selection.ppt

notes of principles of selection

1. Unit: 2
Principles of selection

2. Principles of selection
•Selection is a process in which certain individuals in a population are
preferred to others for the production of next generation.
•Selection is carried on a variety of traits. For example: in dairy cattle
selection is directed to higher milk and butterfat production, in race
horse it is for more speed or in pet stock it is for appearances.
•Selection has been called the keystone of the archin animal breeding.

3. 2.1 Types of selection
•Selection is divided into two general classes:
a)Natural selection
b)Artificial selection
a)Natural selection
•It is defined to be that selection which occurs without the planning of
man.
•It involves the forces of nature that determine which animals will
reproduce and leave viable offspring to continue the process.
•Organisms that are most adapted to their environment will mate each
other to pass on their genes to the next generation.
•Natural Selection has been referred to as survival of the fittest in a
particular environment.

4. b) Artificial selection
•It may be defined as the effort of human to increase the frequency of
desirable genes or combination of genes in the herd/ flock.
•Human decides which animals will be allowed to reproduce and
which will not.
•Animals chosen to produce the next generation may not be chosen for
their ability to survive or adapt to the environment.
•Rather, they are chosen for their apparent superiority for some trait
that humans have decided is useful. e. g. Farmers will breed best
cows to give best milk.
•Since animals involved with artificial selection are usually
domesticated, they are protected from many adversities of the
environment.

5. 2.2 Basis of selection
1.Selection based on individualﹸs performance
•It means that an animal is kept or rejected for breeding purposes on
the basis of its own phenotype for a particular trait or traits.
•The progress made from this type of selection depends on the
heritability of the trait being selected.
•The higher the heritability the higher the correlation between the
genotype and phenotype, and better the genetic progress.
•The value of an individual must be determined by comparing the
individual's phenotype to the average of the group in which it is
selected.

6. 2. Selection based on pedigree performance
•Pedigree isdirect ancestors of individuals, in which the performances
of each ancestor are recorded and individuals are selected based on
these records.
•The more emphasis should be placed on information from more
recent ancestors.
•Pedigree information can be used to support what may already be
known about an individual.
•For example, if performance records on a certain bull are very good
and those of his parents are also favorably this is valuable support for
a conclusion that the bull may be genetically superior.

7. Cont…
•On the other hand, if the bull has a good performance record, but
none of his ancestors do it may be an indication that his superiority is
due to a lucky combination of genes.
•The ultimate conclusion might be not to choose the only good bull in
a bad family.
3. Selection based on progeny testing
•It is an effort to evaluate the genotype of an animal by observing the
phenotypes of its progeny.
•Progeny testing is usually performed on males because they can
produce so many more progeny than females.
•Time is one of the most limiting factors associated with progeny
testing.
•The great attraction of progeny testing is due to the high accuracy,
which can be achieved when many progenies are obtained.

8. 4. Selection based on collateral relatives
•Collateral relatives are those not directly related to an individual as
ancestors or progeny.
•Thus, they are individual’s brothers, sisters, cousins, uncles, aunts,
etc.
•The more closely they are related to the individual, the more valuable
is the information they can supply for selection purposes.

9. 2.3 Methods of selection
•The value of an animal is dependent upon several traits that may not
be of equal economic value or that may be independent of each other.
•For this reason, it is usually necessary to select for more than one
trait at a time.
•If too many traits are selected for at one time, less progress will
obtain in the improvement of any particular one.
•Several methods may be used to determine which animal should be
selected or rejected for breeding purposes.

10. 1.Tandem method
•It involves selection of one trait at a time for genetic progress .
•When a satisfactory level of performance is attained in one trait, the
next trait is considered.
•This method is generally not recommended, because of the added
time needed to achieve progress in several traits.
•It is also difficult to maintain a satisfactory level in one trait while
trying to obtain progress in another.

11. 2. Independent Culling method
•In this method, selection is practiced for two or more traits at a time.
•For each trait a minimum standard is set that an animal must meet in
order for it to be saved for breeding purposes.
•Failure to meet the minimum standard for any one trait causes that
animal to be rejected for breeding purposes.

12. 3. Selection Index/ Total Score
•This method involves theseparate determination of the value for each
of the traits selected for, and addition of these values to give a total
score for all of the traits.
•The animals with the highest total scores are then kept for breeding
purposes.
•It is more efficient (more genetic improvement) than the independent
culling method and tandem method.
•When considering multiple trait selection, it is important to keep the
number of traits as low as possible.
•The more traits being selected means that less progress is made in
any one trait.

13. Cont…
•The selection index is more efficient than the independent culling
method because it allows the individuals that are superior in some
traits to be saved for breeding purposes even though they may be
slightly deficient in one or more of the other traits.