Selection is the differential reproduction of genotypes based on differences in fertility, viability, or choice of mates between individuals. There are two main types of selection: natural selection, where nature selects the most adaptable individuals; and artificial selection, where humans intervene to select superior individuals to be parents of the next generation. The criteria or basis of selection includes individual selection based on own performance; pedigree selection using ancestor performance; family selection based on family mean; and progeny testing using offspring performance. Selection methods include the tandem method selecting traits sequentially; independent culling levels setting minimum standards for traits; and the selection index assigning economic weights and scores to rank individuals.

1. Prepared by Dr. Sushil Neupane (B.V.Sc. and A.H., IAAS, TU)
Unit 6: Principles of selection.
6.1 Importance and basic principles of selection
6.2 Basis of selection
6.3 Methods of selection
Definition:
1. According to Prof. I M Lerner: Selection is the non-random differential
reproduction of different genotypes.
i. Differences in fertility among individuals in parent generation
ii. Differences in viability among the progeny
iii. Choice of mated individuals
2. According to Prof. Jay L Lush: Selection is the differential reproductive rates
within the population whereby some individuals tend to have more offspring than the others.
i. All individuals are not allowed to be the parents.
ii. Difference in fitness from individual to individual.
Types of selection
1. Natural Selection
 Selection operates through natural forces like fertility and viability.
 Nature selects individuals which are more adaptable and fit in a particular
environment.
2. Artificial Selection
 Selection operates through intervention of man (animal breeder)
 Superior individuals will be allowed to be the parents for next generation.
 Natural selection favours artificial selection.
Criteria or Basis of Selection
• Breeding value is the only criteria of selection.
• Value associated with the genes carried by an individual is the B.V.
• Breeding value of an individual is estimated either from the phenotypic value of the
individual itself or from the phenotypic value of its relatives.
• Information from relatives is very important to estimate the breeding value of an
individual. Because there are certain percentage of genes which are common between
individual and its relatives due to common ancestry.
The sources of information based on which the breeding value of an individual is
estimated are called as the basis of selection or Principles of selection or Criteria of
selection.

2. Prepared by Dr. Sushil Neupane (B.V.Sc. and A.H., IAAS, TU)
1. Individual Selection
2. Pedigree selection
3. Family selection
4. Progeny testing
1. Individual Selection
• Selection is made on the basis of own performance or own phenotypic value.
• When breeding value of an individual is estimated based on its own phenotypic
value, then it is known as individual selection.
• Conditions applied for individual selection:
(i) When traits are expressed in the individual
(ii) When mating records are maintained
Limitation of individual selection:
1. Cannot be applied for sex-limited traits.
2. Cannot be selected if the traits are expressed later in life or after death of individual
(slaughter traits)
Advantages of individual selection:
1. Information of the individuals are easily available.
2. It can be applied before progeny testing
3. Used when pedigree information are not available
4. Generation interval is shorter in comparison to progeny testing
2. Pedigree Selection
 Selection of an individual based on performance of its ancestors is known as pedigree
selection.
 Pedigree is the systematic list or records of ancestors of an individual.
 Ancestors are parents, grand-parents, great grand-parents, etc.
 Pedigree having information on the economic traits of ancestors is useful for selection
of an individual.
 Breeding value of an individual is estimated on the basis of performance of ancestors.
 An individual is related by 50% with its parents, 25% with grand parents, 12.5% with
great grand parents and so on.
 The relationship is reduced to half in each generation due to halving nature of
inheritance.
 It is thus more important to consider recent ancestors (parents) than the remote ancestors
for pedigree selection.
Advantages of pedigree selection:
• Allows selection at younger age.
• It is helpful in multistage selection.

3. Prepared by Dr. Sushil Neupane (B.V.Sc. and A.H., IAAS, TU)
• It is helpful when two individuals have similar performance but one belongs to a better
pedigree.
• It is less costly, as only compilation of pedigree is required.
Demerits of pedigree selection:
• Characters with low heritability cannot be improved to the level of expectation.
• Some pedigree gets undue emphasis and favoured. Better environment is provided
to the favoured pedigree.
• Pedigree selection provides no basis of selection among individuals when
descendants are from the same ancestors.
3. Family selection
 Information from collateral relatives may be used for estimation the B.V. of an
individual.
 Among the collateral relatives, full-sibs and half-sibs are more closely related to the
individual.
 Full-sibs and half-sibs constitute full-sib family and half-sib family respectively.
 Selection of an individual on the basis of family mean is called family selection or
sib selection depending upon inclusion or exclusion of individual’s own performance in
the family mean.
 Selection of an individual on the basis of family mean is called family selection when
individual’s own performance is included in the estimation of family mean.
Advantages of family selection:
• It is suitable for traits with low heritability and high reproductive rate. Eg. Litter size
in pig, egg production.
• It reduces generation interval.
• It is an aid to individual selection because it is better to select an individual from a
superior family.
Limitation:
• It leads to inbreeding
• Full-sib family selection can only be applied in species with high reproductive rate
to get large family size.
• Family selection is costly since large number of families are required to be
maintained.
4. Progeny Testing
• Selection of an individual based on its progeny performance is known as progeny
selection or progeny testing.
• It is the most important basis of selection.
• Mean phenotypic value of the progeny is taken as the deviation from mean
phenotypic value of contemporaries.

4. Prepared by Dr. Sushil Neupane (B.V.Sc. and A.H., IAAS, TU)
• Each progeny inherits one-half of the genes from each parent due to halving nature
of inheritance.
• This is taken as transmitting ability of the parent.
Advantages of Progeny testing:
• It increases intensity of selection.
• Accuracy increases with the increase in progeny number.
• Useful for identification of recessive genes, if present, in the bull put under test.
Limitations of Progeny Testing:
i. Time and cost is the main limiting factor.
ii. Increases generation interval
iii. Genetic gain per unit of time is low.
Methods of Selection:
1. Tandem method
2. Independent Culling Level method
3. Selection Index or total score method
1. Tandem Method
 This is a multi-trait selection.
 Tandem method is practiced for improvement of several traits but at different
times i.e., one trait at a time and selection process will continue for several
generations till the improvement is made in that trait up to desired level.
 Thereafter, selection is practiced for improvement in the second trait and the
selection process will also continue for several generations till the goal is
achieved to the desired level.
 In this way selection will continue for all the traits one by one.
2. Independent Culling Level Method of Selection
 Two or more traits are taken at a time for selection of an individual.
 A minimum standard (level) is fixed for each and every trait.
 Each and every animal has to achieve the target fixed for each character for its
selection.
 If an animal fails to meet the minimum standard fixed for any one character will
be rejected even though it is exceptionally good for all other traits.
Example: IC method of selection in buffalo
Traits Standard Set Buffalo No.
1 2
AFC (months) 42 40 44
LMY (Kg) 1800 1850 2200
Fat % 7 7 6.5

5. Prepared by Dr. Sushil Neupane (B.V.Sc. and A.H., IAAS, TU)
Buffalo no. 2 is rejected though it is excellent in LMY.
Buffalo no. 1 is selected as it has achieved the minimum target fixed for all the three
traits.
3. Selection Index
• Selection index is the numerical score assigned to an individual to estimate its
breeding value on the basis of economic weight of the traits considered for selection.
• Several number of traits are considered simultaneously for selection of an individual.
• Selection index is an index of the net merit of an animal for many traits.
• Deficiency in some traits is taken into account by superiority of other traits.
• Certain value (economic weight) is assigned to each trait depending its h2 and
economic value as well as genetic correlation among the traits.
• Values of all the traits are added together to get the total score for an animal. For this
reason selection index is also known as total score method.
• Credits and penalties are given to each animal according to the degree of superiority
or inferiority in each trait.
• The animal with the highest score is then selected for breeding.