Unit 5 Selection

Selection Differential (S)

• The average superiority of the selected parents is called as selection differential, symbolized by “S”.
• It is defined as the difference between the mean phenotypic value of the individuals selected as parents and the mean phenotypic value of all the individuals in the parental generation before selection.

S = (P_s – P)

where,

• P_s = mean of the selected parents
  • P = mean of the Population
• The selection differential may also be expressed in terms of phenotypic standard deviation (standard deviation is the measure of variability) as,

S = i × σ_P

Where,

• i = intensity of the selection
• σ_P = phenotypic standard deviation
• The intensity of the selection is also called as selection pressure. It is the mean deviation of the selected individuals in units of standard deviation.
• The intensity of selection is symbolized by “i”. It depends on the proportion of the individuals selected and it can be determined from the tables of properties of normal distribution.
• i = Selection differential / Phenotypic standard deviation

• Factors Affection Selection Differential 
  • Proportions of the animal selected for breeding; smaller the number larger the selection differential
  • Herd size; larger the herd size, smaller the proportions of animals selected
  • Reproductive rate; in cattle selection differential will be less whereas in pigs, it will be more because of more litter size
  • Use of artificial insemination and frozen semen increases selection differential or selection intensity in case of males and in females, super ovulation and embryo transfer increases the selection differential or selection intensity.

Generation Interval

• Definition: The average age of parents when their offspring are born.
• Varies between species and selection procedures.
• Factors affecting Generation Interval:
  • Early breeding in females reduces GI.
  • Progeny testing increases GI.

Accuracy of Selection

• Directly related to the heritability of the trait.

High Heritability:

• Selection on phenotype provides a good estimation of breeding value.

Low Heritability:

• More errors in selection.
• Accuracy can be improved by:
  1. Comparing animals in controlled environmental conditions.
  2. Using techniques to reduce environmental variation, thus increasing heritability.

Improving Accuracy When Individual Selection is Low:

1. Using additional measurements for the trait from the same individual.
2. Using measurements of correlated traits.
3. Using measurements of relatives.

Relationship Between Heritability and Selection Methods

Heritability	Selection Method Effectiveness
High	Phenotypic selection effective
Low	Need additional information

Selection Limit:

• When the selection is carried out continuously, the response to selection will be more for a few generations, and then it slows down and finally stops.
• When the response to selection has stopped, the population is said to be at “plateau” or “selection limit”.
• The main cause for this is fixation of favourable genes.
• This causes reduction or absence of genetic variation.
• Therefore, further improvement depends on introduction of new genetic variation.
• The new genetic variation can be introduced by cross breeding, mutation and genetic engineering.

Response to selection (R)

• The change produced by selection is the change of the population mean in the offspring.
• Symbolized by “R”
• Response to selection is the difference of mean phenotypic value between the offspring of the selected parents and the whole of the parental generation before selection.
• Also called as the expected genetic gain, symbolized by ∆G.

R = h²S

where,

• H² = heritability
• S = selection differential
• R / year = h²S / GI where,
• GI = generation interval

Factors affecting genetic gain 

The factors affecting the response to selection are

• heritability
• selection differential and
• generation interval
• Maximum gain will result when the selection differential (S) and the heritability (h² ) are high and the Generation Interval is low.

Bases of Selection

The sources of information indicative of an animal’s genetic value w.r.t. a particular phenotype, based on which selection is made, are known as bases of selection.

1. Individual selection
   1. Selection of an individual based solely on its performance
   1. Good for traits with high heritability – a good indicator of its breeding value for the phenotype
   1. Not useful for lowly heritable traits, sex-limited traits and carcass traits

• Pedigree Analysis
  • Pedigree: Record of an individual’s ancestors through its parents.
  • Selection is based on the information from pedigree records
  • Useful for sex-limited traits
  • Closer the relation of the ancestors to the individual, better the selection
  • E.g. selection based on parents and grandparents is more accurate than that based on just its great-great-grand parents

• Family Selection
  • Selection based upon the performance of collateral relatives (full-sibs, half-sibs, cousins)
  • Two types based on the criteria used for selection:
    a) Individual’s records + family records – family selection
    b) Only records of siblings (individual’s own records not used) – sib selection
  • Used more frequently in swine and poultry
  • Advantage:
    • Does not increase generation interval
    • Useful for traits which are hard to measure (viz., meat quality in live animals), sex limited traits and traits related to survival and disease resistance
• Accuracy of Selection
  Factors Affecting Selection Accuracy Based on Collateral Relatives
  • Heritability: Higher heritability increases accuracy.
  • Relationship closeness: Closer relatives provide better accuracy.
  • Number of relatives: More relatives used improves accuracy.
  • Phenotypic correlation: Lower correlation between relatives’ phenotypes increases accuracy.

• Progeny Testing
  • Individual selected as a parent based on the records of its progeny
  • High number of progenies is tested before making the decision
  • The production records of progeny are compared to its contemporaries
  • Useful especially for selection of sires for sex-limited traits (milk production)
  • The result of progeny testing for sires is expressed as sire index.
  • Advantage: High accuracy, identification of recessive genes in the sire
  • Disadvantage: Costly, time consuming (if generation interval is high)

Key Points

• Accuracy increases with more half-sibs and higher heritability.
• Accuracy using half-sibs never exceeds 0.5, regardless of number or heritability.
• Full-sibs can provide higher accuracy than half-sibs.
• Progeny testing offers higher accuracy but increases generation interval.

Methods of Selection

• The net economic value of an animal depends on performance of several characters
• Essential to estimate the total breeding worth (net merit) of an animal
• Multi trait selection: selection practiced for several traits simultaneously to improve overall merit

Requirements and efficiency of multi trait selection:

• Estimation of economic value of the traits
• Genetic significance of the animal in terms of h² of the traits and genetic correlations among the traits
• The methods of selection and number of traits to be included in the selection criteria

1. Tandem Method

• Multi trait selection practiced for improvement of several traits but at different times (one trait at a time)
• Selection continues for all traits one by one
• Efficiency depends on genetic correlation among traits
• Merits: easy to understand and conduct, positive genetic correlation leads to improvement in correlated traits
• Demerits: less efficient, requires more time, undesirable genetic correlation can neutralize progress

2. Independent Culling Level (ICL) Method

• Two or more traits taken at a time for selection
• Minimum standard (level) fixed for each trait
• Animal rejected if it fails to meet minimum standard for any one trait
• Efficiency depends on level fixed for each trait
• Advantages: superior to tandem method, culling can start at early age
• Disadvantages: inferior to selection index, no compensation for excellent traits, emphasis on early life traits, intensity reduced with more traits

3. Selection Index

• Numerical score assigned to estimate breeding value based on economic weight of traits
• Deficiency in some traits compensated by superiority in others
• Economic weight assigned to each trait based on h², economic value, genetic correlations
• Values of all traits added to get total score
• Animal with highest score selected for breeding
• Merits: superior to tandem and ICL, balanced approach, allows superior animals to be selected despite some inferior traits
• Requirements: economic value of traits, genetic and phenotypic variances and covariances of traits