Selection introduction

Selection
Presented by: Dr. Jayesh Vyas

Contents
Selection definition
Forces of selection
Patterns of selection
Multistage selection
Multitrait selection
Multistage-multisource selection
Combined selection
Consequences of selection

The main objective of animal breeder is to bring
change in genetic composition of population such
that the progeny generation is genetically better
than parental population.
2 ways to bring about genetic improvement:-
i. Selection
ii. Mating/ Crossing system

Selection:- Selection is differential propagation of
genetic material to next generation.
The process of differential survival and
reproduction of different individuals (genotypes)
in population is called selection.
2 forces of selection:-
I. Natural selection
II. Artificial selection

Natural selection
1. Principle- “Survival of
fittest” in given
environment.
2. It is measured by
survivability and
reproductive success of the
individual.
3. A very slow process and
usually takes hundreds/
thousands of generations
to produce meaningful
effects.
Artificial selection
1. Standards laid down by the
breeder, which animals
retain and allow to become
parents of next generation.
2. Under control of certain
set rules. To govern the
probability that an
individual reproduce.
3. Fast process and bring
about considerable change
in genetic composition of
population over short
periods.

4. Occur at gametic stage:-
• Segregation of
homologous
chromosomes at meiosis
for gamete formation.
• Gamete competition.
At zygotic stage:-
• Embryonic growth and
development
• Maturity rate
• Mating ability
• Total progeny produced
• Birth to adulthood
4. Can be conducted when
genotypes are more
attractive, productive,
more efficient
functionally and
preferred by the
breeder.

5. Example:- origin and
development of new
species of animals and
plants.
6. Mediated by force of
nature & animal which
best fit in their
environment have higher
survival rate and produce
more no. of progeny.
5. Example:- development of
new varieties/ breeds of
animals and plants.
6. Mediated by man & in this,
choosing the individuals to
become parents of next
generation.

Natural selection may occur simultaneously with
artificial selection.
Intense artificial selection for a trait is practiced
consistently in one direction over successive
generations, natural selection oppose it through
reduced viability and reproduction.
Example-: artificial selection in broilers for last 3
to 4 decades has led to severe decline in their
viability and reproduction.

Patterns/ Modes/ Forms of artificial selection:-
1. Stabilizing selection/ Centripetal/ Unifying
selection on balanced selection:-
• Selecting the individuals having phenotypic value
around population mean and with intermediate
phenotypes are selected to be parents of progeny
generation.
• Discarding the individuals with extreme
phenotypes.
• This type of selection favours heterozygotes.
• This selection keep stable population mean over
generations and reduces variability.
• This selection operates contrary to directional
selection.
• Eg:- Birth wt. on infant mortality

2. Diversifying/ Disruptive/ Bidirectional/
Centrifugal/ 2-way selection:-
• Involves selecting individuals from two extremes
(maximum & minimum) of distribution.
• Selection favours two diverse types at a time and
eliminating the individuals with intermediate
(heterozygotes) phenotypic values.
• Selection results in little change in phenotypic,
breeding value, gene frequencies and avg.
performance. 3 types:-

Disassortative mating:- phenotype of mated
individuals are less similar.
• Variance is reduced in offspring generation.
• Increases heterozygotes at expense of
homozygotes.
• Progeny population less variable.
• Gene frequencies do not change in subsequent
generation.

Assortative mating:- Similar phenotypic
individuals are mated.
• Increased the variance in offspring generation.
• Finally emerge in two separate and distinct sub
populations one with better performance and
other with poor performance.
• Heterozygous genotypes are reduced in
subsequent generations and homozygous
genotypes are increased.
Random mating:- Increases the variability in
offspring generation very slightly.

3. Directional/ One way upward/ Linear/
Dynamic selection:-
• Selecting the individuals with extreme
phenotypic value (greatest or least).
• Not allowing to reproduce individuals with
intermediate phenotypic values.
• Individuals selected are genetically superior
with higher frequency of favourable alleles for
that trait.

• Increasing the frequency of more favourable
alleles and results in improvement of mean
phenotype/ performance of offspring generation
in direction of favoured phenotype.
• This is most common form of selection applied in
farm animals.
• The individual chosen to become parents of next
generations are superior over the population avg.
and constitute the selected group.
• The remaining animal of population are culled
animals and may be used for commercial
purposes.

Multistage selection:- The selection involves the
identification of individuals which are superior
and it is a complex process completed in different
stages/ ages of animal known as multistage
selection.
First stage of selection At the birth of animal
Second stage of
selection
At growth and
maturity
Third stage of
selection
At pedigree of the
animal
Young male selection On libido, semen
quality, semen
freezability
Last stage of selection On breeding value for
the character

Two stages of selection:-
1. Phenotypic selection:- First stage of selection.
• Based on physical attributes of phenotypic
performance.
• Free from any type of genetical or congenital
defect.
• Functional reproductive system.
• Selected animal should be healthy.
• Dams performance.

2. Genetic selection:- Second stage of selection.
• Based on breeding value (B.V.) of animals which
is the final selection.
• B.V. estimated on basis of individual’s own
performance.
• B.V. estimated on performance of the relatives
of the individual.
• Relatives either its direct relatives(ancestor and
progeny) or collateral relatives(full sib, half sib).

• The Source of information based on which the
B.V. of individual is estimated are called as basis
of selection/ aids to selection/ criteria of
selection, these are basis of estimating the B.V.
• The B.V. obtained is known as estimated B.V.
(EBV)/ probable B.V. (PBV)/ predicted B.V.
 Aids to selection/ Basis of selection:-
1. Individual selection/ Mass selection
2. Pedigree selection
3. Progeny testing
4. Family selection

Multi trait selection:- The economic value of
animal depends upon several characters known
as overall performance of total breeding value
(net merit) of animal.
• Net economic value of an animal depends upon
several traits.
• Breeding merit of an animal determined on
several traits simultaneously and not by a single
trait, to improve the overall economic value
based on simultaneous selection for several
character, which is known as multi trait selection.

Requirement of multi trait selection:-
1. Economic value of trait measured as amount by
which each unit of variation in it actually raises
or lowers an individual’s value, known as relative
economic value of the trait.
2. Genetic significance of trait in terms of
heritability of trait and genetic correlation
among the traits.
3. Method of selection chosen and no. of traits to
be included in selection criteria.

Methods of selection/ Multi-trait selection:-
(Given by Hazel and Lush in 1942)
1. Tandem selection
2. Independent culling method
3. Total score card/ Selection index method

Multitrait – Multisource selection index:-
• The accuracy of selection for low heritability
traits can be increased by using information from
other sources (individual’s relatives records) for
such traits.
• The selection index for two or more traits is
obtained from individual and its relative’s
records, such selection index is known as
multitrait – multisource index.
• It is constructed on principle of selection index
theory.

Combined selection:- The selection of an
individual on basis of two or more source of
information is called as combined selection or
index selection.
• An index combining information from various
relatives (dam, sib or progeny) with or without
individual’s own record.
• In this multiple regression technique is used.
• These multiple regression coefficients are to be
used as weighting factors.
• Combined selection uses all the information
available about each individual’s B.V. , combined
into an index of merit.

Consequences of selection:-
 Basic effect of selection is to change gene
frequencies and genetic properties of population.
 Population mean is changed as a result of
selection.
 Long term selection should lead to fixation of
alleles with consequent loss/ exhaustion of
genetic variance at selection limit. This will
reduce the phenotypic variance and at last
selection will not result in any response.

 Due to selection there is disagreement between
expected and observed response to selection
called as insufficient response.
 The selection in opposite direction known as
bidirectional selection or two way selection. Due
to this, response is not equal and this inequality
of selection called as asymmetry of response to
selection.
 The selection acts on additive genetic variance
which goes on diminishing in future generations
as a result of continuous selection.
 Due to continuous selection there will be no
genetic variance and hence there will be no
response to further selection.

 When there is no response to selection it is said
that selection limit has reached and such
population is called at selection limit or plateaued
population, this is also known as selection
plateau.
 The phenotypic level of a trait at which the
response ceases is called as selection limit.
 Selection limits reached between 20-30
generations.
 Selection limit can be overcomed by introducing
new genetic material either through mutation or
through outcrossing.

Selection introduction

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