Selection, type of selection, patterns of selection, and their effect on population

1. SELECTION, TYPE OF SELECTION, PATTERNS OF
SELECTION AND THEIR EFFECT ON POPULATION
Submitted to :
Dr. P. C. Patel
Assistant Professor
Dept. Of Genetics And Plant Breeding
C. P. College of Agriculture
SDAU
Submitted by :
Kalpeshkumar M. Damor
Reg.no.04-AGRMA-01983-2019
C. P. College of Agriculture
SDAU

2. SELECTION :
 The selection is the differential propagation of
genetic material to the next generation.
 Process of differential survival and reproduction of
individuals is known as selection.

3. TYPE OF SELECTION
1. Natural selection
2. Artificial selection

4. 1. NATURAL SELECTION
 The first type of selection is the natural selection whose
principle is the "survival of fittest" in a given
environment.
 The individual which are not fit/adapted to survive and
to produce their progeny died of their genetic death
without any offspring. This is known as natural selection.
 The survival of the fittest is determined measured by the
survivability and the reproductive success of the
individual.
 There fore, survivability and the reproductive status are
known as the fitness or adaptive characters.

5.  The natural selection is based on the theory that certain
genotypes in a population have advantage over others in
survival and reproduction.
 Natural selection is a term used to indicate that some
genotypes leave more offspring than others.
 The natural selection (has no purpose except that it) is a
consequence of the differences between individuals for
their capacity to produce progeny.
 Large population have all the time a great store of
potential genetic variability know as the plasticity of a
population.

6.  Stages of life cycle of natural selection
 Gametic stage:
• Segregation of homologous chromosomes at meiosis for gamete formation
(Segregation distortion).
• Gametic competition in the way of differential penetration or elimination of
male gamete and compatibility of ova and sperm before fertilization.
 Zygotic stage:
• Embryonic growth and development .
• Maturity rate (Early or late puberty).
• Fecundity, age-specific fertility, gamete production, care and protection of
young.
• Mating ability
• Total progeny produced .
• Survival at different ages viz embryonic stage, birth to maturity and
adulthood.

7.  The factors affect survival are
• Resistance to disease, outbreak of disease.
• Protection from danger wild animals (predation) and
natural calamities.
• Accident, hunger.
• Competitive ability to use resources.
• Differential ability to cope with climate and other
environmental conditions.
• Differential survival of certain genotypes.
• Lethal genes.

8. 2. ARTIFICIAL SELECTION
 The second type of selection is man made which depends
upon the will or choice of the breeder to allow the animals to
produce the next generation. This is called as artificial
selection.
 It is under the control of breeder who decides that to which
animals he wants to retain and to allow to become parents of
next generation.
 The choice the breeder is objective specific and hence the
artificial selection bas purpose.
 Some genotypes are either more attractive, productive or more
efficient functionally and hence preferred by the breeder.
 The artificial selection is under the control of certain rules to
govern the probability that an individual reproduces .

9. PATTERNS OF SELECTION
 Three pattern of selection
1. Directional selection
2. Stabilizing selection
3. Disruptive selection

10. 1. DIRECTIONAL SELECTION
 This is also called as one way upward selection or linear
selection.
 It is done to change the mean by selecting the individuals
with extreme phenotypic value (greatest or least) and not
allowing to reproduce the individuals whose phenotypic
value is intermediate for the trait under selection.
 This results a change in the mean value of the offspring
generation in the direction of the favored phenotype
compared with parent population.

11.  The individual selected are genetically superior with
higher frequencies of favorable alleles for that trait and
thus these individuals contribute these favorable alleles
to the next generation.
 On the other hand, the individuals which have less
favorable alleles arc not allowed to contribute (the less
favorable alleles) to the next generation.
 Thus, the frequency of alleles in the next generation is
changed, increasing the frequencies of more favorable
alleles and it results in an improvement in the average
phenotype of next generation.

12. BASIS OF DIRECTIONAL SELECTION
 The individuals to become parents of the next generation
are selected on the basis of their breeding value for the
trail under selection.
 The breeding value is estimated on the basis of
individual's own performance or on the basis of the
information on the performance of their relatives.
1. Individual selection
2. Selection based on relatives performance

13. I. INDIVIDUAL SELECTION
 When the individual are selected based on their higher rank
for phenotypic/breeding values is called the "individual
selection" or "mass selection".
 The variant of this system of selection is the “truncation
selection”.
 when all the individuals above a specific value are selected
and individuals below the value are rejected.
 The individuals may he selected for improvement either in
one trait or for more traits simultaneously.
 The selection for more than one trait simultaneously is known
as multi trait selection or methods of selection.
 Therefore, the methods of selection come under the individual
selection.

14. II. SELECTION BASED ON RELATIVES
PERFORMANCE
 The other method of ranking the individuals to be selected
is to rank them on the basis of the performance of their
relatives.
 The relatives on the basis of whose performance the
individuals are selected may be either parents (called as
pedigree selection) progeny (called as progeny selection or
progeny testing) or their collateral relatives (called as
family selection). The families may he of full sibs or of half
sibs.
 There are three forms of family selection
1. Sib selection
2. Family selection
3. Within family selection

15. 1. Sib selection:- The sib selection is when the individuals
selected are not measured to estimate family mean and
selection of individuals is based on the values of their
sibs.
2. Family selection:- The selected individual are also
measured and included in the family mean. However,
the selection is based on family mean which implies
deviation of individuals' family mean from population
mean (pf)
3. Within family selection:- In which is a procedure when
the better individual within each family are selected
based on the deviation of each individual from the
mean value of their family (pw) and the individuals
with the maximum deviation are selected.

16. 2. DISRUPTIVE SELECTION
 This is also called as two way selection or bidirectional
selection or diversifying or centrifugal.
 The individuals are selected with extreme phenotypes
from both sides (maximum and minimum) and those
having their phenotypes near the average the population
there are selected for reproduce.
 selection favor's two diverse types at a time and
eliminating the individuals with intermediate phenotypic
values.

17.  Thus individuals of intermediate phenotypes
(heterozygotes) are at a selective disadvantage relative to
those of both extremes.
 Disruptive selection is the selection for the tails of a bell-
shaped statistical distribution and once it is started in the
population it then becomes directional selection of
opposite side in the two resulting populations.
 This type of selection result in little change in the
phenotypic/breeding values or gene frequencies in the
next generation.

18. 3. STABILIZING SELECTION
 This is also called as centripetal selection on balanced
selection.
 This is third type of selection when the individuals with
intermediate phenotypes (having their phenotype values
near the population mean) are selected the parents of
progeny generation and those having extreme
phenotypes on either side are not allowed to reproduce.
 This does not change the mean of the progeny generation
and to some extent reduces the variance.

19.  The phenotype breeding value and the gene frequency of
progeny generation under this type of selection are near
the average because the parents selected were also near
the average There is no decrease in phenotypic variance
when the selection of replacements is entirely random,
 The effect of birth weight on infant mortality is an
example of stabilizing selection.
 This selection operates contrary to directional selection.
 This selection favours heterozygotes.