About selection

1. Selection
Nirvana Sharma
M. sc. Ag Genetics And Plant Breeding
PLB-10M-2023
Agriculture And Forestry University

2. Contents
• Selection
• Selection for crop improvement
• Fitness
• Response to Selection
• Gametic And Zygotic Selection
• Limit to selection

3. Introduction
• Selection is the process of choosing plants with desirable
characteristics.
• The individual plants or group of plants are sorted out from
mixed population thus eliminating undesirable plants.
• In any crop improvement program, creation of genetic
variation followed by selection are fundamental steps.
• The effectiveness of selection depends upon the degree of
which phenotype reflects the genotype.

4. Importance of selection in crop improvement
• In the crop improvement program selection seems to be important step.
The main purpose is to better the quality and yield of upcoming
generation.
• Selection allows breeders to identify individuals that are genetically
superior for specific traits and to use them in breeding programs.
• This helps in producing plants that have a better resistance to pests and
diseases.
• By selecting plants with superior traits, breeders can increase the
likelihood that the resulting offspring will also posses these desirable traits.
• Helps to speed up breeding process by allowing breeders to quickly
identify superior plants resulting in improved crop species.

5. Fitness
• The capacity of a genotype to be propagated into future
generations. It is relative measure of individual reproductive
success.
Also known as adaptive value or selective value.
• An individual’s fitness is the organisms ability to pass its genetic
material to its offspring.
• Example ;
If genotype A individual produce 100 offspring that reach maturity
while genotype B produce only 90 offspring in the same environment.
The reproductive success of B is reduced by a fraction 0.1 thus
genotype A possess superior adaptive value relative to B. This adaptive
value is W i.e. Fitness.

6. Selection Coefficient
Population genetical parameter which denotes the measure of differences
in the relative fitness of different alleles and genotypes.
It is important for selection as fitness differences determine the change in
genotype frequencies.
• It is denoted by s.
• The contribution of favored genotype, measured in terms of fitness is
taken to be 1, the fitness of unfavored genotype , which is reduced by s,
is measured as 1-s.
Mathematically,
W = 1-s Here, W= fitness and s denotes selection coefficient.
s = 1-w

7. Response to selection
• The change of population mean is the response to selection.
• Response to selection is the difference of mean phenotypic
value between the offspring of the selected parents and whole
of the parental generation before selection.
• The change in performance of progeny generation due to
artificial selection is known as response to selection.
• It is also called as the expected genetic gain, symbolized by R
or G.

8. R or G =h2s Where, h2= heritability
S= selection differential
R or G = h2iσp
Where, i= intensity of the selection
σp= phenotypic standard deviation
R or G / year = h2S/ GI Where, GI = generation interval

9. Factors governing response to selection :
• Heritability: If h2 is high the genetic gain will also be more and
vice versa because environment variation will be less.
• Generation Interval: Time interval between generations ,
average age of parents when the offspring is born. Annual
response will increase with lower GI. We can minimize
generation interval by using better management practices.
• Intensity of Selection (i) : When the population is small, the
selection is more intense. The R will be more when population
will be small. Response will be higher when we select lower
proportion from a population.

10. Selection Differential
• Selection differential (S) : The superioty of the selected parents
over population before selection.
• Difference between mean phenotypic value of the individuals
selected as parents and the mean phenotypic value of all the
individuals in the parental generation before selection.
• Mathematically,
S= (Ps-P) Where,
Ps= mean of selected parents
P= mean of the population

11. Xs
S
Xp
Xo
R
Xp
Xo
R
S
Xo Xs
Xp-Xo = B ( Xs-Xo)
Xo

12. Selection Limit
• When the selection is continuous, the response to selection
will be more for a few generations, then it slows down and
finally stops.
• The point where further change is no longer feasible.
• When the response to selection has stopped, the population is
said to be at “plateau” or “selection limit”.
• Main cause for this fixation of favorable genes is reduction or
absence of genetic variation.
• Further improvement depends on the introduction of new
genetic variation.

13. Gametic Selection
• Gametic selection is the preferential selection of the gametes
(germ cells- sperm or egg cells) that can occur during
fertilization.
• Those gametes selected possess alleles influencing survival,
mating capacity or fertilization success.
• gametes influencing best character or showing some
advantages for particular individual will be selected.
• With no selection in gametes, we may assume gametes with
dominant and recessive alelles would be equally efficient.

14. Zygotic Selection
• Unlike gametic selection, the degree of dominance also
becomes important.
• Genotypes selected that will have capability to add more
offspring to next generation.
• Those genotypes selected will influence the mating process,
successful in mating, i.e. mating success.
• Zygote with good mating performance will be selected.
• Here, genotype selected will have best character.

15. Selection at gametic stage
• Gene A (alleles A and a), fitness of gene A= 1 and a=1-s. Frequency of A=p and a=q. If a gene is
lethal, i.e., s=1, this gene will be eliminated in the next generation.
A a
Initial frequency p0 q0
Fitness 1 1-s
Frequency after
selection
p0 q0(1-s)
Total frequency: 1-sq0
Frequency of gene, a after one generation of selection will be :
q1 = (1-s)q0/ (1-sq0)
Change in gene frequency( q) = q1-q0
= -sq(1-q)/(1-sq)

16. Selection at zygotic stage
• In most of the higher plants and animals, selection operates at
zygotic stage. Gene A ( alleles A and a), among other things,
effectiveness of selection at zygotic stage depends upon
degree of dominance. A>a.

17. Situation Ist
Selection operated against aa, suppose gene a is lethal, i.e., s=1
AA Aa aa
Initial Frequency p2
0 2p0q0 q2
0
Fitness 1 1 S = 1
Frequency after selection p2
0 2p0q0 0
Total frequency: p0
2 (1+q0)
Frequency of a gene after one generation of selection(q1) will be ,
q1 = q0 / 1+ q0
Change of gene frequency ( q) resulting from one generation of selection will be , ( q) =q1-qo
=q2/1+q.

18. Situation 2nd
when s not equal to 1.
AA Aa aa
Initial Frequency p2
o 2poqo q2
o
Fitness 1 1 1-s
Frequency after
selection
p2
o 2poqo q2
o(1-s)
Total frequency : 1-sq2
o
Frequency of gene a after one generation of selection (q1) will be,
q1= (pq+ q2(1-s))/ 1-sq2
Change of gene frequency (-q) resulting from one generation of selection will be, ( q) = q1-qo
= -sq2 (1- q)/ 1- sq2

19. Situation 3rd
dominant allele not favored but recessive allele favored
• In this case selection will be more effective. If dominant gene is
completely lethal, its frequency will be reduced to zero after one
generation of selection.
• When selection operates against A genotype, change in frequency of
allele A will be,
( p) = (-sp(1-p)2)/ 1- sp(2-p)
( q) = sp2( 1-q)/ (1-s(1-p2))

20. Situation 4th
• Selection against gene a and there is no dominance.
• Fitness of AA, Aa and aa will be 1, 1-0.5s and 1-s resp.
• Change in the frequency of gene a, i.e. ( q) resulting from one
generation of selection against aa will be,
( q) = -0.5 sq (1-q)/1-sq

21. Situation 5th
If heterozygote has superior fitness.
• If heterozygote has superior fitness.
• Fitness of AA, Aa and aa will be 1-s1, 1 and 1-s2.
• Frequency of the gene A at equilibrium = s2/(s1+s2)
• Expected frequency of the gene a at equilibrium is q= s1/(s1+s2)

22. Indirect Selection
• If breeders find to select a character say y, instead of x ( in
which improvement is desired) which is more convenient or
economical, known as indirect selection.
• The character selected is known as secondary character.
• Mostly useful in perennial crops; fruit trees as they start to
produce fruits only after 4 to 5 years so as an alternative plant
breeder start to use vigor and trunk diameter as a selection
criteria.