Response to selection is the change in the population mean from one generation to the next due to selection. It is represented by R or the expected genetic gain (ΔG). R is influenced by factors like heritability (h2), selection differential (S), and generation interval. Higher h2, S, and shorter generation intervals result in greater response to selection and genetic gain per year. Selection differential depends on proportion selected and herd size, while generation interval varies by species from 1-2 years in pigs and chickens to 8-12 years in horses.

1. Response to Selection
Topic: Response to selection,
Selection Differential,
Selection Intensity
»
Dr Syed Shanaz

2. RESPONSE TO SELECTION
• The change produced by selection is the change of 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 whole of the parental
generation before selection
• Response to selection is also called as the expected genetic gain,
symbolized by G.
R or G = h2 S
where,
h2 = heritability
S = selection differential
R or G/year = h2 S / GI
where,
h2 = heritability
S = selection differential
GI = generation interval

3. FACTORS AFFECTING IT
• Generation interval: It is the time interval between generations and
is defined as the average age of the parents when the offspring is
born. Varies between species and selection procedure
• Management practices for early breeding in females reduces GI and
breeding practices like progeny testing increases the GI
The average generation intervals for different species are:
Species Generation Interval (in years)
Males Females Average
Dairy cattle 3 - 4 4.5 - 6.0 4 - 5
Beef cattle 3 - 4 4.5 - 6.0 4 - 5
Sheep 2 - 3 4.0 - 4.5 3 - 4
Swine 1.5 - 2 1.5 - 2.0 1.5 - 2.0
Chicken 1 - 1.5 1 - 1.5 1.0 - 1.5
Horse 8 - 12 8 - 12 8 - 12

4. • Accuracy for selection is directly related to the heritability of the trait
If heritability is high, the selection on phenotype will permit an
average estimation of breeding value
• If heritability is low, many errors are possible. Increased accuracy in
selection can be obtained by comparing the animals in controlled
environmental conditions
• Corrections may be made for the age of the individual, age of the dam
and sex to remove non-genetic variations
• The techniques may increase the heritability of the trait by reducing
the environmental variation
• Accuracy can be increased by
 using additional measurements for the trait from the same
individual
 using measurements of correlated traits and
 using measurements of relatives.
Accuracy of selection:

5. Selection limit
• When selection is continuous, the response to selection will be
more for a few generations, 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”.
• Main cause for this is fixation of favourable genes.  reduction or
absence of genetic variation
• Further improvement depends on introduction of new genetic
variation
• New genetic variation can be introduced by cross breeding,
mutation and genetic engineering.

6. 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) is high and the Generation Interval is low
• Heritability: Genetic gain depends on the h2 of the character in
generation from which the parents are selected and if the h2 is high,
genetic gain will also be more, because the environmental variation will be
less
• Selection differential: The average superiority of the selected parents is
called as selection differential, symbolized by “S”
• It is 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

7. S = (Ps - P)
where
Ps = mean of the selected parents
P = mean of the population
S = i sp
where
i = intensity of the selection
sp = phenotypic standard deviation
 The selection differential may also be expressed in terms of
phenotypic standard deviation
 standard deviation is the measure of variability

8. Intensity of the selection
• Also called as selection pressure and 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
i = Selection differential / Phenotypic standard
deviation

9. FACTORS AFFECTING 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
• Selection differential will be less in cattle; more in pigs because of
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

10. Species Percentage of animals to be selected
Females Males
Dairy cattle 4 - 5 50 - 60
Beef cattle 4 - 5 40 - 50
Sheep 2 - 4 45 - 55
Swine 1 - 2 10 - 15
Chicken 1 - 2 10 - 15
Horse 2 - 4 40 - 50
Percentage of males and females to be selected for breeding to
maintain a constant herd size for different species