Selection intensity and frequency-based selection are two important concepts in evolutionary biology, particularly in the study of how populations change over time due to various selective pressures. These concepts help explain differences in survival and reproductive success among individuals within a population, which are key to understanding evolutionary dynamics. population. This concept is used to quantify how much a population's genetic makeup is altered by natural selection for or against a specific trait. High Selection Intensity: When a trait significantly increases or decreases an organism's chances of survival and reproduction, selection intensity is said to be high. This typically results in rapid changes in allele frequencies within the population, driving quick evolutionary responses. Low Selection Intensity: Conversely, if the trait has a smaller impact on survival and reproduction, selection intensity is low, resulting in slower evolutionary changes. Selection intensity can be affected by environmental factors, predation pressures, competition for resources, and changes in population size. Frequency-based selection (or frequency-dependent selection) occurs when the fitness of a phenotype depends on its frequency relative to other phenotypes in the population. There are two main types: Positive Frequency-Dependent Selection: Here, the fitness of a phenotype increases as it becomes more common. An example is the selection for common warning colors in poisonous or distasteful animals, where predators more easily recognize and avoid commonly seen patterns. Negative Frequency-Dependent Selection: In this case, the fitness of a phenotype increases as it becomes rarer. This can help maintain genetic diversity within a population. A classic example is seen in host-parasite interactions, where rare genotypes of the host may be less likely to be recognized and targeted by parasites. Importance in Evolutionary Biology Both selection intensity and frequency-based selection are crucial for understanding how populations adapt to their environments and how biodiversity is maintained. Selection intensity helps explain the speed and direction of evolution, while frequency-based selection helps explain the maintenance of diverse phenotypes within populations.

1. Selection Intensity &
Frequency based Selection
GPB 607 (3+0) - Crop Evolution
Dr. K N Ganeshaiah

2. Background
Peppered moth
Biston betularia
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

3. Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Lichens
Not seen under polluted conditions

4. Background
Industrialization
London / Europe
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

5. Biston betularia F.typica
light-coloured species with dark patches, that
help them to camouflage against the lichens on
the barks of the trees
Biston betularia F.carbonaria
sub species of F.typica mutated into dark
coloured moth with light-coloured patches.
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

6. The proportion of the
white (Bb) and black (Bb)
after the predation will
be used to re-populate
next generation
Assumptions
Predation rate
The rate at which the
birds prey on the moth
Biston betularia is
constant for all
generations
Repopulation No Death
There will be no decline
in the population of the
Bb in any generation due
to any other factors like
disease or natural
calamities
10% wn:bn::Wn+1:Bn+
1
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

7. BIRD ‘A’
Results of demonstration / experiment conducted by group A
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

8. Study - 1% Frequency
How selection against white (Bb)
increased black (Bb) proportion
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

9. ● Population curves (frequency)
remain stable until predation of
Black (Bb)
● Predation of Black (Bb) in
13th generation
● No individual of Black (Bb) to
repopulate the 14th generation
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

10. ● Population curves (frequency)
remain stable until predation of
Black (Bb)
● Predation of Black (Bb) in
13th generation
● No individual of Black (Bb) to
repopulate the 14th generation
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

11. Study - 5%Frequency
How selection against white (Bb)
increased black (Bb) proportion
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

12. Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Faint lines represent the normalized polynomial curve
( to the power of 3) for which the equations are
represented below in the graph
1. White (Bb): Observe the sharp
decline in the population curve
after 10th generation
1. Black (Bb): Observe the sharp
increase in the population
curve after 10th generation
We shall discuss this later - collectively
10th
generation

13. Study - 10% Frequency
How selection against white (Bb)
increased black (Bb) proportion
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

14. Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Faint lines represent the normalized polynomial curve
( to the power of 2) for which the equations are
represented below in the graph
1. White (Bb): Observe the sharp
decline in the population curve
after 12th generation
1. Black (Bb): Observe the sharp
increase in the population
curve after 12th generation
We shall discuss this later - collectively
12th
generation

15. Study - 20% Frequency
How selection against white (Bb)
increased black (Bb) proportion
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

16. Faint lines represent the normalized polynomial curve
(to 4th degree) for which the equations are
represented below in the graph
1. White (Bb): Observe the
gradual decline in the
population curve
1. Black (Bb): Observe the
gradual increase in the
population curve
We shall discuss this later - collectively
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

17. Study - 50% Frequency
How selection against white (Bb)
increased black (Bb) proportion
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

18. Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Faint lines represent the normalized linear curve for
which the equations are represented below
There is a sharp increase in
selection against the White (Bb)
The Black (Bb) population is
reaching the saturation and stability
from the 14th generation onwards

19. Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

20. Collective discussion
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Trying to explain
SELECTION INTENSITY and
FREQUENCY based SELECTION

21. Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
12
Dif. Population White (Bb) Black (Bb)
0 G0 =
(initial population)
90 10
12 G12 =
(12th generation)
80 20
5 G17 =
(17th generation)
70 30
3 G20 =
(20th generation)
60 40
10 G31 =
(31st generation)
1 99
17 20 31

22. Difference Population White (Bb) Black (Bb)
0 G0 =
(initial population)
90 10
12 G12 =
(12th generation)
80 20
5 G17 =
(17th generation)
70 30
3 G20 =
(20th generation)
60 40
10 G31 =
(31st generation)
1 99
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
10

23. Difference Population White (Bb) Black (Bb)
0 G0 =
(initial population)
90 10
12 G12 =
(12th generation)
80 20
5 G17 =
(17th generation)
70 30
3 G20 =
(20th generation)
60 40
10 G31 =
(31st generation)
1 99
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
10

24. Difference Population White (Bb) Black (Bb)
0 G0 =
(initial population)
90 10
12 G12 =
(12th generation)
80 20
5 G17 =
(17th generation)
70 30
3 G20 =
(20th generation)
60 40
10 G31 =
(31st generation)
1 99
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
10

25. Difference Population White (Bb) Black (Bb)
0 G0 =
(initial population)
90 10
12 G12 =
(12th generation)
80 20
5 G17 =
(17th generation)
70 30
3 G20 =
(20th generation)
60 40
10 G31 =
(31st generation)
1 99
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
60

26. Difference Population White (Bb) Black (Bb)
0 G0 =
(initial population)
90 10
12 G12 =
(12th generation)
80 20
5 G17 =
(17th generation)
70 30
3 G20 =
(20th generation)
60 40
10 G31 =
(31st generation)
1 99
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
60
10
Initially where it took ≅ 10 generations (G0-G12)to reduce the population by 10 individuals,
later the 10 generations (G20-G31) reduced the population by 60 individuals (6x increase)

27. Which means that -
‘with increasing number of
generations, there is incremental
selection against the White (Bb)
population’
i.e ., favouring Black (Bb) population
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
12 17 20 31
This constitutes
SELECTION INTENSITY

28. This selection intensity is in-turn
dependent on the frequency of the
population of
Whites and Blacks
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
12 17 20 31

29. Population White (Bb) Selection Intensity
G5 =
(5th generation)
86 -
G6 =
(6th generation)
85 1
G7 =
(7th generation)
84 1
G26 =
(26th generation)
34 -
G27 =
(27th generation)
28 6
G28 =
(28th generation)
22 6
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Therefore, when there is lower frequency of white population, there is higher probability that the white
moths are preyed upon - frequency based selection

30. FREQUENCY based
SELECTION
{
The selection intensity and frequency
based selection are interdependent
“The lower
frequency of the
population
increases the
selection intensity”
{
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

31. BIRD ‘B’
Results of demonstration / experiment conducted by group B
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

32. 20% Population frequency
10% Selection intensity
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
40 (White):10 (Black)
I case
White – Chickpea
Black - Horsegram
II case
White – Chickpea
Black - Soybean

33. 40:10 (White: Black)
● Gradual changes till 6th
generation
● After 25:25 changes were
sharp at 6th generation
● By the 15th generation, Black
moths were more than white
moths
● Stability may be achieved
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

34. Selection intensity >>>>> Distinction between moths
I case
- Predominant distinction between moths
- Discrimination also increases
- Selection intensity is more
II case
- Distinction between moths is less
- Discrimination is gradual (The earliest stage of
melanization
- Selection intensity is less
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

35. Study - 1% Frequency
99 (White):1 (Black)
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

36. 99:1 (White : Black)
● Stabilization till the 5th
generation
● Random chance
● Leads to random genetic drift
● Is there any chance of the
occurrence of black moth
again??
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

37. Study - 5% Frequency
95 (White):5 (Black)
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

38. 95:5 (White : Black)
● Gradual changes
● 22nd generation >>> sharp
changes
● Converge at 50:50
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

39. Study - 10% Frequency
90 (White):10 (Black)
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

40. 90:10 (White : Black)
● Sharp decrease and increase
in white moths and black moths
population
● Converge at 50:50
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

41. Study - 50% Frequency
50(White):50 (Black)
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

42. 50:50 (White : Black)
● Initial sharp changes in both
the populations
● Later the changes were gradual
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah

43. ● The rate of reproduction is more
when white moths is 90 and the
rate of elimination of white
moths when it is 10 is lesser due
to the reproduction
● But when both white and black
moths are at the same
population composition (50: 50)
the chance of elimination and
chance of reproduction will be
equal then there will sharp/
exponential change.
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Generations No. of generations White Black
G1 – G8 8 90 - 80 10- 20
G8- G18 10 80-60 20-40
G-18 – G21 3 60-50 40-50
>>>G-21 - 50 50
Generations No. of generations White Black
G1-G4 4 50-40 50-60
G4-G6 3 40-30 60-70
G6-G9 3 30-20 70-80
G9-G12 3 20-10 80-90
G12-G21 10 10-0 90-100
90:10 50:50

44. The black moths
population rate will be
sharp as the number of
black moths in the initial
population increases

45. THANK YOU
Department of Genetics and Plant Breeding
GPB 607 (3+0) - Dr. K N Ganeshaiah
Brahmesh Reddy B R
Aishwarya G
Sinchana K
PhD Scholars
UAS,GKVK, Bangalore