1
1

•
A group of the
• same species living in an area
No two individuals
• are exactly alike
(variations)
More Fit individuals survive & pass on their traits
2
2

•
Different species
• do NOT exchange genes by interbreeding
Different species that interbreed often produce sterile or less viable offspring e.g. Mule
3
3

•
Formation of new
• species
One species may
• split into 2 or more species
A species may
• evolve into a new species
Requires very periods of time long
4
4

5

Combines Darwinian selection and
Mendelian inheritance
Population genetics study of genetic variation within a population
Emphasis on quantitative characters
6
6

•
• Today’s theory on evolution
Recognizes that GENES are responsible
• for the inheritance of characteristics
Recognizes that POPULATIONS , not
• individuals, evolve due to natural selection & genetic drift
Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes
7
7

•
Changes occur in gene pools due to
• mutation, natural selection, genetic drift, etc.
Gene pool changes cause more
•
•
VARIATION in individuals in the population
This process is called
MICROEVOLUTION
Example: Bacteria becoming unaffected by antibiotics (resistant)
8
8

•
•
9
9

Allele Frequencies Define Gene Pools
500 flowering plants
480 red flowers 20 white flowers
320 RR 160 Rr 20 rr
As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females):
320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8
(80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2
(20%) r
10
10

•
•
11
11

•
•
.
12
12

•
13
13

Sources of genetic variation
(Disruption of H-W law)
1. Mutations
- if alleles change from one to another, this will change the frequency of those alleles
2. Genetic recombination
- crossing over; independent assortment
3. Migration
- immigrants can change the frequency of an allele by bringing in new alleles to a population.
- emigrants can change allele frequencies by taking alleles out of the population
14
14

Sources of genetic variation
(Disruption of H-W law)
4. Genetic Drift
small populations can have chance fluctuations in allele frequencies (e.g., fire, storm).
bottleneck; founder effect
5. Natural selection
if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.
15
15

Hardy-Weinberg Equilibrium
The gene pool of a non-evolving population remains constant over multiple generations; i.e., the allele frequency does not change over generations of time.
The Hardy-Weinberg Equation:
1.0 = p 2 + 2pq + q 2 where p 2 = frequency of AA genotype; 2pq = frequency of
Aa plus aA genotype; q 2 = frequency of aa genotype
16
16

17
17

18
18

But we know that evolution does occur within populations.
Evolution within a species/population = microevolution.
Microevolution refers to changes in allele frequencies in a gene pool from generation to generation. Represents a gradual change in a population.
Causes of microevolution:
1) Genetic drift
2) Natural selection (1 & 2 are most important)
3) Gene flow
4) Mutation
19
19

1) Genetic drift
Genetic drift = the alteration of the gene pool of a small population due to chance.
Two factors may cause genetic drift: a) Bottleneck effect may lead to reduced genetic variability following some large disturbance that removes a large portion of the population. The surviving population often does not represent the allele frequency in the original population.
b) Founder effect may lead to reduced variability when a few individuals from a large population colonize an isolated habitat.
20
20

21
21

22
22

*Yes, I realize that this is not really a cheetah.
23
23

2) Natural selection
As previously stated, differential success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance).
The only agent that results in adaptation to environment.
3) Gene flow
-is genetic exchange due to the migration of fertile individuals or gametes between populations.
24
24

25
25

4) Mutation
Mutation is a change in an organism’s DNA and is represented by changing alleles.
Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool.
The original source of variation.
26
26

Genetic Variation, the Substrate for Natural Selection
Genetic (heritable) variation within and between populations: exists both as what we can see (e.g., eye color) and what we cannot see (e.g., blood type).
Not all variation is heritable.
Environment also can alter an individual’s phenotype [e.g., the hydrangea we saw before, and…
…Map butterflies (color changes are due to seasonal difference in hormones)].
27
27

28
28

Variation within populations
Most variations occur as quantitative characters (e.g., height); i.e., variation along a continuum, usually indicating polygenic inheritance.
Few variations are discrete (e.g., red vs. white flower color).
Polymorphism is the existence of two or more forms of a character, in high frequencies, within a population. Applies only to discrete characters.
29
29

Variation between populations
Geographic variations are differences between gene pools due to differences in environmental factors.
Natural selection may contribute to geographic variation.
It often occurs when populations are located in different areas, but may also occur in populations with isolated individuals.
30
30

Geographic variation between isolated populations of house mice.
Normally house mice are
2n = 40. However, chromosomes fused in the mice in the example, so that the diploid number has gone down.
31
31

Cline, a type of geographic variation, is a graded variation in individuals that correspond to gradual changes in the environment.
Example: Body size of North American birds tends to increase with increasing latitude. Can you think of a reason for the birds to evolve differently?
Example: Height variation in yarrow along an altitudinal gradient. Can you think of a reason for the plants to evolve differently?
32
32

33
33

Mutation and sexual recombination generate genetic variation a. New alleles originate only by mutations (heritable only in gametes; many kinds of mutations; mutations in functional gene products most important).
- In stable environments, mutations often result in little or no benefit to an organism, or are often harmful.
- Mutations are more beneficial (rare) in changing environments. (Example: HIV resistance to antiviral drugs.) b. Sexual recombination is the source of most genetic differences between individuals in a population.
- Vast numbers of recombination possibilities result in varying genetic make-up.
34
34

Diploidy and balanced polymorphism preserve variation a. Diploidy often hides genetic variation from selection in the form of recessive alleles.
Dominant alleles “hide” recessive alleles in heterozygotes.
b. Balanced polymorphism is the ability of natural selection to maintain stable frequencies of at least two phenotypes.
Heterozygote advantage is one example of a balanced polymorphism, where the heterozygote has greater survival and reproductive success than either homozygote
(Example: Sickle cell anemia where heterozygotes are resistant to malaria).
35
35

36
36

Frequency-dependent selection = survival of one phenotype declines if that form becomes too common.
(Example: Parasite-Host relationship. Co-evolution occurs, so that if the host becomes resistant, the parasite changes to infect the new host. Over the time, the resistant phenotype declines and a new resistant phenotype emerges.)
37
37

38
38

39
39

Neutral variation is genetic variation that results in no competitive advantage to any individual.
- Example: human fingerprints.
40
40

A Closer Look: Natural Selection as the Mechanism of
Adaptive Evolution
Evolutionary fitness - Not direct competition, but instead the difference in reproductive success that is due to many variables.
Natural Selection can be defined in two ways: a. Darwinian fitness- Contribution of an individual to the gene pool, relative to the contributions of other individuals.
And,
41
41

b. Relative fitness
- Contribution of a genotype to the next generation, compared to the contributions of alternative genotypes for the same locus.
- Survival doesn’t necessarily increase relative fitness; relative fitness is zero (0) for a sterile plant or animal.
Three ways (modes of selection) in which natural selection can affect the contribution that a genotype makes to the next generation. a. Directional selection favors individuals at one end of the phenotypic range. Most common during times of environmental change or when moving to new habitats.
42
42

Directional selection
43
43

Diversifying selection favors extreme over intermediate phenotypes.
- Occurs when environmental change favors an extreme phenotype.
Stabilizing selection favors intermediate over extreme phenotypes.
- Reduces variation and maintains the current average.
- Example = human birth weights.
44
44

Diversifying selection
45
45

46
46

Natural selection maintains sexual reproduction
-Sex generates genetic variation during meiosis and fertilization.
-Generation-to-generation variation may be of greatest importance to the continuation of sexual reproduction.
-Disadvantages to using sexual reproduction: Asexual reproduction produces many more offspring.
-The variation produced during meiosis greatly outweighs this disadvantage, so sexual reproduction is here to stay.
47
47

All asexual individuals are female (blue). With sex, offspring = half female/half male. Because males don’t reproduce, the overall output is lower for sexual reproduction.
48
48

Sexual selection leads to differences between sexes a. Sexual dimorphism is the difference in appearance between males and females of a species.
-Intrasexual selection is the direct competition between members of the same sex for mates of the opposite sex.
-This gives rise to males most often having secondary sexual equipment such as antlers that are used in competing for females.
-In intersexual selection (mate choice), one sex is choosy when selecting a mate of the opposite sex.
-This gives rise to often amazingly sophisticated secondary sexual characteristics; e.g., peacock feathers.
49
49

50
50

51
51

Natural selection does not produce perfect organisms a. Evolution is limited by historical constraints (e.g., humans have back problems because our ancestors were 4-legged). b. Adaptations are compromises. (Humans are athletic due to flexible limbs, which often dislocate or suffer torn ligaments.) c. Not all evolution is adaptive. Chance probably plays a huge role in evolution and not all changes are for the best.
d. Selection edits existing variations. New alleles cannot arise as needed, but most develop from what already is present.
52
52

Chapter 21
53

Natural selection and evolutionary change
Some individuals in a population possess certain inherited characteristics that play a role in producing more surviving offspring than individuals without those characteristics.
The population gradually includes more individuals with advantageous characteristics.
54
54

Measuring levels of genetic variation blood groups – 30 blood grp genes
Enzymes – 5% heterozygous
Enzyme polymorphism
A locus with more variation than can be explained by mutation is termed polymorphic .
Natural populations tend to have more polymorphic loci than can be accounted for by mutation.
15% Drosophila
5-8% in vertebrates
55
55

Population genetics - study of properties of genes in populations blending inheritance phenotypically intermediate (phenotypic inheritance) was widely accepted new genetic variants would quickly be diluted
56
56

Hardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generation
Sexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions.
57
57

Population of cats n=100
16 white and 84 black bb = white
B_ = black
Can we figure out the allelic frequencies of individuals BB and Bb?
58
58

Necessary assumptions
Allelic frequencies would remain constant if… population size is very large random mating no mutation no gene input from external sources no selection occurring
59
59

Calculate genotype frequencies with a binomial expansion
(p+q) 2 = p 2 + 2pq + q 2 p2 = individuals homozygous for first allele
2pq = individuals heterozygous for alleles q2 = individuals homozygous for second allele
60
60

p 2 + 2pq + q 2 and p+q = 1 (always two alleles)
16 cats white = 16bb then (q 2 = 0.16)
This we know we can see and count!!!!!
If p + q = 1 then we can calculate p from q 2
Q = square root of q 2 = q √.16 q=0.4
p + q = 1 then p = .6 (.6 +.4 = 1)
P 2 = .36
All we need now are those that are heterozygous (2pq)
(2 x .6 x .4)=0.48
.36 + .48 + .16
61
61

62
62

Mutation
Mutation rates are generally so low they have little effect on
Hardy-Weinberg proportions of common alleles.
ultimate source of genetic variation
Gene flow movement of alleles from one population to another tend to homogenize allele frequencies
63
63

Nonrandom mating assortative mating - phenotypically similar individuals mate
Causes frequencies of particular genotypes to differ from those predicted by Hardy-Weinberg.
64
64

Genetic drift – statistical accidents.
Frequencies of particular alleles may change by chance alone.
important in small populations founder effect - few individuals found new population (small allelic pool) bottleneck effect - drastic reduction in population, and gene pool size
65
65

66
66

Selection – Only agent that produces adaptive evolutionary change artificial - breeders exert selection natural - nature exerts selection variation must exist among individuals variation must result in differences in numbers of viable offspring produced variation must be genetically inherited natural selection is a process, and evolution is an outcome
67
67

Selection pressures: avoiding predators matching climatic condition pesticide resistance
68
68

Fitness is defined by evolutionary biologists as the number of surviving offspring left in the next generation.
relative measure
Selection favors phenotypes with the greatest fitness.
69
69

Levels of variation retained in a population may be determined by the relative strength of different evolutionary processes.
Gene flow versus natural selection
Gene flow can be either a constructive or a constraining force.
Allelic frequencies reflect a balance between gene flow and natural selection.
70
70

Frequency-dependent selection
Phenotype fitness depends on its frequency within the population.
Negative frequency-dependent selection favors rare phenotypes.
Positive frequency-dependent selection eliminates variation.
Oscillating selection
Selection favors different phenotypes at different times.
71
71

Heterozygote advantage will favor heterozygotes, and maintain both alleles instead of removing less successful alleles from a population.
Sickle cell anemia
Homozygotes exhibit severe anemia, have abnormal blood cells, and usually die before reproductive age.
Heterozygotes are less susceptible to malaria.
72
72

73
73

Disruptive selection
Selection eliminates intermediate types.
Directional selection
Selection eliminates one extreme from a phenotypic array.
Stabilizing selection
Selection acts to eliminate both extremes from an array of phenotypes.
74
74

75
75

Guppies are found in small northeastern streams in
South America and in nearby mountainous streams in
Trinidad.
Due to dispersal barriers, guppies can be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.
76
76

77
77

High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.
Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.
In the absence of predators, larger, more colorful fish may produce more offspring.
78
78

79
79

Genes have multiple effects pleiotropy
Evolution requires genetic variation
Intense selection may remove variation from a population at a rate greater than mutation can replenish.
thoroughbred horses
Gene interactions affect allelic fitness epistatic interactions
80
80

81
81

Population genetics
• alleles
• genotypes group of individuals of the same species that can interbreed
Patterns of genetic variation in populations
Changes in genetic structure through time
82
82

Describing genetic structure
rr = white
Rr = pink
RR = red
83
83

Describing genetic structure
200 white
500 pink genotype frequencies:
200/1000 = 0.2 rr
300 red
500/1000 = 0.5 Rr
300/1000 = 0.3 RR total = 1000 flowers
84
84

Describing genetic structure
200 rr = 400 r
500 Rr = 500 r
= 500 R
300 RR = 600 R allele frequencies:
900/2000 = 0.45 r
1100/2000 = 0.55 R total = 2000 alleles
85
85

for a population with genotypes:
100 GG
160 Gg
140 gg
Genotype frequencies
Phenotype frequencies
Allele frequencies
86
86

for a population with genotypes:
100 GG
160 Gg
140 gg
Genotype frequencies
260
100/400 = 0.25 GG
160/400 = 0.40 Gg
140/400 = 0.35 gg
0.65
Phenotype frequencies
260/400 = 0.65 green
140/400 = 0.35 brown
Allele frequencies
360/800 = 0.45 G
440/800 = 0.55 g
87
87

another way to calculate allele frequencies:
100 GG
160 Gg
Genotype frequencies
0.25 GG
0.40 Gg
0.35 gg g g
G
G
0.25
0.40/2 = 0.20
0.40/2 = 0.20
0.35
140 gg
Allele frequencies
360/800 = 0.45 G
440/800 = 0.55 g
OR [0.25 + (0.40)/2] = 0.45
[0.35 + (0.40)/2] = 0.65
88
88

Population genetics – Outline


- genotype frequencies
- allele frequencies
89
89

Genetic variation in space and time
Frequency of Mdh-1 alleles in snail colonies in two city blocks
90
90

Genetic variation in space and time
Changes in frequency of allele F at the Lap locus in prairie vole populations over 20 generations
91
91

Genetic variation in space and time
Why is genetic variation important?
• adaptation to environmental change
- conservation
•divergence of populations
- biodiversity
92
92

Why is genetic variation important?
variation no variation global warming survival
EXTINCTION!!
93
93

Why is genetic variation important?
variation no variation
94
94

Why is genetic variation important?
divergence variation no variation
NO DIVERGENCE!!
95
95

Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
96
96

Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
97
97

Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant mutation!
98
98

Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
99
99

Natural selection
Resistance to antibacterial soap
Generation 1: 1.00 not resistant
0.00 resistant
Generation 2: 0.96 not resistant
0.04 resistant
Generation 3: 0.76 not resistant
0.24 resistant
Generation 4: 0.12 not resistant
0.88 resistant
100
100

Natural selection can cause populations to diverge divergence
101
101

Selection on sickle-cell allele aa – abnormal ß hemoglobin sickle-cell anemia very low fitness
AA – normal ß hemoglobin vulnerable to malaria intermed.
fitness
Aa – both ß hemoglobins resistant to malaria high fitness
Selection favors heterozygotes ( Aa ).
Both alleles maintained in population ( a at low level).
102
102

How does genetic structure change?
• mutation
• migration
• genetic drift genetic change by chance alone
• natural selection
• sampling error
• misrepresentation
• small populations
• non-random mating
103
103

Genetic drift
Before:
8 RR
8 rr
0.50 R
0.50 r
After:
2 RR
6 rr
0.25 R
0.75 r
104
104

How does genetic structure change?
• mutation
• migration
• natural selection
• genetic drift
• non-random mating cause changes in allele frequencies
105
105

How does genetic structure change?
mating combines alleles into genotypes
• non-random mating
non-random allele combinations
106
106

A
A A
A a
A
A
A
0.8
A
0.8
a
0.2
AA
0.8 x 0.8
Aa
0.8 x 0.2
aa
AA allele frequencies:
A = 0.8
A = 0.2
a
0.2
aA
0.2 x 0.8
aa
0.2 x 0.2
genotype frequencies:
AA = 0.8 x 0.8 = 0.64
Aa = 2(0.8 x0.2) = 0.32
aa = 0.2 x 0.2 = 0.04
107
107

B__ = black bb = red
Herd of 200 cows:
100 BB, 50 Bb and 50 bb
108
108

No. B alleles = 2(100) + 1(50) = 250
No. b alleles = 2(50) + 1(50) = 150
Total No. = 400
Allele frequencies: f(B) = 250/400 = .625 f(b) = 150/400 = .375
109
109

genotypic frequencies f(BB) = 100/200 = .5 f(Bb) = 50/200 = .25 f(bb) = 50/200 = .25 phenotypic frequencies f(black) = 150/200 = .75
f(red) = 50/200 = .25
110
110

Previous example: counted alleles to compute frequencies. Can also compute allele frequency from genotypic frequency. f(A) = f(AA) + 1/2 f(Aa) f(a) = f(aa) + 1/2 f(Aa)
111
111

Previous example, we had f(BB) = .50, f(Bb) = .25, f(bb) = .25. allele frequencies can be computed as: f(B) = f(BB) + 1/2 f(Bb)
= .50 + 1/2 (.25) = .625
f(b) = f(bb) + 1/2 f(Bb)
= .25 + 1/2 (.25) = .375
112
112

B_ = brown bb = platinum (blue-gray)
Group of females (.5 BB, .4 Bb, .1 bb) bred to heterozygous males (0 BB, 1.0 Bb, 0 bb).
113
113

Allele frequencies among the females?
f(B) = .5 + 1/2(.4) = .7
f(b) = .1 + 1/2(.4) = .3
Allele frequencies among the males?
f(B) = 0 + 1/2(1) = .5
f(b) = 0 + 1/2(1) = .5
114
114

Expected genotypic, phenotypic and allele frequencies in the offspring?
sires
.5 B .5 b
________________ dams .7 B .35 BB .35 Bb
.3 b .15 Bb .15 bb
115
115

Expected frequencies in offspring
Genotypic
.35 BB
.50 Bb
.15 bb
Phenotypic
.85 brown
.15 platinum
116
116

f(B) = f(BB) + .5 f(Bb)
= .35 + .5(.50) = .6
f(b) = f(bb) + .5 f(Bb)
= .15 + .5(.50) = .4
117
117

Also note: allele freq. of offspring = average of sire and dam f(B) = 1/2 (.5 + .7) = .6
f(b) = 1/2 (.5 + .3) = .4
118
118

Population gene and genotypic frequencies don’t change over generations if is at or near equilibrium.
Population in equilibrium means that the populations isn’t under evolutionary forces
(Assumptions for Equilibrium*)
119
119

large population (no random drift)
Random mating no selection no migration (closed population) no mutation
120
120

Under these assumptions populations remains stable over generations.
It means: If frequency of allele A in a population is =.5, the sires and cows will generate gametes with frequency =.5 and the frequency of allele
A on next generation will be =.5!!!!!
121
121

Therefore:
It can be used to estimate frequencies when the genotypic frequencies are unknown.
Predict frequencies on the next generation.
122
122

If predicted frequencies differ from observed frequencies Population is not under Hardy-
Weinberg Equilibrium.
Therefore the population is under selection, migration, mutation or genetic drift.
Or a particular locus is been affected by the forces mentioned above.
123
123

Hardy-Weinberg Theorem
(2 alleles at 1 locus)
Allele freq.
f(A) = p f(a) = q p + q = 1
Sum of all alleles = 100%
Genotypic freq.
f(AA) = p 2 Dominant homozygous f(Aa) = 2 pq Heterozgous f(aa) = q 2 Recessive homozygous p 2 + 2 pq + q 2 = 1
Sum of all genotypes = 100%
124
124

Hardy-Weinberg Theorem
Allele freq.
f(A) = p f(a) = q p + q = 1
Sum of all alleles = 100%
Genotypic freq.
f(AA) = p 2 Dominant homozygous f(Aa) = 2 pq Heterozgous f(aa) = q 2 Recessive homozygous p 2 + 2 pq + q 2 = 1
Sum of all genotypes = 100%
Gametes A (p) a(q)
A(p) a(q)
AA
(pp) aA
(qp)
Aa
(pq) aa
(qq)
AA = p*p = p 2
Aa = pq + qp = 2pq
Aa = q*q = q 2
125
125

1000-head sheep flock. No selection for color. Closed to outside breeding.
910 white (B_)
90 black (bb)
126
126

Start with known: f(black) = f(bb) = .09 =q 2 q
 q
2 
.
09

Then, p = 1 – q = .7 = f(B)
.
3
 f ( b ) f(BB) = p 2 = .49
f(Bb) = 2pq = .42
f(bb) = q 2 = .09
127
127

Allele freq.
f(B) = p = .7 (est.) f(b) = q = .3 (est.)
Phenotypic freq.
f(white) = .91 (actual) f(black) = .09 (actual)
Genotypic freq.
f(BB) = p 2 = .49 (est.) f(Bb) = 2pq = .42 (est.) f(bb) = q 2 = .09 (actual)
128
128

Group of 2000 (1920 brown, 80 platinum) in equilibrium. We know f(bb) = 80/2000 = .04 = q 2 f(b) =  (q 2 ) =  .04 = .2
f(B) = p = 1- q = .8 f(BB) = p 2 = .64
f(Bb) = 2pq = .32
129
129

1. Mutation
2. Migration
3. Selection
4. Random (genetic) drift
Selection and migration most important for livestock breeders.
130
130

Change in base DNA sequence.
Source of new alleles.
Important over long time-frame.
Usually undesirable.
131
131

Introduction of allele(s) into a population from an outside source.
Classic example: introduction of animals into an isolated population.
others: new herd sire.
opening herd books.
“under-the-counter” addition to a breed.
132
132

Change in allele freq. due to migration
 p mig
= m(P m
-P o
) where
P m
P o
= allele freq. in migrants
= allele freq. in original population m = proportion of migrants in mixed pop.
133
133

100 Red Angus (all bb, p
0 purchase 100 Bb (p m
 p = m(p m
- p o
= .5)
= 0)
) = .5(.5 - 0) = .25
new p = p
0 new q = q
0
+  p = 0 + .25 = .25 = f(B)
+  q = 1 - .25 = .75 = f(b)
134
134

Changes in allele frequency due to random segregation.
Aa  .5 A, .5 a gametes
Important only in very small pop.
135
135

Some individuals leave more offspring than others.
Primary tool to improve genetics of livestock.
Does not create new alleles. Does alter freq.
Primary effect  change allele frequency of desirable alleles.
136
136

Example: horned/polled cattle
100-head herd (70 HH, 20 Hh, and 10 hh).
Genotypic freq.: f(HH) = .7, f(Hh) = .2, f(hh) = .1
Allele freq.: f(H) = .8, f(h) = .2
Suppose we cull all horned cows. Calculate allele and genotypic frequencies after culling?
137
137

f(HH) = .7/.9 = .778
f(Hh) = .2/.9 = .222
f(hh) = 0 f(H) = .778 + .5(.222) = .889
f(h) = 0 + .5(.222) = .111
 p = .889 - .8 = .089
138
138

Cow herd with 20 HH, 20 Hh, and 60 hh
Initial genotypic freq.: .2HH, .2Hh, .6hh
Initial allele frequencies: f(H) = .2 + 1/2(.2) = .3
f(h) = .6 + 1/2(.2) = .7
Again, cull all horned cows.
139
139

Genotypic freq (HH) = .2/.4 = .5
freq (Hh) = .2/.4 = .5
freq (hh) = 0
 p = .75 - .3 = .45
Allele freq.
f(H) = .5 + 1/2(.5) = .75
f(h) = 0 + 1/2(.5) = .25
Note: more change can be made when the initial frequency of desirable gene is low.
140
140

initial genotypic freq. .2 HH, .2 Hh, .6 hh.
Initial allele freq. f(H) = .3 and f(h) = .7
Cull half of the horned cows.
141
141

Genotypic f
(HH) = .2/.7 = .2857
f (Hh) = .2/.7 = .2857
f (hh) = .3/.7 = .4286
 p = .429 - .3 = .129
Allele freq.
f(H) = .2857 + 1/2(.2857) = .429
f(h) = .4286 + 1/2(.2857) = .571
Note: the higher proportion that can be culled, the more you can change allele freq.
142
142

Selection Against Recessive Allele
.5
.7
.9
Allele Freq.
A a
.1
.3
.9
.7
.5
.3
.1
Genotypic Freq.
AA Aa
.01
.09
.18
.42
.25
.49
.81
.50
.42
.18
aa
.81
.49
.25
.09
.01
143
143

Factors affecting response to selection
1. Selection intensity
2. Degree of dominance
(dominance slows progress)
Initial allele frequency (for a one locus)
Genetic Variability (Bell Curve)
144
144