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The Evolution of Population
Figure 23.2
Average beak depth (mm)
Natural selection acts on individuals within a population,
population evolve!!!
10
9
8
0
1978
1976
(similar to the (after
prior 3 years) drought)
• Microevolution is a change in allele
frequencies in a population over generations
• Three mechanisms cause allele frequency
change:
– Natural selection
– Genetic drift
– Gene flow
• Only natural selection causes adaptive
evolution
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Driving Force of Evolution:
Genetic Variation
• Mutation, mutation,
mutation!!
• Variation in
heritable traits is a
prerequisite for
evolution
– Result of variations in
DNA sequence
– Cause variations in
phenotype
– Natural selection acts
on phenotype
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Variation Between Populations
1
2.4
8.11
9.12
3.14
5.18
10.16 13.17
6
7.15
19
XX
1
2.19
3.8
4.16 5.14
9.10 11.12 13.17 15.18
6.7
XX
Cline: a graded change in a trait along a
Ldh-Bb allele frequency
geographic axis as a result of natural selection
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Latitude (ºN)
Maine
Cold (6°C)
Georgia
Warm (21ºC)
Formation of New Alleles
• Mutation can cause a change in an allele
• Only mutations in cells that produce gametes
can be passed to offspring
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Three mechanisms for shuffling alleles
• Point mutation
• Chromosomal mutation
• Sexual reproduction
– Crossing over
– Independent assortment
– fertilization
Rapid Reproduction
• The average rate of eukaryotic mutation is
about one in every 100,000 genes per
generation
• Mutations rates are often lower in prokaryotes
and higher in viruses
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The Hardy-Weinberg equation can be used
to test whether a population is evolving
key terminology:
• Population: a localized
group of individuals capable
of interbreeding and
producing fertile offspring
• Gene pool: all the alleles for
all loci in a population
– A locus is fixed if all
individuals in a
population are
homozygous for the
same allele
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MAP
AREA
CANADA
ALASKA
Figure 23.6
Beaufort Sea
Porcupine
herd range
Porcupine herd
Fortymile
herd range
Fortymile herd
Hardy-Weinberg Equation
Allelic Frequency: the percentage of the frequency of an allele
at a locus in a population
-The frequency of all alleles in a population will add up to 1
-For example, p + q = 1
Practice Problem: consider a population of wildflowers
that is incompletely dominant for color:
320 red flowers (CRCR)
160 pink flowers (CRCW)
20 white flowers (CWCW)
• Calculate the number of copies of each allele:
– CR  (320  2)  160  800
– CW  (20  2)  160  200
• To calculate the frequency of each allele:
– p  freq CR  800 / (800  200)  0.8
– q  freq CW  200 / (800  200)  0.2
• The sum of alleles is always 1
– 0.8  0.2  1
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The Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a
population remain constant from generation to
generation
-In a given population where gametes contribute to
the next generation randomly, allele frequencies will
not change
-Mendelian inheritance preserves genetic variation in
a population Alleles in the population
Gametes produced
Frequencies of alleles
p = frequency of
CR allele
= 0.8
Each egg:
Each sperm:
q = frequency of
CW allele
= 0.2
20%
80%
chance chance
20%
80%
chance chance
• Hardy-Weinberg equilibrium describes the
constant frequency of alleles in such a gene
pool
• Consider, for example, the same population
of 500 wildflowers and 100 alleles where
– p  freq CR  0.8
– q  freq CW  0.2
• The frequency of genotypes can be
calculated
– CRCR  p2  (0.8)2  0.64
– CRCW  2pq  2(0.8)(0.2)  0.32
– CWCW  q2  (0.2)2  0.04
• The frequency of genotypes can be
confirmed using a Punnett square
© 2011 Pearson Education, Inc.
Figure 23.8b
Sperm
CR (80%)
CW (20%)
CR
(80%)
64% (p2)
CRCR
Eggs
CW
16% (pq)
CRCW
4% (q2)
CWCW
16% (qp)
CRCW
(20%)
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
+ 16% C R W
(from C C plants)
= 80% CR = 0.8 = p
4% CW
(from CWCW plants)
W
+ 16% C R W
(from C C plants)
= 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
Remember that the Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a population remain
constant from generation to generation
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• Natural populations can evolve at some loci,
while being in Hardy-Weinberg equilibrium at
other loci
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Mechanisms that alters allele frequencies:
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Figure 23.9-1
Genetic drift
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Figure 23.9-2
Genetic drift
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.9-3
Genetic drift
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
2
plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Founder Effect
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Figure 23.10-1
The Bottleneck Effect
Original
population
Figure 23.10-2
The Bottleneck Effect
Original
population
Bottlenecking
event
Figure 23.10-3
The Bottleneck Effect
Original
population
Bottlenecking
event
Surviving
population
Case Study: Impact of Genetic Drift on the
Greater Prairie Chicken
• Loss of prairie habitat caused a severe
reduction in the population of greater prairie
chickens in Illinois
• The surviving birds had low levels of genetic
variation, and only 50% of their eggs hatched
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Figure 23.11
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken
(a)
Location
Illinois
1930–1960s
1993
Population
size
Percentage
Number
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
Effects of Genetic Drift: A Summary
1. Genetic drift is significant in small populations
2. Genetic drift causes allele frequencies to
change at random
3. Genetic drift can lead to a loss of genetic
variation within populations
4. Genetic drift can cause harmful alleles to
become fixed
© 2011 Pearson Education, Inc.
Gene Flow
60
Survival rate (%)
50
Population in which the
surviving females
eventually bred
Central
Eastern
Central
population
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
40
2 km
30
20
10
0
Females born
in central
population
Females born
in eastern
population
Parus major
• Gene flow can increase the fitness of a population
• Consider, for example, the spread of alleles for
resistance to insecticides
– Insecticides have been used to target mosquitoes
that carry West Nile virus and malaria
– Alleles have evolved in some populations that
confer insecticide resistance to these mosquitoes
– The flow of insecticide resistance alleles into a
population can cause an increase in fitness
© 2011 Pearson Education, Inc.
Natural selection is the only mechanism that
consistently causes adaptive evolution
Evolution by natural
selection involves
both change and
“sorting”
-New genetic variations
arise by chance
-Beneficial alleles are
“sorted” and favored by
natural selection
© 2011 Pearson Education, Inc.
Original
population
Evolved
population
(a) Directional selection
Frequency of
individuals
Directional,
Disruptive,
and
Stabilizing
Selection
Original population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
Figure 23.15
Sexual Selection
Preserving genetic variation in a
population:
• Diploidy maintains genetic variation
– Heterozygotes carry recessive alleles hidden from the
effects of selection
• Balancing selection: natural selection
maintains stable frequencies of two or more
phenotypic forms in a population
– Heterozygote advantage
– Frequency-dependent selection
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Figure 23.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Figure 23.18
“Left-mouthed”
P. microlepis
Frequency of
“left-mouthed” individuals
1.0
“Right-mouthed”
P. microlepis
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Why Natural Selection Cannot Fashion
Perfect Organisms
1.
2.
3.
4.
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the
environment interact
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