Download (a) Directional selection

CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
21
The Evolution
of Populations
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
© 2014 Pearson Education, Inc.
Overview: The Smallest Unit of Evolution
 Natural selection acts on individuals, but only
populations evolve
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Figure 21.1
© 2014 Pearson Education, Inc.
Average beak depth (mm)
Figure 21.2
10
9
8
0
1976
1978
(similar to the (after
prior 3 years) drought)
© 2014 Pearson Education, Inc.
 Microevolution …a change in allele frequencies in
a population over generations
 3 mechanisms
 Natural selection
 Genetic drift
 Gene flow
 Only natural selection causes adaptive evolution
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Figure 21.3
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Genetic Variation
 Genetic variation can be measured at whole gene
level as gene variability
 Gene variability can be quantified as the average %
of loci that are heterozygous
 Natural selection only acts on phenotypic variations
with genetic component
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Figure 21.5
(a) Caterpillars raised on a diet of
oak flowers
© 2014 Pearson Education, Inc.
(b) Caterpillars raised on a diet of
oak leaves
Altering Gene Number or Position
 Duplicated genes can take on new functions by
further mutation
 An ancestral odor-detecting gene has been
duplicated many times: Humans have 350 functional
copies of the gene; mice have 1,000
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Concept 21.2: The Hardy-Weinberg equation can
be used to test whether a population is evolving
 population ya localized group of individuals
capable of interbreeding and producing fertile
offspring
 A gene pool yall the alleles for all loci in a
population
 An allele for a particular locus is fixed if all individuals
in a population are homozygous for the same allele
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MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
© 2014 Pearson Education, Inc.
CANADA
Porcupine herd
ALASKA
Figure 21.6
The Hardy-Weinberg; Calculating Allele
Frequencies
 For diploid organisms, the total number of alleles at a
locus is the number of individuals times 2
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Conditions for Hardy-Weinberg Equilibrium
 Hardy-Weinberg equilibrium describes a population
that is not evolving - allele frequencies will not change
 In real populations, allele and genotype frequencies
do change over time
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 The five conditions for nonevolving populations are
rarely met in nature
1. No mutations
2. Random mating
3. No natural selection
4. large population size
5. No gene flow
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 Natural populations can evolve at some loci while
being in Hardy-Weinberg equilibrium at other loci
© 2014 Pearson Education, Inc.
The Hardy-Weinberg equation
 p2  2pq  q2  1
where p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the
frequency of the heterozygous genotype
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Factors that change allele frequencies
 Natural selection
 Genetic drift
 Gene flow
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Genetic Drift
 Genetic drift random changes in allele frequencies
especially in small populations
Animation: Causes of Evolutionary Changes
Animation: Mechanisms of Evolution
© 2014 Pearson Education, Inc.
Figure 21.9-1
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR)  0.7
q (frequency of CW)  0.3
© 2014 Pearson Education, Inc.
Figure 21.9-2
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
5 plants
leave
offspring
CRCR
CRCW
CRCW
CRCW
Generation 1
p (frequency of CR)  0.7
q (frequency of CW)  0.3
© 2014 Pearson Education, Inc.
CWCW
CRCR
CRCW
CRCR
CRCR
CWCW
CRCW
CRCR
CRCW
Generation 2
p  0.5
q  0.5
Figure 21.9-3
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
5 plants
leave
offspring
CRCR
CRCR
CRCW
CRCW
CRCW
Generation 1
p (frequency of CR)  0.7
q (frequency of CW)  0.3
© 2014 Pearson Education, Inc.
CWCW
CRCR
CRCW
CWCW
CRCW
CRCR
CRCR
2 plants CRCR
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p  0.5
q  0.5
CRCR
CRCR
CRCR
Generation 3
p  1.0
q  0.0
The Founder Effect
 founder effect - when a few individuals become
isolated from a larger population
 Allele frequencies in founder population can be
different from original population
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The Bottleneck Effect
 bottleneck effect results from a drastic reduction in
population size
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Figure 21.10
Original
Surviving
Bottlenecking
population
population
event
(a) By chance, blue marbles are overrepresented in
the surviving population.
(b) Florida panther (Puma concolor coryi)
© 2014 Pearson Education, Inc.
Figure 21.10a-1
Original
population
(a) By chance, blue marbles are overrepresented in the
surviving population.
© 2014 Pearson Education, Inc.
Figure 21.10a-2
Original
population
Bottlenecking
event
(a) By chance, blue marbles are overrepresented in the
surviving population.
© 2014 Pearson Education, Inc.
Figure 21.10a-3
Original
population
Bottlenecking
event
Surviving
population
(a) By chance, blue marbles are overrepresented in the
surviving population.
© 2014 Pearson Education, Inc.
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 21.11a
Pre-bottleneck Post-bottleneck
(Illinois, 1820) (Illinois, 1993)
Greater prairie chicken
(a)
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Range
of greater
prairie
chicken
Figure 21.11b
Population
size
Number
of alleles
per locus
Percentage
of eggs
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
Location
Illinois
1930–1960s
1993
(b)
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 Researchers using DNA from museum specimens
showed a loss of alleles at several loci after the
bottleneck
 Introduction of greater prairie chickens from
populations in other states introduced new alleles
and increasing hatch rate to 90%
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Gene Flow
 Gene flow - movement of alleles among
populations
 reduces genetic variation between populations
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Concept 21.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
 Only natural selection consistently results in
adaptive evolution, (increase in frequency of alleles
that improve fitness)
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Directional, Disruptive, and Stabilizing Selection
 3 modes of natural selection

Directional selection favors individuals at one
end of the phenotypic range

Disruptive selection favors individuals at both
extremes of the phenotypic range

Stabilizing selection favors intermediate
variants and acts against extreme
phenotypes
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Frequency of
individuals
Figure 21.13
Original
population
Original
Evolved
population population
Phenotypes (fur color)
(a) Directional selection
(b) Disruptive selection
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(c) Stabilizing selection
Directional Selection
© 2014 Pearson Education, Inc.
Fig. 18-6a1, p.288
Directional Selection
© 2014 Pearson Education, Inc.
Fig. 18-6a2, p.288
Directional Selection
© 2014 Pearson Education, Inc.
Fig. 18-6b1, p.288
Directional Selection
© 2014 Pearson Education, Inc.
Fig. 18-6b2, p.288
Directional Selection
© 2014 Pearson Education, Inc.
Fig. 18-7a, p.289
Directional Selection
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Fig. 18-7b-e, p.289
Figure 21.14
Bones shown in
green are movable.
Ligament
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Figure 21.14a
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Sexual Selection
 Sexual selection is natural selection for mating
success
 can result in sexual dimorphism, marked
differences between the sexes
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Sexual
Selection
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Fig. 18-12, p.292
Sexual
Selection
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Figure 21.15
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The Preservation of Genetic Variation
 Neutral variation -genetic variation giving no
selective advantage or disadvantage
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Balancing Selection
 Balancing selection - natural selection maintains
stable frequencies of two or more phenotypes in a
population
 Balancing selection includes
 Heterozygote advantage
 Frequency-dependent selection
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 Heterozygote advantage occurs when
heterozygotes have a higher fitness than do both
homozygotes
 example, sickle-cell allele
© 2014 Pearson Education, Inc.
Figure 21.17
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)
© 2014 Pearson Education, Inc.
5.0–7.5%
7.5–10.0%
10.0–12.5%
12.5%
 Frequency-dependent selection occurs when the
advantage of a phenotype declines if it becomes too
common
 example, frequency-dependent selection selects for
equal numbers of “right-mouthed” and “leftmouthed” scale-eating fish
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Figure 21.18
“Left-mouthed”
P. microlepis
Frequency of
“left-mouthed” individuals
1.0
0.5
0
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“Right-mouthed”
P. microlepis
1981
’85
’87
’83
Sample year
’89
Figure 21.19
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Adaptation to What?
Fig. 18-19a, p.297
Adaptation
to What?
Fig. 18-19b, p.297
Fig. 18-20, p.299
Physician
during the
Plague
p.283