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The Evolution of Populations
Ch. 23
Lecture Objectives
1. Microevolution & Natural Selection
2. Genetic Drift
3. Founder & Bottleneck Effects
4. Sexual Selection
Recall from our lecture on natural
selection…..
 Charles Darwin presented evidence to support
 Descent with Modification (aka evolution)
 Natural Selection (driving force behind D w/ M)
 Finch Population on Galapagos Islands
Average beak depth (mm)
Figure 23.2
10
9
8
0
1976
1978
(similar to the (after
prior 3 years) drought)
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Microevolution
 Change in gene frequencies in a population over
generations
 Population is a localized group of individuals capable of
interbreeding & producing fertile offspring
 Three mechanisms cause frequency changes
 Natural selection
 Genetic drift
 Gene flow
Figure 23.3
Variation in coat color (PHENOTYPE) is influenced by genes
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Figure 23.5
Not all traits are heritable (although we will
focus mainly on heredity
(a)
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(b)
1. Natural Selection
 Individuals in a population exhibit variations in their heritable
traits  best suited traits tend to produce more offspring.
 Adaptive Evolution: consistently favoring some traits over
others (not coincidental)
 improvement in the match between organisms and their environment
Natural Selection
Directional, Disruptive, and Stabilizing
Selection
 There are three modes of selection
 Directional selection favors individuals at one extreme
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
Frequency of
individuals
Figure 23.13
Original
population
Original
Evolved
population population
Phenotypes (fur color)
(a) Directional selection
(b) Disruptive selection
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(c) Stabilizing selection
2. Genetic Drift
 Describes how gene frequencies fluctuate

unpredictably from one generation to the next
Tends to reduce genetic variation through losses of genes
 The smaller a sample, the greater the chance of random
deviation from a predicted result
Figure 23.9–3
5 plants
leave
offspring
CRCR
CRCW
CWCW
CRCW
CRCR
CRCR
CRCR
CRCR
CWCW
2 plants
leave
offspring
CRCR
CRCR
CWCW
CRCR
CWCW
CRCW
CRCR
CRCW
CRCR
CRCW
CRCR
CRCR
CRCR
CRCR
CRCW
CRCR
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
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CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
Generation 3
p = 1.0
q = 0.0
Ex. Of Genetic Drift
The Founder Effect (under umbrella of genetic drift)
 Occurs when a few individuals become isolated from a
larger population
 Allele frequencies in the small founder population can
be different from those in the larger parent population
The Bottleneck Effect (under umbrella of genetic drift)
 A sudden reduction in population size due to a change in
the environment
 Resulting gene pool may no longer be reflective of the
original population’s gene pool
 If the population remains small, it may be further
affected by genetic drift
 Understanding the bottleneck effect can increase
understanding of how human activity affects other species
Figure 23.10–3
Original
population
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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
Figure 23.11
Pre-bottleneck
(Illinois, 1820)
Greater prairie
chicken
(a)
Post-bottleneck
(Illinois, 1993)
Range
of greater
prairie
chicken
Population
size
Number
of alleles
per locus
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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Percentage
of eggs
hatched
Effects of Genetic Drift: A Summary
Genetic drift is significant in small populations
2. Genetic drift can cause 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 genes to become fixed
1.
Gene Flow – AKA Migration
 Movement of genes among populations
 Genes can be transferred through the movement of fertile

individuals or gametes (for example, pollen)
Tends to reduce variation between populations over time
 Can increase or decrease the fitness of a population
Gene Flow
To summarize..
 Natural selection increases the frequencies of alleles that enhance
survival and reproduction
 Adaptive evolution occurs as the match between a species and its
environment increases
 Because the environment can change, adaptive evolution is a
continuous process
 Genetic drift and gene flow do not consistently lead to adaptive
evolution as they can increase or decrease the match between an
organism and its environment
Sexual Selection
 Natural selection for mating success
 It can result in sexual dimorphism, marked
differences between the sexes in secondary sexual
characteristics
1. Intersexual Selection: Members of the competitive sex show
off for mates and the opposite sex chooses the best display. Some
examples include dancing, singing, or showing bright colors.
Figure 23.15
http://dragonflyissuesinevolution13.wikia.com/wiki/Intrasexual_S
election_vs._Intersexual_Selection?file=The_Mating_Dance
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2. Intrasexual Selection: Members of the competitive sex
fight amongst themselves and the key event determines
reproductive success