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Chapter 23
The Evolution of Populations
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• One common misconception about evolution is
that individual organisms evolve, in the
Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but
populations evolve
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• Genetic variations in populations
– Contribute to evolution
Figure 23.1
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Where does variation come from?
• Mutation and sexual recombination produce
the variation that makes evolution possible
– Produce the variation in gene pools that
contributes to differences among individuals
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Mutation Rates
• Mutation rates
– Tend to be low in animals and plants
– Average about one mutation in every 100,000
genes per generation
– Are more rapid in microorganisms
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Sexual Recombination
• In sexually reproducing populations, sexual
recombination
– Is far more important than mutation in
producing the genetic differences that make
adaptation possible
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Altering allele frequencies in a population
• Three major factors alter allele frequencies and
bring about most evolutionary change
– Natural selection
– Genetic drift
– Gene flow
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• Genetic drift
–
Chance events cause allele frequencies to fluctuate unpredictably
from one generation to the next
–
Tends to reduce genetic variation
–
More likely in small populations
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
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Only 2 of
10 plants
leave
offspring
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect (one example of genetic drift)
• In the bottleneck effect
– A sudden change in the environment may
drastically reduce the size of a population
– The gene pool may no longer be reflective of
the original population’s gene pool
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Original
population
Figure 23.8 A
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Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect
– Can increase understanding of how human
activity affects other species
(b) Similarly, bottlenecking a population
of organisms tends to reduce genetic
variation, as in these northern
elephant seals in California that were
once hunted nearly to extinction.
Figure 23.8 B
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The Founder Effect (a second example of genetic drift)
• The founder effect
– Occurs when a few individuals become
isolated from a larger population
– Can affect allele frequencies in a population
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Gene Flow
• Gene flow
– Causes a population to gain or lose alleles
– Results from the movement of fertile
individuals or gametes
– Tends to reduce differences between
populations over time
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Natural Selection
• Natural selection is the primary mechanism of
adaptive evolution
• Natural selection
– Accumulates and maintains favorable
genotypes in a population
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A Closer Look at Natural Selection
• From the range of variations available in a
population
– Natural selection increases the frequencies of
certain genotypes, fitting organisms to their
environment over generations
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Directional, Disruptive, and Stabilizing Selection
• Selection
– Favors certain genotypes by acting on the
phenotypes of certain organisms
• Three modes of selection are
– Directional
– Disruptive
– Stabilizing
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• 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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• The three modes of selection
Original population
Original
population
Phenotypes (fur color)
Evolved
population
(a) Directional selection shifts the overall
makeup of the population by favoring
variants at one extreme of the
distribution. In this case, darker mice are
favored because they live among dark
rocks and a darker fur color conceals them
from predators.
(b) Disruptive selection favors variants
at both ends of the distribution. These
mice have colonized a patchy habitat
made up of light and dark rocks, with the
result that mice of an intermediate color are
at a disadvantage.
Fig 23.12 A–C
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(c) Stabilizing selection removes
extreme variants from the population
and preserves intermediate types. If
the environment consists of rocks of
an intermediate color, both light and
dark mice will be selected against.
Sexual Selection
• Sexual selection
– Is natural selection for mating success
– Can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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• Intrasexual selection
– Is a direct competition among individuals of
one sex for mates of the opposite sex
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• Intersexual selection
– Occurs when individuals of one sex (usually
females) are choosy in selecting their mates
from individuals of the other sex
– May depend on the showiness of the male’s
appearance
Figure 23.15
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The Evolutionary Enigma of Sexual Reproduction
• Sexual reproduction
– Produces fewer reproductive offspring than asexual
reproduction  a so-called reproductive handicap
Sexual reproduction
Asexual reproduction
Generation 1
Female
Female
Generation 2
Male
Generation 3
Generation 4
Figure 23.16
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• If sexual reproduction is a handicap, why has it
persisted?
– It produces genetic variation that may aid in
disease resistance
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The Preservation of Genetic Variation
• Various mechanisms help to preserve genetic
variation in a population
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Diploidy
• Diploidy
– Maintains genetic variation in the form of
hidden recessive alleles
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Balancing Selection
• Balancing selection
– Occurs when natural selection maintains
stable frequencies of two or more phenotypic
forms in a population
– Leads to a state called balanced polymorphism
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Heterozygote Advantage
• Some individuals who are heterozygous at a
particular locus
– Have greater fitness than homozygotes
• Natural selection
– Will tend to maintain two or more alleles at that
locus
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• The sickle-cell allele
– Causes mutations in hemoglobin but also
confers malaria resistance
– Exemplifies the heterozygote advantage
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
Figure 23.13
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Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
• Frequency-Dependent Selection
• In frequency-dependent selection
– The fitness of any morph declines if it becomes
too common in the population
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• An example of frequency-dependent selection
On pecking a moth image
the blue jay receives a
food reward. If the bird
does not detect a moth
on either screen, it pecks
the green circle to continue
to a new set of images (a
new feeding opportunity).
Parental population sample
Experimental group sample
Phenotypic diversity
0.06
0.05
0.04
Frequencyindependent control
0.03
0.02
0
Plain background
Patterned background
Figure 23.14
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20
60
40
80
Generation number
100
Neutral Variation
• Neutral variation
– Is genetic variation that appears to confer no
selective advantage
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Why Natural Selection Cannot Fashion Perfect Organisms
• Evolution is limited by historical constraints
• Adaptations are often compromises
• Chance and natural selection interact
• Selection can only edit existing variations
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