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Biology
A Guide to the Natural World
Chapter 17 • Lecture Outline
The Means of Evolution: Microevolution
Fifth Edition
David Krogh
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17.1 What Is It That Evolves?
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Microevolution
• The smallest unit that evolves is a
population.
• A population is defined as all the members
of a single species living in a defined
geographical area at a given time.
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original
coloration
(a) Original
environment
(b) Altered environment
lighter
coloration
population
B
expanse of
barren terrain
darker
coloration
population
A
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Figure 17.1
Genetic Basis of Evolution
• Genes exist in variant forms called alleles.
• In most species, no individual will possess
more than two alleles for a given gene.
• One allele comes from the individual’s
father, the other allele comes from the
individual’s mother.
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Genetic Basis of Evolution
• A population, however, is likely to possess
many alleles for a given gene.
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Genetic Basis of Evolution
• The sum total of alleles in a population is
referred to as that population’s gene pool.
• The basis of evolution is a change in the
frequency of alleles in a gene pool.
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17.2 Evolution as a Change in the
Frequency of Alleles
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maternal
paternal
chromosome 3 chromosome 3
maternal
paternal
chromosome 3 chromosome 3
a2 a4
a1 a2
alleles
alleles
dark coloration
light coloration
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Figure 17.2
Genetic Basis of Evolution
• To the extent that a given set of alleles
increases in frequency from one generation
to the next within a population, the
phenotypes, or observable characteristics,
produced by those alleles will be exhibited
to a greater extent within the population.
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Genetic Basis of Evolution
• With such a change, a population can be
said to have evolved.
• Evolution at this level is referred to as
microevolution: a change of allele
frequencies within a population over a
relatively short period of time.
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Genetic Basis of Evolution
• Conversely, macroevolution, a product of
microevolution, is evolution on a larger
scale.
• Macroevolution is evolution that results in
the formation of new species or other large
groupings of living things.
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17.3 Five Agents of Microevolution
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Five Agents of Microevolution
• Five evolutionary forces can result in
changes in allele frequencies within a
population.
• These agents of microevolution are:
•
•
•
•
•
mutation
gene flow
genetic drift
sexual selection
natural selection
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Table 17.1
Agents of Change: Five Forces That Can Bring about Change in Allele Frequencies
in a Population
Agent
Description
Mutation
Alteration in an organism’s DNA; generally has no effect or a harmful effect. But beneficial
or “adaptive” mutations are indispensable to evolution.
Gene flow
The movement of alleles from one population to another. Occurs when individuals move
between populations or when one population of a species joins another, assuming the
second population has different allele frequencies than the first.
Genetic drift
Chance alteration of gene frequencies in a population. Most strongly affects small
populations. Can occur when populations are reduced to small numbers (the bottleneck
effect) or when a few individuals from a population migrate to a new, isolated location
and start a new population (the founder effect).
Sexual selection
Occurs when some members of a population mate more often than other members.
Natural selection
Some individuals will be more successful than others in surviving and hence reproducing,
owing to traits that give them a better “fit” with their environment. The alleles of those
who reproduce more will increase in frequency in a population.
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Table 17.1
Mutations
• A mutation is any permanent alteration in
an organism’s DNA, and some mutations
are heritable, meaning they can be passed
on from one generation to the next.
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Mutations
(a)
(b)
Normal
Point mutation
Normal
correct
nucleotide sequence
incorrect
nucleotide sequence
complete
chromosome 5
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Deletion
incomplete
chromosome 5
Figure 17.3
Mutations
• Mutation happens fairly infrequently, and
most mutations either have no effect or are
harmful.
• Yet rare adaptive mutations are vital to
evolution in that they are the only means by
which entirely new genetic information
comes into being.
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Gene Flow
• Gene flow is the movement of genes from
one population to another.
• It takes place through migration, meaning
the movement of individuals from one
population into the territory of another.
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(a) Hawaiian silversword
(b) Tarweeds in California
North
America
Hawaiian
Islands
Pacific
Ocean
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Figure 17.4
Genetic Drift
• Genetic drift, the chance alteration of allele
frequencies in a population, has its greatest
effects on small populations.
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Genetic Drift
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Figure 17.5
Genetic Drift
• Genetic drift can have large effects on small
populations through two common scenarios:
• the Bottleneck Effect
• the Founder Effect
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Genetic Drift: Bottleneck Effect
• The first of these is the bottleneck effect,
defined as a change in allele frequencies
due to chance during a sharp reduction in a
population’s size.
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Genetic Drift: Bottleneck Effect
“bottleneck”
only allows a few
individuals through
Original population,
original allele
frequency.
Hunting of seals in
late 1800s greatly
reduced population
size.
Surviving population
had different allele
frequency and little
genetic diversity.
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This different allele
frequency is reflected
in today's population.
Figure 17.6
Genetic Drift: Founder Effect
• The second is the founder effect: the fact
that when a small subpopulation migrates to
a new area to start a new population, it is
likely to bring with it only a portion of the
original population’s gene pool.
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Sexual Selection
• Sexual selection is a form of natural
selection that can affect the frequency of
alleles in a gene pool.
• It occurs when differences in reproductive
success arise because of differential success
in mating.
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Sexual Selection
• A given male in a population may, for
example, sire many more offspring than the
average male in the population.
• If so, this male’s alleles will increase in
frequency in the next generation of the
population.
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Sexual Selection
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Figure 17.7
Natural Selection
• In a population, some individuals will be
more successful than others in surviving,
and hence reproducing, owing to traits that
better adapt them to their environment.
• This phenomenon is known as natural
selection.
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Natural Selection
• Natural selection is the only agent of
microevolution that consistently acts to
adapt organisms to their environments.
• As such, it is generally regarded as the most
powerful force underlying evolution.
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80
Number of individuals
beak depth
1976
60
40
average beak
depth, 1976
20
1978
average beak
depth, 1978
0
5
6
7
8
9
10
11
Beak depth (mm)
12
13
14
Shift of average beak
depth following drought
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Figure 17.9
17.4 Natural Selection and
Evolutionary Fitness
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Evolutionary Fitness
• The phrase “survival of the fittest” is
misleading because it implies that evolution
works to produce generally superior beings
who would be successful competitors in any
environment.
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Evolutionary Fitness
• Evolutionary fitness, however, has to do
only with the relative reproductive success
of individuals in a given environment at a
given time.
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Evolutionary Fitness
• One individual is said to be more fit than
another to the extent that it has more
offspring than another.
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Evolutionary Fitness
• Individuals are not born with invariable
levels of fitness; instead, fitness can change
in accordance with changes in the
surrounding environment.
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17.5 Three Modes of Natural Selection
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Stabilizing Selection
• Natural selection has three modes:
• stabilizing selection
• directional selection
• disruptive selection
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Directional selection
Disruptive selection
Time (many generations)
Stabilizing selection
Range of a particular characteristic (in this instance, lightness or darkness of coloration)
In stabilizing selection,
individuals that possess
extreme values of a
characteristic are selected
against and die or fail to
reproduce. Over succeeding
generations, an increasing
proportion of the population
becomes average in
coloration.
In directional selection,
one of the extremes of a
characteristic is better suited
to the environment, meaning
that individuals at the other
extreme are selected against.
Over succeeding generations,
the coloration of the population
moves in a direction–in this
case toward darker coloration.
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In disruptive selection,
individuals with average
coloration are selected
against and die. Over
succeeding generations,
part of the population
becomes lighter, while part
becomes darker, meaning the
range of color variation in the
population has increased.
Figure 17.10
Three Modes of Natural Selection
• Stabilizing selection moves a given
character in a population toward
intermediate forms and hence tends to
preserve the status quo.
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Stabilizing Selection
Percent of infant deaths
70
infant
deaths
60
infant
births
15
50
40
Infant mortality
is lowest among
infants of average
birth weight
30
20
10
5
10
Percent of births in population
20
0
0
2
3
4
5
6
7
8
9
10
11
Birth weight in pounds
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Figure 17.11
Directional Selection
• Directional selection moves a given
character toward one of its extreme forms.
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Directional Selection
Cranial capacity (the volume of the skull)
has increased in hominins over time
H. sapiens
H. erectus
1400
H. habilis
1200
A. afarensis
A. africanus
1000
800
600
400
200
4
3
2
1
Earliest fossil record (millions of years ago)
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Cranial capacity (cubic centimeters)
H. ergaster
Present
Figure 17.12
Disruptive Selection
• Disruptive selection moves a given
character toward two extreme forms.
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Disruptive Selection
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Figure 17.13