Chapter 10: Natural Selection

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Chapter 10
Natural Selection
An Evolving Enemy
Copyright © 2010 Pearson Education, Inc.
Chapter 10 Section 1
Tuberculosis
Return of an Ancient Killer
Copyright © 2010 Pearson Education, Inc.
10.1 Return of an Ancient Killer
Tuberculosis
 Tuberculosis (TB) has infected humans for
1000s of years
 Evidence of TB has been found in Egyptian
mummies
 Hippocrates described a TB-like condition
 In 1906, TB accounted for 2 out of every
1000 deaths in the U.S.
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10.1 Return of an Ancient Killer
What is Tuberculosis?
 TB is caused by
bacterium,
Mycobacterium
tuberculosis
 2 billion people carry TB
 New infections occur at
rate of 1 per second
 TB causes roughly 2
million deaths per year
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Figure 10.1
10.1 Return of an Ancient Killer - What is
Tuberculosis?
Symptoms of TB include:




Cough that produces blood
Fever
Fatigue
Period of wasting – patient becomes weaker
and thinner
 Led to calling the disease “consumption”
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10.1 Return of an Ancient Killer - What is
Tuberculosis?
 Consumptive symptoms occur because of
damage to lung tissues.
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Figure 10.2
10.1 Return of an Ancient Killer - What is
Tuberculosis?
 Testing is by x-ray of
lungs and skin test
 Transmission of TB
occurs through the air,
from infected individuals
 A single sneeze can
contain 40,000 droplets,
all containing infectious
bacteria
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Figure 10.4
10.1 Return of an Ancient Killer
Treatment – and Treatment Failure
 19th and early 20th century – treatment
consisted of “sanitariums”
 Discovery of antibiotics revolutionized TB
treatment
 Since the 1980s, scientists have noticed
an increase in TB that is resistant to
antibiotics.
 Because of resistance to antibiotics, the
number of TB cases worldwide is
increasing.
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END Chapter 10 Section 1
Tuberculosis
Return of an Ancient Killer
Copyright © 2010 Pearson Education, Inc.
Chapter 10 Section 2
Natural Selection Causes Evolution
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10.2 Natural Selection Causes Evolution
Darwin discussed two ideas in Origin of Species
1. Common descent
2. Natural selection


Natural selection is considered to be the primary
cause of evolution
Other factors include genetic drift and sexual
selection (chapter 11)
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10.2 Natural Selection Causes Evolution
Natural selection is an inference based on
four observations
1. Individuals within populations vary
2. Some of the variation within individuals can
be passed on to their offspring
3. Populations of organisms produce more
offspring than will survive
4. Survival and reproduction are not random
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10.2 Natural Selection Causes Evolution
1. Individuals within populations vary.
 This is true of human and non-human
populations.
 Variation can include traits other than
appearance, such as blooming time in
flowers.
Copyright © 2010 Pearson Education, Inc.
Figure 10.6
10.2 Natural Selection Causes Evolution
2. Some of the variation within individuals
can be passed on to their offspring.
Darwin noticed that animal
breeders could get
exaggerated traits through
selective breeding.
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Figure 10.7
10.2 Natural Selection Causes Evolution
3. Populations of organisms produce
more offspring than will survive.
 Even slow-breeding animals can produce
large populations quickly.
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10.2 Natural Selection Causes Evolution
3. Populations of organisms produce more
offspring than will survive.
If a female elephant (colored pink) lives a full fertile lifetime, she will bear about six calves
in about 90 years. On average, half of her calves will be female.
Shelf = Available resources
Generation 0 =
2 elephants
Generation 1 =
6 elephants
Generation 2 =
18 elephants
Generation 3 =
54 elephants
Copyright © 2010 Pearson Education, Inc.
Figure 10.8
10.2 Natural Selection Causes Evolution
4. Survival and reproduction are not random
 Fitness: Relative survival and reproduction of
one variant
 Adaptation: Traits that increase individual
fitness in an environment
 Individuals with adaptations for a particular
environment are more likely to survive and
reproduce
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10.2 Natural Selection Causes Evolution
4. Survival and reproduction are not random.
Number of individuals
1976
Bill
depth
Average
bill depth
of drought
survivors
Average bill
depth before
drought
1978
Bill depth (mm)
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Figure 10.9
10.2 Natural Selection Causes Evolution
Adaptations do not only
affect survival.
 Any trait that increases the
number of offspring
produced is an adaptation
 A flower’s reproduction is
impacted by traits that
affect the number of
pollinators it receives.
 Therefore, color or nectar
production might be
adaptations.
Copyright © 2010 Pearson Education, Inc.
Figure 10.10
10.2 Natural Selection Causes Evolution
Darwin’s Inference: Natural Selection Causes
Evolution
 Result of natural selection is
 Favorable inherited variations tend to
increase in frequency
 Unfavorable variations tend to be lost
 End result is a change in the traits of
individuals in a population over generations
(i.e. evolution)
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10.2 Natural Selection Causes Evolution
Testing Natural Selection
 Artificial Selection:
Selection imposed by human
choice
 Breeds of pigeons studied by
Darwin arose through artificial
selection
 Breeds of dog have been
artificially selected by humans
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10.2 Natural Selection Causes Evolution
Testing Natural Selection in the Lab
 In laboratory, environmental conditions
can be manipulated and effects on
population examined
 Example: alcohol in fruit flies
 Scientists examined alcohol metabolism in
fruit flies
 All animals have enzymes to metabolize
alcohol
 Variations in ability to metabolize alcohol
exist in populations
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10.2 Natural Selection Causes Evolution Testing Natural Selection
Percent of population that
metabolized alcohol rapidly
 Natural selection in the Lab
Increase in percent of
fast-metabolizing
flies
No change in
alcohol-metabolizing rate
Generation Generation
1
57
Fly population in
normal laboratory
environment
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After 57 generations, all flies
in the high-alcohol environment
are fast processors of alcohol.
As a result, the average rate
of alcohol metabolism is twice
the rate of the unmodified
population.
Generation Generation
1
57
Fly population in a
high-alcohol
laboratory environment
Figure 10.12
10.2 Natural Selection Causes Evolution
Natural Selection in Wild Populations
 Many examples exist:
 Evolution of resistance of M. tuberculosis to antibiotics
 Many other disease-causing pathogens have also
evolved resistance
 Galapagos finches provide another classic example
Number of individuals
1976
Bill
depth
Average
bill depth
of drought
survivors
Average bill
depth before
drought
1978
Bill depth (mm)
Copyright © 2010 Pearson Education, Inc.
Figure 10.9
10.2 Natural Selection Causes Evolution
PLAY
Animation—Natural Selection
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Figure 10.9
End Chapter 10 Section 2
Natural Selection Causes Evolution
Copyright © 2010 Pearson Education, Inc.
Chapter 10 Section 3
Natural Selection Since Darwin
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10.3 Natural Selection Since Darwin
The Modern Synthesis
 The union of genetics and evolution is
called “the modern synthesis”
 Knowledge of genetics facilitates
understanding of the mechanisms of
evolution
 Alleles are the basis of variation of traits
 Mutations can create new alleles and
provide the basis for new traits
 Natural selection provides a filter that
selects for or against new traits
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10.3 Natural Selection Since Darwin
The Modern Synthesis
 Mutation gives rise to new alleles
 Generates raw material for natural selection
 Natural Selection alters the frequency of
alleles within a population over
generations
 Evolution of a population = an increase
or decrease in the frequency of an allele of
a particular gene
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10.3 Natural Selection Since Darwin
Mutation, Natural Selection & Evolution
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Figure 10.13
10.3 Natural Selection Since Darwin Overview: The Subtleties of Natural Selection
PLAY
Animation—Drug Resistance and Natural Selection
Copyright © 2010 Pearson Education, Inc.
10.3 Natural Selection Since Darwin
A Closer Look: Subtleties of Natural Selection
1. Natural selection cannot cause new
traits to arise
 Not an issue of choice or “will” of organisms
 Selection can ONLY act on variations that
ALREADY exist
 Mutation creates new alleles RANDOMLY
 In fly example, alcohol-rich environment did
not cause gene to arise, differential survival
caused allele to become more common
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10.3 Natural Selection Since Darwin
A Closer Look: Subtleties of Natural Selection
2. Natural selection does not result in
perfection
 Natural selection does cause organisms to
become a better fit to their environment
 Organisms are not necessarily “better”, just
better fit to a particular situation
 Adaptation that is beneficial in one situation
might be a liability in another
 Adaptations are trade-offs between better fit
in one situation versus another
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10.3 Natural Selection Since Darwin
A Closer Look: Subtleties of Natural Selection
 Adaptations are
constrained by
underlying biology
 Result is “jury-rigged
design”
 Example is panda’s
“thumb”
 Actually grown from
wrist bones
 Not as efficient as
opposable thumb
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Figure 10.14
10.3 Natural Selection Since Darwin
A Closer Look: Subtleties of Natural Selection
3. Natural selection does not cause
progression towards a goal
 Natural selection favors variants with the
most appropriate adaptations for current
environment
 Organisms do not choose to change or
adapt
 Natural selection depends on the situation of
the population
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10.3 Natural Selection Since Darwin
Patterns of Selection
 Different environmental conditions can
lead to different changes in populations
 Directional selection – change in
population traits by favoring one allele over
another
 Stabilizing selection – selection for the
average traits
 Diversifying selection – selection for
extremes
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10.3 Natural Selection Since Darwin
Directional Selection
Not favored by
pollinator = low fitness
Preferred by
pollinator = high fitness
Color range
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Number of individuals in the
population of each type
Number of individuals in the
population of each type
(a) Directional selection
Population evolves
in the direction of a
darker pink color.
Color range
Figure 10.16a
10.3 Natural Selection Since Darwin
Stabilizing Selection
Pale flowers
not recognized
by pollinators
= low fitness
Pollinators more likely
to visit similar-colored
flowers = high fitness
Dark flowers not
recognized by
pollinators = low fitness
Color range
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Number of individuals in the
population of each type
Number of individuals in the
population of each type
(b) Stabilizing selection
Population
stabilizes;
nearly all the
individuals are
same color.
Color range
Figure 10.16b
10.3 Natural Selection Since Darwin
Diversifying Selection
Neither pollinator chooses
intermediate color = low fitness
One type of
pollinator
specializes in
pale flowers
= high fitness
Another pollinator
specializes in dark
flowers = high fitness
Color range
Copyright © 2010 Pearson Education, Inc.
Number of individuals in the
population of each type
Number of individuals in the
population of each type
(c) Diversifying selection
Population diversifies
into pale variety and
dark variety.
Color range
Figure 10.16
END Chapter 10 Section 3
Natural Selection Since Darwin
Copyright © 2010 Pearson Education, Inc.
Chapter 10 Section 4
Natural Selection and Human Health
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10.4 Natural Selection and Human Health
Tuberculosis Fits Darwin’s Observations
 Mycobacterium tuberculosis has evolved
resistance to antibiotics because it fulfills
the same observations Darwin made




Bacteria in the population vary
Variation can be passed on to offspring
More bacteria are produced than survive
Bacterial survival is not random
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10.4 Natural Selection and Human Health
Selecting for Drug Resistance
 Simple antibiotic
treatment can result in
directional selection
in bacteria.
Single drug therapy
1 Start with different variants
of M. tuberculosis.
2 Single drug reduces fitness
of most variants.
3 Resistant variants proliferate.
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Figure 10.17
10.4 Natural Selection and Human Health
Stopping Drug Resistance
 Continue treatment
until infection is
completely cured
(maybe months)
 Combination therapy
(aka drug cocktail) is
a powerful tool against
drug resistance.
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Figure 10.18
10.4 Natural Selection and Human Health
Can Natural Selection Save Us From Superbugs
 If bacteria can evolve resistance to
antibiotics, can humans evolve resistance
to bacteria?
 Humans do vary in their immune capacity
 To evolve resistance to superbugs would
require many humans to die over many
generations
 By using modern drugs are we allowing
the “survival of the weakest”?
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END Chapter 10 Section 4
Natural Selection and Human Health
Copyright © 2010 Pearson Education, Inc.
END Chapter 10
Natural Selection
An Evolving Enemy
Copyright © 2010 Pearson Education, Inc.
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