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12.4
DID YOU
KNOW
?
Selection of Another
Hemoglobin Allele?
In November 2001, geneticists
reported in the journal, Nature, about
another mutated form of the hemoglobin gene that provides resistance
to malaria. Unlike the sickle-cell
allele (HbS), individuals with two
copies of the HbC allele show no
signs of disease and are very
resistant to even the most serious
form of malaria. Interestingly, the
mutation in the -hemoglobin that
produced the HbC allele is in the
identical location as the HbS allele.
Patterns of Selection
What is the source of variation? How are subtle differences passed from generation to generation? These questions that puzzled Darwin have been answered by the scientific
understanding of genetics and mutations. Mutations provide a continuous supply of
new genetic variations, which may be inherited and expressed as different phenotypes.
Natural selection leads to a variety of outcomes when this genetic variation occurs within
competitive populations living under diverse environmental conditions. Sickle-cell
anemia, a potentially serious blood disorder, is a useful example of how mutation, genetic
variation, and the environment result in different patterns of natural selection.
The allele for sickle-cell anemia differs from the normal hemoglobin gene by having
a single base-pair mutation. Individuals homozygous for the sickle-cell allele are severely
afflicted with this disorder. Heterozygous individuals are only mildly affected by sicklecell anemia; however, they are much more resistant to malaria than are people with
normal hemoglobin. In regions where malaria is uncommon, individuals with the sicklecell allele are at a disadvantage and their phenotypes are less likely to contribute alleles
to the gene pool. But in regions where malaria is common (Figure 1), heterozygous
individuals are strongly favoured; they are much more likely to survive and pass on their
genes to the next generation. The environment selects the best-adapted phenotype and,
in so doing, favours a particular set of alleles.
The sickle-cell allele is only common where it provides an overall advantage to the
individual. In populations where it has an overall harmful effect, it does not persist. This
pattern establishes an important relationship between mutations and evolution:
•
Harmful mutations occur frequently but they are selected against and, therefore,
these mutant alleles remain extremely rare.
•
Beneficial mutations are rare but they are selected for and, therefore, these
mutant alleles accumulate over time.
Although genes provide the source of variation, natural selection acts on individuals
and their phenotypes. As a result, particular alleles are most successful and passed on when
they enhance the phenotype of the individual and, thereby, contribute to their reproductive
success. Selective forces can favour particular variations in the phenotype of individuals in a number of ways.
(a)
Figure 1
Of the 120 million new cases of
malaria each year, about 1 million are
fatal. The prevalence of malaria in
Africa (a) closely matches the distribution of the sickle-cell allele (b).
556 Chapter 12
(b)
P. falciparum
malaria in Africa
malaria
sickle-cell
allele in Africa
1– 5%
5 –10%
10 – 20%
Adapted from Biology 5/E, fig. 20.4 p. 407 Peter H. Raven and George B. Johnson, the McGraw-Hill Companies, Inc.
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Section 12.4
Types of Selection
DID YOU
Many factors influence how selection can operate on individual phenotypes in a population. Some populations live for long periods of time under stable conditions, while
others live in a constantly changing environment. At times, a variable or diverse environment may favour multiple phenotypes. Within populations themselves, competition
for mates adds further selective pressures that can and do influence the evolutionary
pathway followed by populations. These different sets of conditions result in a number
of general selection types.
Stabilizing Selection
(a)
Range of Values for Bill Length,
Time 1
(b)
?
Bigger Babies
In societies with a high standard
of nutrition and medical care, birth
weights may be changing. Healthy
mothers are producing bigger
babies, while hospitals providing
Caesarean sections and incubators
reduce many risks to heavier and
smaller babies. Selective pressures
are no longer as pronounced. What
long-term effects might result from
these conditions?
stabilizing selection selection
against individuals exhibiting variations in a trait that deviate from the
current population average
Figure 2
Number of Individuals
in the Population
Number of Individuals
in the Population
Number of Individuals
in the Population
In your lifetime, you are unlikely to see a dramatic change in the appearance of any
species as a result of natural selection. Indeed, most species show little change over
periods lasting thousands of years. Although these observations suggest the absence of
evolutionary processes, theoretical models predict that once species become well adapted
to their environment, selection pressures will tend to prevent them from changing.
Stabilizing selection occurs when the most common phenotypes within a population
are most favoured by the environment. For example, human birth weights are variable
and part of this variability is inheritable. According to the theory of natural selection, babies
born at weights that offer the best chance of surviving birth should be more numerous,
and research shows that far more human babies are born weighing just over 3 kg than
any other weight. Babies with significantly lower weights are often developmentally premature and less likely to survive, while heavier babies often experience birth-related
complications that threaten the life of both baby and mother. The evidence suggests
that natural selection eliminates extreme variations of a particular trait, resulting in a population in which most human births are near the ideal weight.
Stabilizing selection is by far the most common form of selection. Once a species
becomes adapted to its environment, selective pressures maintain their evolved features
and traits. For example, the hummingbird draws nectar from flowers with a long bill
and tongue (Figure 2). Most birds of this species will succeed if their bill and tongue
length are well adapted for the size of flowers that they feed on in their local environment.
A longer bill requires more nutrients and energy to grow and carry around, while a
shorter bill may reduce a bird’s ability to reach food. The ideal bill length also increases
the success of flower pollination, thereby enhancing the reproductive success of the
plant species as well as that of the hummingbird. The environment will select against mutations that occur to produce birds with a bill length that differs from the best-adapted
length. The graphs in Figure 3 show the effects of stabilizing selection based on bill
length over successive generations of hummingbirds.
KNOW
Range of Values for Bill Length,
Time 2
(c)
Range of values for Bill Length,
Time 3
Figure 3
Stabilizing selection in a population of hummingbirds. The bell-shaped curve shows the frequency and variation in hummingbird bill lengths. The arrows indicate the less successful forms.
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Mechanisms of Evolution
557
Directional Selection
(a)
Number of Individuals
in the Population
Number of Individuals
in the Population
Number of Individuals
in the Population
Figure 4
Directional selection occurs when the environment favours individuals with more
extreme variations of a trait. When an organism migrates to a new environment, or when
aspects of its habitat change, it will encounter new forces of natural selection. If the hummingbird population moves to a new habitat with longer flowers, individuals with bills
that were best adapted to medium-length flowers will no longer be ideal (Figure 4). Birds
that inherited longer bills will then be more successful than those with medium-length
and short bills; longer-billed birds will obtain more food and contribute more offspring
to later generations (Figure 5). Directional selection may result in an observable change
in a population.
(b)
Range of Values for Bill Length,
Time 1
Range of Values for Bill Length,
Time 2
(c)
Range of Values for Bill Length,
Time 3
Figure 5
Directional selection shown in a population of hummingbirds. In a new environment with longer
flowers, which hummingbirds have an advantage?
directional selection selection
that favours an increase or decrease
in the value of a trait from the current population average
In species with large populations and short generation times—such as salmon—many
offspring are produced. In such species, the amount of genetic variation from both
recombination and mutation is increased. Gill-net fishing in the Bella Coola River and
the Upper Johnston Strait from 1950 to 1974 had a dramatic effect on the pink salmon
population: the average weight of the salmon decreased by about one third (Figure 6).
Bella Coola
6.0
odd years
5.0
Figure 6
Gill-net fishing substantially
decreased the size of adult salmon
on a British Columbia river over
25 years. Larger fish were less likely
to escape gill nets and, therefore,
less likely to contribute to later
generations. Distinctly different
salmon populations breed in alternating years in each location. At the
time of this study imperial measurements were in use.
558 Chapter 12
Mean Weight (lb)
4.0
3.0
even years
Upper Johnston Strait
odd years
6.0
5.0
4.0
even years
3.0
0
’51 ’52 ’53 ’54 ’55 ’56 ’57 ’58 ’59 ’60 ’61 ’62 ’63 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72
Year
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Section 12.4
In these years, about 75% of all adult salmon were being caught and, as a result, smaller
adults were the most likely to escape the nets and contribute offspring to the next generation. In this way, human activity resulted in directional selection.
Disruptive Selection
Number of Individuals
in the Population
Number of Individuals
in the Population
(a)
Range of Values for Bill Length,
Time 1
(b)
Figure 7
Number of Individuals
in the Population
Disruptive selection favours individuals with variations at opposite extremes of a trait
over individuals with intermediate variations. Sometimes, environmental conditions may
favour more than one phenotype. For example, two species of plants with different-sized
flowers may be available as a food source for the hummingbird population (Figure 7). Each
species is a good source of nectar, but neither is well suited to a hummingbird with an
average-length bill. Birds with longer and shorter bills will be more successful and will contribute more offspring to later generations (Figure 8).
Range of Values for Bill Length,
Time 2
(c)
Range of Values for Bill Length,
Time 3
Figure 8
Disruptive selection in a hummingbird population. In a habitat with both shorter and longer
flowers, which birds have the advantage?
A living example of disruptive selection in a bird population is provided by the blackbellied seedcracker finch (Pyrenestes ostrinus). These African finches depend on the seeds
of two different types of sedge, one that produces a soft seed and the other a much
harder seed. Finches with small bills, shown in Figure 9(a) are efficient at feeding on
soft seeds, while birds with larger bills, shown in Figure 9(b) are able to crack the hard
seeds. A study of 2700 finches produced the findings depicted in the graph shown in
Figure 9, which appears to be observable evidence for disruptive selection. Disruptive selection is a significant evolutionary mechanism for the formation of distinctive forms
within a population. Distinctive groups may eventually become isolated breeding populations with separate gene pools.
disruptive selection selection that
favours two or more variations or
forms of a trait that differ from the
current population average
Number of Individuals
60
50
40
30
20
10
0
10
(a) narrow bill
12.8
15.7
18.5
Width of Lower Bill
(mm)
(b) wide bill
From Biology: The Unity and Diversity of Life w/CD ROM, 9/E, by C. Starr and Taggart © 2001. Reprinted with permission of
Wadsworth, an imprint of the Wadsworth Group, a division of Thomson Learning. Fax 800-730-2215.
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Figure 9
The environment has strongly
favoured the evolution of two different bill sizes among black-bellied
seedcracker finches. They display
variation in bill sizes, but during the
dry season, survival is limited to two
narrow size ranges (orange bars on
graph). This is an observable product
of disruptive natural selection.
Mechanisms of Evolution
559
Sexual Selection
sexual selection differential
reproductive success that results
from variation in the ability to
obtain mates; results in sexual
dimorphism and mating and
courtship behaviours
sexual dimorphism striking differences in the physical appearance
of males and females not usually
applied to behavioural differences
between sexes
Individuals that mate and reproduce frequently make a substantial contribution to the
gene pool of later generations. Sexual selection favours the selection of any trait that influences the mating success of the individual. The traits favoured in sexual selection include
sexual dimorphism (i.e., striking differences in the physical appearance of males and
females) and behavioural differences between the sexes. The most common forms of
sexual selection result from female mate choice and from male-versus-male competition.
In some species, females choose mates based on physical traits, such as bright colouration, or behavioural traits, such as courtship displays and song. In other species, males
are equipped with physical features that assist them in establishing control of and
defending their territory against other males (Figure 10). This territory provides an area
to which they can attract, and sometimes forcibly detain, the females with which they mate.
Such traits are not produced by selective pressures from environmental conditions; if
they were, both sexes would be expected to possess them.
Figure 10
Sexual dimorphism may take the
form of a physical advantage, such
as a much larger size in males, or an
enlarged limb, as in fiddler crabs.
DID YOU
KNOW
?
Penguin Puzzlement
Not all species show obvious sexual
dimorphism. In some species of penguin, males and females look so similar that even they have a hard time
telling each other apart (Figure 11).
A male picks up a stone and drops it
at the feet of a would-be mate. If the
other penguin happens to be a male,
the gift is firmly rejected.
Figure 11
560 Chapter 12
Many species have evolved features that are a compromise between different selective pressures. Sexual selection has produced traits that are beneficial for mating but are
otherwise detrimental. Avoiding predators is not made easier, for instance, by brilliant
plumage or a distinctive song (Figure 12). A bizarre extreme—sometimes called runaway
selection—can be illustrated by the stalk-eyed fly (Figure 13). Although males and
females both have eye stalks, females have preferentially mated with males with the
longest eye stalks to the point where the feature has become very exaggerated.
Sexual diversity is not limited to animal populations. Most plants do not select mates
but they do need to attract or use various agents—such as insects, birds, and bats—to
assist in pollination. Flowers and scents are the most obvious sexual features that have
evolved through natural selection. By maximizing their chances of being pollinated, plants
have a greater likelihood of contributing more alleles to the next generation’s gene pool.
Chance and selection play important roles in evolution. Scientists accept that anatomical, morphological, and behavioural traits of organisms evolve chiefly through selection
mechanisms. These traits directly influence the daily survival and reproductive success
of the individual; at the molecular level, the roles of random chance and selection are less
clear. Some biologists argue that most of the differences in genomes result from neutral
mutations, while others hold that most differences are responses to subtle effects from
selective pressures. Although the debate over the details of molecular diversity continues,
the basis for evolution is agreed: the ultimate fate of all genetic combinations rests in their
ability to produce individuals best adapted to survive and reproduce in their habitat.
Natural selection is the mechanism that drives the evolution of species.
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Section 12.4
DID YOU
KNOW
?
Something Smells Fishy
A study suggests that female
sticklebacks detect a scent
to select males with the most
diverse array of genes for fighting
disease. Apparently, females can
use their sense of smell to find
the mate that will give them the
most disease-resistant offspring.
Figure 12
The mating game is risky for male túngara
frogs. When calling for a mate in the dark,
they run the risk of giving away their location
to the deadly frog-eating bat.
Figure 13
This remarkable male fly continues to contribute alleles for eyes on excessively long
stalks because they attract females of the
species. What might change this situation?
Case Study The Evolution of Antibiotic
Resistance in Bacteria
An article in the Canadian Medical Association Journal (July 2001) reported an alarming
six-fold increase in the rate of antibiotic resistance in Canada between 1995 and 1999.
In addition to added health risks, fighting antibiotic resistance can be expensive.
Sunnybrook and Women’s College hospitals in Toronto reported spending $525 000
in two years on fighting resistant bacteria and, across Canada, the cost is estimated at
$50 to $60 million a year.
In 1996, doctors took samples of bacteria from a patient suffering from tuberculosis,
a lung infection caused by the bacterium Mycobacterium tuberculosis. Cultures of the bacteria found it to be sensitive to a variety of antibiotics, including rifampin. The patient was
treated with rifampin and initially responded so well that the lung infection seemed to be
over. Soon after, however, the patient had a relapse and died. An autopsy revealed that
bacteria had invaded the lungs again in large numbers. Cultures of these bacteria were found
to be sensitive to many antibiotics, but resistant to rifampin. DNA sequencing revealed
that a certain bacteria’s gene had a single base-pair mutation that was known to confer resistance to rifampin. Doctors compared the new bacteria culture with the original culture and
found that the sequences were identical except for this single mutation. Researchers then
examined more than 100 strains of bacteria from other tuberculosis patients living in
the same city at the same time. None of these bacteria had the same genetic code as the
rifampin-resistant bacteria obtained in the autopsy. When doctors had begun administering rifampin, the bacteria in the patient had been subjected to a new environmental selective agent, one that gave the mutant strain a major adaptive advantage.
The pattern in this story is not uncommon, but evolution offers some hope as well
as alarm. Many traits that provide antibiotic resistance are harmful to the bacteria. For
example, a strain of E. coli bacteria possesses a plasmid with a gene that enables it to produce an enzyme called -lactamase. This enzyme gives the bacterium resistance to the
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Mechanisms of Evolution
561
antibiotic ampicillin. However, there is a cost for this resistance: to maintain its antibiotic resistance, the bacterium must devote cellular resources to producing the enzyme
and to making copies of the plasmid before cell division. In another example, the bacterium Mycobacterium tuberculosis normally produces catalase, a beneficial but nonessential enzyme. This enzyme, however, activates the antibiotic isoniazid, which destroys
the bacterium. Bacteria that have a defective catalase gene are, therefore, resistant to
isoniazid—as it cannot be activated in the absence of catalase—but they lack the benefits normally provided by the enzyme. As a result of these costs of resistance, when
an antibiotic is not present, natural selection often favours those bacteria that do not
carry antibiotic-resistant alleles.
DID YOU
KNOW
?
Linezolid
Linezolid, sold under the trademark
name Zyvoxam, was first approved
for use in Canada in May 2001. It
was the first entirely new antibiotic
on the market in 35 years. In the
United States, where the drug had
been in use for only a few months, a
strain of Linezolid-resistant bacteria
had already developed.
Case Study Questions
Understanding Concepts
1. Did the rifampin-resistant bacteria found in the autopsy evolve within the patient’s
lungs or did they result from a brand new infection? Explain the evidence.
2. Most antibiotics are derived from microorganisms that do not occur naturally in the
human body. Most infectious bacteria showed no resistance to these antibiotics when
they were first used in the 1940s, because pathogens (disease-causing organisms)
did not already have antibiotic resistance to them then. Why?
3. Bacteria that are sensitive to antibiotics usually out-compete resistant strains in the
absence of antibiotics. Account for this observation.
Applying Inquiry Skills
4. Tuberculosis patients are now routinely given two different antibiotics at the same
time. Why might this approach be more effective than administering a different
antibiotic only after bacteria develop resistance to the first?
Making Connections
multidrug-resistant tuberculosis
Mycobacterium tuberculosis that is
resistant to both antibiotics usually
used to treat it: isoniazid and rifampin
5. Suggest some strategies that could help reduce the incidence of antibiotic resistance
in your own home, your school, and in society at large.
EXPLORE an issue
Debate: Treatment of
Drug-Resistant Tuberculosis
In October 1999, the Harvard Medical School reported that a
deadly strain of tuberculosis (TB) was spiralling out of control.
This was six years after TB had been declared a global health
crisis. TB has been around for a long time; evidence of the
bacterium that causes the lung infection Mycobacterium
tuberculosis has been found in 4000-year-old Egyptian
mummies. It used to have a 50% mortality rate. Presently, it
is unlikely to develop in people who have a healthy immune
system capable of keeping the bacterium in check, but TB
is common among people with poor nutrition, a weakened
immune system, and inconsistent access to shelter and medical
care. As much as one third of the global human population is
thought to be infected by TB. Until the late 1990s, modern drug
treatments—usually with the use of the antibiotics isoniazid and
562 Chapter 12
Decision-Making Skills
Define the Issue
Defend the Position
Analyze the Issue
Identify Alternatives
Research
Evaluate
rifampin—had produced a 98% cure rate. The 1999 report was
of a worldwide outbreak of a strain of TB that was resistant to
this antibiotic treatment (Figure 14).
Modern medical treatment of TB requires antibiotic use from a
few months to more than a year. Because these long exposures
to antibiotic microorganisms increase the opportunity for TB bacteria to develop resistance, doctors have been giving patients at
least two antibiotics from the outset. For people most at risk from
TB, this lengthy treatment can be difficult to maintain and a
challenge for medical officials to monitor, especially in regions
with few clinics and trained staff. In their efforts to prevent the
spread of the lethal multidrug-resistant tuberculosis, some
authorities in North America are resorting to legally enforced
quarantine in patients’ homes.
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Section 12.4
EXPLORE an issue continued
Statement
Individuals being treated for multidrug-resistant TB should be
placed under medical house arrest to ensure they complete their
entire treatment under medical supervision.
•
In a group, share responsibility for researching this issue,
using written and electronic resources.
GO
•
most effective antibiotics, which increases the cost for
treating such patients at a time when medical expenses are
escalating. Find out how house-arrest programs in North
America operate and how effective they have been in the
past. Can individual rights be balanced against society’s
need to prevent the spread of antibiotic-resistant diseases?
Consider other alternatives to enforced confinement that
could guarantee public safety.
www.science.nelson.com
You may want to consider the current status of the TB outbreak. Some TB strains are resistant to four or more of the
•
Prepare arguments and counterarguments to debate your
position on this issue.
Figure 14
The purple areas on the map indicate countries reporting cases of
multidrug-resistant tuberculosis
as of 1999
SUMMARY
Patterns of Selection
•
Neutral mutations—the most common type in eukaryotic organisms—usually
occur in noncoding regions of genetic material but can provide additional
genetic material.
•
Harmful mutations occur frequently but the environment selects against them
and, therefore, alleles that result from them remain rare.
•
Beneficial mutations are rare but the environment selects them and, therefore,
alleles resulting from them accumulate over time.
•
•
The frequency of mutations in a large population is substantial.
•
Directional selection occurs when an extreme of an inheritable trait is favoured,
usually when a population experiences a change in habitat or environmental
conditions.
•
Disruptive selection favours two or more distinct forms, often as a result of a
change in habitat or environmental conditions.
•
Sexual selection favours inherited traits that enhance mating success but may
reduce an individual’s chances of survival.
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Stabilizing selection tends to maintain allele frequencies in stable environments
over long periods of time.
Mechanisms of Evolution
563
Section 12.4 Questions
Understanding Concepts
1. Identify and explain the type of selection that accounts for
each of the following:
(a) the hollow bones of birds
(b) the light emitted by fireflies
(c) the smell of a skunk
(d) the body diameter of snakes
2. Suggest how large antlers or bright colouration could be a
disadvantage for males of some species.
3. Identify and explain whether stabilizing, directional, or
disruptive selection is the most likely cause of each of the
following:
(a) the evolution of bill size in black-bellied seedcrackers
(b) the consistent colour of blue jays for the past 100 years
(c) the evolution of the elephant’s trunk
(d) the evolution of human brain size
4. Study Figure 15. Describe the sexual dimorphism shown
and its evolutionary role.
Applying Inquiry Skills
6. Consider the example of natural selection and evolution in
the African seedcracking finches. What specific technologies do you think might have been used by the researchers
to gather, analyze, and interpret appropriate data? Be as
detailed as possible.
Making Connections
7. Many insect species have evolved resistance very rapidly
to a range of pesticides. Like other species, insects exhibit
variation in physical, chemical, and behavioural traits.
(a) Describe how an insect species would evolve resistance
to a pesticide newly introduced into its environment.
(b) Identify and describe the type of selection the insects
exhibit.
(c) How might high rates of reproduction and the short
duration of insect generations affect their evolution?
(d) How might an understanding of the evolution of pesticide resistance influence how you use pesticides or
alternative methods of insect control?
8. Many Africans who are carriers of the allele for sickle-cell
anemia have emigrated from malaria-stricken areas in
Africa to North America. Has this influenced the biological
role of the sickle-cell allele? Explain.
9. Although, in theory, an individual could mate at random
Figure 15
Male (with inflated throat sac) and female frigate birds
5. Respond to this statement: Harmful mutations are more
common than beneficial ones, and as the many harmful
effects of mutations accumulate, species become weaker
and less able to adapt to changes in their environment.
564 Chapter 12
with other members of a large population, this seldom
occurs. Under most natural conditions, individuals tend to
mate with nearby members of the same species, especially
if they are not very mobile. Alternatively, individuals choose
mates that share similar traits; for example, toads (and
often humans) tend to pair according to size.
(a) How might inbreeding (the mating of closely related
individuals) lead to an increase in recessive phenotypes? Relate your answer to either a population of
cheetahs in the wild or a population of golden
retrievers in a breeding kennel.
(b) Does nonrandom mating result in changes to population phenotype frequencies, genotype frequencies, or
allele frequencies?
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