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Chapter 13: Evolution and
Natural Selection
Lecture Outline
Enger, E. D., Ross, F. C., & Bailey, D. B. (2012). Concepts in biology (14th ed.). New York: McGrawHill. 1
13-1
The Scientific Concept
of Evolution
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Evolution is the change in the frequency of
genetically determined characteristics within a
population over time.
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Microevolution involves changes in allele frequencies
between populations of the same species.
Macroevolution involves major genetic changes that occur
over long periods of time that generate new species.
The mechanisms of micro- and macroevolution are
the same.
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Microevolution vs.
Macroevolution
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Early Thinking About Evolution
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Mid 1700’s
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Buffon observed fossils were so different from modern
animals, implying that animals have changed over time
(evolution).
Early 1800’s
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Lamarck suggested a process by which the changes could
occur.
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He proposed that acquired characteristics could be passed on
to offspring.
Example: Giraffes have long necks because their ancestors
stretched to reach leaves in trees.
Lamarck was wrong, but his ideas stimulated thought about
evolution.
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The Theory of Natural Selection
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1858
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The theory of natural selection
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Darwin and Wallace suggested the theory of
natural selection as a mechanism for evolution.
Darwin wrote Origin of Species by Means of
Natural Selection.
The idea that some individuals have genetic
combinations that allow them to survive, reproduce
and pass their genes on to the next generation
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Assumptions of the Theory
of Natural Selection
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All organisms produce more offspring than can
survive.
No two organisms are exactly alike.
Among organisms, there is a constant struggle for
survival.
Individuals that possess favorable characteristics for
their environment have a higher rate of survival and
produce more offspring.
Favorable characteristics become more common in
the species.
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Unfavorable characteristics are lost over time.
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Natural Selection vs. Acquired
Characteristics
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Modern Interpretations
of Natural Selection
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Darwin and Wallace’s theories were not immediately
accepted because meiosis, genes, and inheritance
were poorly understood.
Mendel’s discovery provided an explanation for how
characteristics could be transmitted from one
generation to the next.
Knowledge of mutation, gene flow and reproductive
isolation supported Darwin/Wallace’s theory.
Now we can integrate Darwin and Wallace’s
hypothesis with what we know about meiosis, genes,
and inheritance.
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Darwin and Wallace’s Basic
Assumptions can be Updated
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An organism’s ability to overproduce results
in surplus organisms.
Due to mutation, new traits enter the gene
pool and due to meiosis new combinations of
alleles can be generated.
Resources such as food, soil nutrients,
water, etc. are in short supply, so some
individuals will not survive.
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Disease, predators, etc. will also affect survival.
These are called selecting agents.
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Darwin and Wallace’s Basic
Assumptions can be Updated
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Selecting agents favor individuals with the
best combinations of alleles.
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They will be more likely to survive and reproduce
and pass their alleles on to the next generation.
Allele combinations that favor survival and
reproduction will be more common in a
population.
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The Role of Natural Selection
in Evolution
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Natural selection will select for individuals with
certain alleles.
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Natural selection works on individuals, but only
populations evolve.
Three factors work together to determine how a
population changes over time.
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When allele frequencies change over time, evolution has
occurred.
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Environmental factors that affect individuals
Sexual reproduction among the individuals
Genetic diversity within the gene pool
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Reproductive Success
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Individuals that have the combinations of
alleles that allow them to successfully
reproduce will pass on their alleles.
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This success is measured as fitness.
Fitness is a relative measure.
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An individual can successfully reproduce, but be less fit
than another individual.
Involves the number of offspring produced and their
viability
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The Importance of Genetic
Diversity
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A large gene pool with great genetic diversity
is more likely to contain genetic combinations
that will allow some individuals to adapt to
changing environments.
Characteristics that may allow individuals to
adapt include
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Structural, behavioral, biochemical, or metabolic
characteristics
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Common Misunderstandings
About Natural Selection
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Survival of the fittest
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Struggle for life
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Individual survival is not as important as
reproductive success (# of descendants).
Does not refer to conflict
Refers to finding and utilizing resources
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Common Misunderstandings
About Natural Selection
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The significance of acquired characteristics
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The relationship between the mechanism of natural
selection and the outcomes of the selection process
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Phenotypes acquired during the life of the individual will not
be passed on.
These will not be important during natural selection.
The effects of natural selection are seen in the population.
The outcome of natural selection (death, reproductive
success, mate choice) involves individuals.
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What influences natural
selection?
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Genetic diversity within a species
Genetic recombination as a result of
sexual reproduction
Gene expression
The ability of a species to reproduce
excess offspring
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Mechanisms that Affect Genetic
Diversity
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Natural selection cannot occur in a
population in which all of the individuals are
genetically identical.
Genetic diversity is essential for natural
selection.
Several mechanisms generate genetic
diversity in a population.
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Mutation, migration, and sexual reproduction
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Mutations and Migration
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Spontaneous mutations are changes in DNA
that cannot be tied to a specific cause.
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Mutations alter existing genes and generate new
alleles.
Most mutations are harmful.
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Some mutations are helpful.
Mutations are only relevant to natural selection if
they happen in cells that give rise to gametes.
Migration results in alleles entering and
leaving a population.
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Sexual Reproduction and
Genetic Recombination
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Sexual reproduction generates new combinations of
alleles in individuals (genetic recombination).
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Crossing over during meiosis generates new combinations
of alleles in homologous chromosomes.
Independent assortment generates new combinations of
alleles from non-homologous chromosomes.
Random fertilization results in a genetically unique
individual.
Genetic recombination can generate a new
combination of alleles that would give an individual a
selective advantage.
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The Role of Gene Expression
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In order for genes to be selected for, they
must be expressed in phenotype.
Genes are expressed to different degrees in
different individuals.
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Different Degrees of Expression
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Penetrance describes how often an allele is
expressed.
Expressivity describes situations in which the
allele is penetrant, but not expressed equally
in all individuals.
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Why Some Genes May Avoid
Natural Selection
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Some alleles are only expressed during certain
stages of life.
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Some genes need environmental triggers to be
expressed.
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If an organism dies before the allele is expressed, then it
cannot contribute to fitness.
If the trigger is not encountered, then the gene will not be
expressed and will not contribute to fitness.
In heterozygotes, recessive alleles are not
expressed.
Some alleles will not be expressed because an
unrelated gene is required.
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Natural Selection Works
on the Total Phenotype
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One good allele is not enough to give an
individual greater fitness.
Natural selection acts on the total phenotype
which involves a combination of
characteristics.
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The Importance of Excess
Reproduction
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Successful organisms reproduce at a rate in excess
of that necessary to merely replace them when they
die.
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Even though a population remains constant in
number, the individuals that make up the population
change.
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However, population size remains relatively constant.
A high death rate offsets the high reproductive rates.
Therefore, there are many genetically unique individuals.
Some of these individuals will survive and reproduce, even
if the environment changes.
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Processes that Drive Selection;
Differential Survival
 “Survival of the fittest”
– Survival is a prerequisite of
reproduction.
– Individuals that do not survive cannot
reproduce.
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Darwin’s finches
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The finches’ beak size correlated with
the type of seeds they ate.
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Small beaks - softer seeds
Larger beaks - harder seeds
During drought, as soft seeds became
scarce, only birds with larger beaks
survived.
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Darwin’s Finches
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Differential Survival of
Herbicide-resistant Weeds
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When herbicides are used on weeds
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Only the individual plants that are resistant will
survive.
As these reproduce, more and more individuals in
the species are resistant to the herbicide.
Over time, the herbicide is ineffective.
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Differential Survival of
Herbicide-resistant Weeds
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Processes that Drive Selection;
Differential Reproductive Rates
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Survival is necessary to reproductive
success, but does not guarantee
reproductive success.
Reproductive success is measured by the
number of offspring an individual leaves.
Clover fields
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Cows eat taller plants first.
Flowers are at the top of the tall plants.
Tall plants didn’t die, but were not able to reproduce.
Therefore, shorter plants were more reproductively
successful.
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Processes that Drive Selection;
Differential Mate Choice
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Sexual selection occurs when some
individuals are chosen as mates more often
than others.
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More frequently, chosen individuals are
usually larger, more aggressive, more
colorful, etc.
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Relevant only in animal populations
Those that are chosen pass on more genes than
those that are not chosen.
All of these features attract mates.
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Examples of Sexual Selection
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Male red-winged blackbirds establish
territories in which they only allow females.
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Male peacocks and chickens have elaborate
tail feathers.
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The ability to establish and maintain a territory
determines which males will mate.
Males with the most spectacular tails are chosen
by females more frequently.
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Mate Selection
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Patterns of Selection
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Natural selection changes allele frequencies
in a population.
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This actually reduces genetic diversity.
Over time, the individuals become more alike.
Selection can favor different phenotypes in
different situations.
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leads to three different forms of selection.
Stabilizing, directional, and disruptive
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Stabilizing Selection
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Occurs when individuals at the extremes of the
range of characteristic are selected against
This means that the “average” individuals are
selected for.
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Brown mice selected for, white and black selected against
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White and black are more likely to be noticed and eaten by
predators.
Over time the population will have mostly brown mice.
Stable environments favor stabilizing selection.
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Directional Selection
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Occurs when individuals of one extreme of the range
of characteristic are selected for
Usually occurs when the environment is changing
Insecticide use will select for individuals that are
resistant.
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Originally there were probably only a few resistant
individuals.
After several generations, the population will contain mostly
resistant individuals.
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Disruptive Selection
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Occurs when both extremes of a range of
characteristic is selected for
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Insects that live on plants with dark green or
light green leaves
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The intermediate is selected against.
Medium green insects will be noticed and eaten
by predators.
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Patterns of Selection
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Evolution Without Selection —
Genetic Drift
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Genetic drift involves a significant change in
allele frequency that is not a result of natural
selection.
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Results from chance events
More likely to impact small populations
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In a population of 100 plants, 10 of them have red spots
on their leaves.
If these 10 are randomly trampled by an animal or are
killed by a late frost
The red spot allele would be lost, but not due to natural
selection.
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Genetic Drift
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Cougars
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Divided by human
colonization
Small populations
became isolated and
experienced genetic drift.
Now the populations are
endangered because
their gene pools are not
diverse.
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Gene-frequency Studies and
Hardy-Weinberg Equilibrium
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G.H. Hardy and Wilhelm Weinberg described
a simple mathematical relationship that can
be used to study allele frequencies.
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They recognized that under certain conditions,
allele frequencies would not change over time.
In this case, alleles would mix randomly and the
same proportion of genotypes would result.
Their formula uses allele frequencies and
genotypic frequencies to detect an
equilibrium.
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Hardy-Weinberg Equation
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p2 + 2pq + q2 = 1
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If the proportion of genotypes matches this equation,
then the population is not evolving.
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p = frequency of the dominant allele
q = frequency of the recessive allele
p2 = frequency of homozygous dominants
2pq = frequency of heterozygotes
q2 = frequency of homozygous recessives
If gene frequencies do not match this equation, then the
population is evolving.
Certain conditions have to be met for a population to
be in Hardy-Weinberg equilibrium.
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The Conditions Necessary
for H-W Equilibrium
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If these five conditions are met, then gene
frequencies will not change.
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Hardy-Weinberg equilibrium provides a null
hypothesis for evolution in a population.
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Random mating
No mutation
No migration
Very large population size
No natural selection
Gives us a way to compare two populations, or two
generations of a population to determine if evolution is
occurring.
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Determining Genotype
Frequencies
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Genotypic frequencies in a population can be
calculated with a modified Punnett Square.
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In this Punnett square, allele frequencies must be
used to determine the likelihood of particular
genotypes occurring in the next generation.
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Why Hardy-Weinberg Conditions
Rarely Exist
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Since the gene pools of most populations are
changing over time, H-W equilibrium rarely
exists.
Why?
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Random mating rarely occurs.
Spontaneous mutations occur.
Immigration and emigration introduce new alleles.
Populations are not infinitely large.
Natural selection does occur.
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Using the Hardy-Weinberg Concept
to Show Allele-frequency Changes
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The Effect of Changing Allele
Frequencies
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Summary of the Causes
of Evolutionary Change
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