Transcript Section 2: Energy Flow in Ecosystems

Population Genetics and Speciation
Section 2
Bellringer
Brainstorm about two examples of
mutations. One mutation would be
useful and beneficial, while the
other would be harmful. Discuss
how the two different mutations
would most likely be affected by
natural selection.
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Key Ideas
• What does the Hardy-Weinberg principle
predict?
• How does sexual reproduction influence
evolution?
• Why does population size matter?
• What are the limits of the force of natural
selection?
• What patterns can result from natural selection?
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Equilibrium and Change
• A population in which no genetic change
occurred would be in a state of genetic
equilibrium.
• Genetic change in a population can be
measured as a change in genotype frequency or
allele frequency.
• A change in one doesn’t necessarily mean a
change in the other.
Population Genetics and Speciation
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Equilibrium and Change
• The Hardy-Weinberg principle predicts that
the frequencies of alleles and genotypes in
a population will not change unless at least
one of five forces acts upon the population.
• The forces that can act against genetic
equilibrium are gene flow, nonrandom
mating, genetic drift, mutation, and natural
selection.
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Hardy-Weinburg Equation
• The Hardy-Weinberg principle can be expressed
as an equation that can be used to predict stable
genotype frequencies in a population.
• p2 + 2pq + q2 = 1
• p2 = frequency of homozygous dominant
individuals
• pq = frequency of heterozygous individuals
• q2 = frequency of homozygous recessive
individuals
Population Genetics and Speciation
Visual Concept
H-W Principle
Copy this visual into your notes.
Section 2
Population
Genetics
and Speciation
Section
These
caribou
are migrating
from one place to another.
If 2
they meet other groups of caribou and interbreed, gene
Equilibrium
flow
may occur. and Change, continued
Gene Flow
• Gene flow occurs when genes are added to or
removed from a population.
• Gene flow can be caused by migration, the
movement of individuals from one population to
another
Nonrandom Mating
• In sexually reproducing populations, any limits or
preferences of mate choice will cause nonrandom
mating.
Visual: Mating
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Equilibrium and Change, continued
Genetic Drift
• Chance events can cause rare alleles to be lost
from one generation to the next, especially when
populations are small.
• Such random effects on allele frequencies are
called genetic drift.
Visual Concept: Genetic Drift
Mutation
• Mutation can add a new allele to a population.
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Equilibrium and Change, continued
Natural Selection
• Natural selection acts to eliminate individuals
with certain traits from a population.
• As individuals are eliminated, the alleles for
those traits may become less frequent in the
population.
• Thus, both allele and genotype frequencies may
change.
Visual Concept: Natural Selection
Sexual
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selection
Sexual Reproduction and Evolution
favors the
development
• Sexual reproduction creates chances
of extreme
to recombine alleles and thus phenotypic
traits in some
increase variation in a population.
species. The
• Sexual reproduction creates the
vibrant red
possibility that mating patternsstripe
or on the
blue muzzle
behaviors can influence the gene
of this male
pool.
mandrill
baboon does
not appear in
females.
Population Genetics and Speciation
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Sexual Reproduction and Evolution,
continued
• For example, in animals, females sometimes select
mates based on the male’s size, color, ability to gather
food, or other characteristics.
• This kind of behavior is called sexual selection and is an
example of nonrandom mating.
• Another example of nonrandom mating is inbreeding, in
which individuals either self-fertilize or mate with others
like themselves.
• Inbreeding is more likely to occur if a population is small.
In a small population, all members are likely to be closely
related.
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Population Size and Evolution
• Population size strongly affects the
probability of genetic change in a
population.
• Allele frequencies are more likely to remain
stable in large populations than in small
populations.
• Genetic drift is a strong force in small
populations and occurs when a particular
allele disappears.
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Population Genetics and Speciation
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What is the genetic effect of inbreeding?
Increased
homozygosity
Population Genetics and Speciation
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Natural Selection and Evolution
• Charles Darwin proposed natural selection as a
mechanism that could drive evolution. Many
examples of natural selection in action have
been studied.
• Natural selection is a result of the following facts:
– All populations have genetic variation.
– Individuals tend to produce more offspring
than the environment can support.
– Populations depend upon the reproduction of
individuals.
Population Genetics and Speciation
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Natural Selection
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Population Genetics and Speciation
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Natural Selection and Evolution,
continued
How Selection Acts
• Natural selection causes evolution in
populations by acting on individuals.
• Natural selection acts when individuals
survive and reproduce (or fail to do so).
• Less “fit” individuals are less likely to pass
on their genes.
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Natural Selection and Evolution,
continued
Genetic Results of Selection
• The result of natural selection is that each
allele’s frequency may increase or
decrease depending on the allele’s effects
on survival and reproduction.
• Although natural selection is not the only
force that can cause evolution, it is a
powerful force.
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How is “fitness” measured in
evolutionary terms?
success in leaving future
generations of offspring
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Natural Selection and Evolution,
continued
Why Selection is Limited
• The key lesson that scientists have learned
about evolution by natural selection is that the
environment does the selecting.
Natural selection
is indirect
What
might happen to a colorless crayfish
placed inthe
a well-lit
pond?
• It acts only to change
relative
frequency of
alleles that exist in a population.
A colorless crayfish that was previously
• It acts on genotypes
byforremoving
unsuccessful
adapted
a
dark
environment
might
be
Crayfish species exist in a variety of colorations. In many
cases,
phenotypes
a population.
by predators
easily
the coloration
helps from
thefound
crayfish
hide from more
predators
orwhen
attract
a well-lit
mates. But for crayfish placed
speciesinthat
live inenvironment.
lightless caves, having
color gives no fitness advantage.
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Natural Selection and Evolution,
continued
The Role of Mutation
• Only characteristics that are expressed can be
targets of natural selection. If a mutation results
in rare recessive alleles, for example, selection
cannot operate against it.
• For this reason, genetic disorders (such as
cystic fibrosis in humans) can persist in
populations.
Population Genetics and Speciation
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How can unfavorable
alleles persist?
by remaining unexpressed,
recessive alleles can be
inherited and yet not acted
upon by selection
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Patterns of Natural Selection
• Three major patterns are possible in the
way that natural selection affects the
distribution of polygenic characters over
time.
• These patterns are:
– directional selection
– stabilizing selection, and
– disruptive selection.
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Patterns of Natural Selection, continued
Directional Selection
• In directional selection, the “peak” of a
normal distribution moves in one direction
along its range.
• In this case, selection acts to eliminate on
extreme from a range of phenotypes,
making them less common.
• Visual
Population Genetics and Speciation
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Patterns of Natural Selection, continued
Stabilizing Selection
• In stabilizing selection, the bell-curve
shape becomes narrower. In this case,
selection eliminates individuals that have
alleles for any extreme type.
• Stabilizing selection is very common in
nature.
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Patterns of Natural Selection, continued
Disruptive Selection
• In disruptive selection, the bell curve is
“disrupted” and pushed apart into two
peaks.
• In this case, selection acts to eliminate
individuals with average phenotype
values.
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Which form of selection
increases the range of variation
in a distribution?
disruptive selection
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Natural Selection of Anole Lizard Species
Population Genetics and Speciation
Exit Question
Natural selection indirectly affects
______ by removing unsuccessful
______ from a population.
A. phenotypes, genotypes
B. populations, genotypes
C. genotypes, phenotypes
D. individuals, genotypes
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Population Genetics and Speciation
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Population Genetics and Speciation
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