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Genes and Variation
16-1 Genes and Variation
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16-1 Genes and Variation
How Common Is Genetic Variation?
How Common Is Genetic Variation?
Many genes have at least two forms, or alleles.
All organisms have genetic variation that is
“invisible” because it involves small differences in
biochemical processes.
An individual organism is heterozygous for many
genes.
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16-1 Genes and Variation
Variation and Gene Pools
Variation and Gene Pools
A population is a group of individuals of the same
species that interbreed.
A gene pool consists of all genes, including all the
different alleles, that are present in a population.
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16-1 Genes and Variation
Variation and Gene Pools
The relative frequency of an allele is the number of times
the allele occurs in a gene pool, compared with the number
of times other alleles for the same gene occur.
Relative frequency is often expressed as a percentage, and
it is not related to whether an allele is dominant or recessive.
In genetic terms, evolution is any change in the
relative frequency of alleles in a population.
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16-1 Genes and Variation
Variation and Gene Pools
Gene Pool for Fur Color in Mice
Sample Population
Frequency of Alleles
allele for
brown fur
allele for
black fur
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16-1 Genes and Variation
Sources of Genetic Variation
Sources of Genetic Variation
The two main sources of genetic
variation are mutations and the genetic
shuffling that results from sexual
reproduction.
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16-1 Genes and Variation
Sources of Genetic Variation
Mutations
A mutation is any change in a sequence of DNA.
Mutations occur because of mistakes in DNA
replication or as a result of radiation or chemicals
in the environment.
Mutations do not always affect an organism’s
phenotype.
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16-1 Genes and Variation
Sources of Genetic Variation
Gene Shuffling
Most heritable differences are due to gene shuffling.
Crossing-over increases the number of genotypes that
can appear in offspring.
Sexual reproduction produces different phenotypes, but
it does not change the relative frequency of alleles in a
population.
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16-1 Genes and Variation
Single-Gene and Polygenic Traits
Single-Gene and Polygenic Traits
The number of phenotypes produced for
a given trait depends on how many
genes control the trait.
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16-1 Genes and Variation
Single-Gene and Polygenic Traits
A single-gene trait is controlled by one gene that
has two alleles. Variation in this gene leads to only
two possible phenotypes.
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16-1 Genes and Variation
Single-Gene and Polygenic Trait
Many traits are controlled by two or more genes and
are called polygenic traits.
One polygenic trait can have many possible
genotypes and phenotypes.
Height in humans is a polygenic trait.
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16-1 Genes and Variation
Single-Gene and Polygenic Trait
• A bell-shaped curve is typical of polygenic
traits.
• A bell-shaped curve is also called normal
distribution.
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16-1 Genes and Variation
16-2
16-2
Evolution
Evolution
as Genetic
as Genetic
Change
Change
Natural selection affects which individuals
survive and reproduce and which do not.
Evolution is any change over time in the
relative frequencies of alleles in a population.
Populations, not individual organisms, can
evolve over time.
Natural selection on single-gene traits can
lead to changes in allele frequencies and
thus to evolution.
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16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Natural selection can affect the distributions of
phenotypes in any of three ways:
• directional selection
• stabilizing selection
• disruptive selection
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16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Directional Selection
When individuals at one end of the curve have
higher fitness than individuals in the middle or at
the other end, directional selection takes place.
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16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Stabilizing Selection
When individuals near the center of the curve
have higher fitness than individuals at either
end of the curve, stabilizing selection takes
place.
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16-1 Genes and Variation
Natural Selection on
Polygenic Traits
Disruptive Selection
When individuals at the upper and lower ends of
the curve have higher fitness than individuals
near the middle, disruptive selection takes
place.
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16-1 Genes and Variation
Genetic Drift
Genetic drift may occur when a small group of
individuals colonizes a new habitat.
Individuals may carry alleles in different relative frequencies
than did the larger population from which they came.
When allele frequencies change due to migration of a small
subgroup of a population it is known as the founder effect.
Descendents
Sample of original population
Founding Populations
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16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Evolution Versus Genetic Equilibrium
The Hardy-Weinberg principle states that allele
frequencies in a population will remain constant
unless one or more factors cause those
frequencies to change.
When allele frequencies remain constant it is called
genetic equilibrium.
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16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Five conditions are required to maintain genetic
equilibrium from generation to generation:
• there must be random mating,
• the population must be very large,
• there can be no movement into or out of the
population,
• there can be no mutations, and
• there can be no natural selection.
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16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Random Mating
Random mating ensures that each individual has
an equal chance of passing on its alleles to
offspring.
In natural populations, mating is rarely completely
random. Many species select mates based on
particular heritable traits.
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16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
Large Population
Genetic drift has less effect on large populations
than on small ones.
Allele frequencies of large populations are less
likely to be changed through the process of
genetic drift.
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16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
No Movement Into or Out of the Population
Because individuals may bring new alleles into a
population, there must be no movement of
individuals into or out of a population.
The population's gene pool must be kept together
and kept separate from the gene pools of other
populations.
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16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
No Mutations
If genes mutate, new alleles may be introduced
into the population, and allele frequencies will
change.
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16-1 Genes and Variation
Evolution Versus Genetic
Equilibrium
No Natural Selection
All genotypes in the population must have equal
probabilities of survival and reproduction.
No phenotype can have a selective advantage over
another.
There can be no natural selection operating on the
population.
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16-1 Genes and Variation
16-3 The Process of Speciation
Natural selection and chance events can change the
relative frequencies of alleles in a population and
lead to speciation.
Speciation is the formation of new species.
A species is a group of organisms that breed with
one another and produce fertile offspring.
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16-1 Genes and Variation
Isolating Mechanisms
What factors are involved in the formation of new
species?
The gene pools of two populations must become
separated for them to become new species.
Isolating Mechanisms
As new species evolve, populations become
reproductively isolated from each other.
When the members of two populations cannot
interbreed and produce fertile offspring,
reproductive isolation has occurred
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16-1 Genes and Variation
Isolating Mechanisms
Behavioral Isolation
Behavioral isolation occurs when two populations
are capable of interbreeding but have differences
in courtship rituals or other reproductive
strategies that involve behavior.
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16-1 Genes and Variation
Isolating Mechanisms
Geographic Isolation
Geographic isolation occurs when two
populations are separated by geographic barriers
such as rivers or mountains.
Kaibab
Abert
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16-1 Genes and Variation
Isolating Mechanisms
Temporal Isolation
Temporal isolation occurs when two or more
species reproduce at different times.
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