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Genetic Variation
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PACKET #60
CHAPTER #23
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Introduction
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 Genetic variation is necessary for natural selection
 There are multiple reasons for genetic variability in
populations
Mutations
 Crossing over
 Independent assortment
 Random fertilization of eggs by sperm

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Examination of Genetic Variation
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Examination I
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 One way of evaluating genetic variation is to examine
genetic polymorphism.

The presence of two or more alleles for a given locus
 To examine genetic polymorphism, biologists
compare different forms of a particular protein.

Each form consists of a slightly different amino acid sequence
that is coded for by a different allele.

Gel electrophoresis
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Balanced Polymorphsim
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Balanced Polymorphism
 Maintenance in a population
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of two or more alleles in
equilibrium at frequencies too
high to be explained,
particularly for the rarer of
them, by mutation
 Balanced polymorphism, a
special type of genetic
polymorphism, may be a
result of natural selection.
 The mechanisms that allow
balanced polymorphism to
exist while allow natural
selection to occur is: 

Hetereozygote Advantage
Frequency Dependent
Selection.
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Hetereozygote Advantage
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 Hetereozygote exhibits
greater “fitness” than
either homozygotes.

Example

Hetereozygote carriers of
the sickle cell allele.
 Provides individuals
resistance to malaria.
• Allows individuals living
in areas where malaria
is high to survive.
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Frequency Dependent Selection
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 The “fitness” of a particular phenotype depends on
how often it appears within a population.


The ability of a phenotype to be “fit” depends on the how often
it appears within a population.
Often a phenotype has a greater selective value when rare then
when common in a population.

If a phenotype falls within this category, they lose their selective
advantage as they become more common.
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Frequency Dependent Selection
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
Example

Scale-eating cichids
 Two forms
• Right pointing mouths & left
pointing mouths
 Right pointing allele is
dominant

Get food by biting off the scales of
other fish
• Those with right pointed mouths
attack prey’s left flanks


If the prey is constantly being
attacked from a particular side,
then they become more wary to
such attacks
When the prey become wary, the
advantage then swings to those
predators that attack from the
opposite side
• The reason for the frequency of
both forms of cichids have
osculating over the last 20 – 30
years.
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Neutral Variation
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Neutral Variation
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 Observed in a population where variation does not
alter the ability of an individual to survive and
reproduce.
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Geographic Variation
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TYPE OF GENETIC VARIATION
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Geographic Variation
 Genetic variation among
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individuals of different
populations of the same species.

Cline



Type of geographic variation
Gradual change in a species’
phenotype and genotype frequencies
through a series of geographically
separate populations due to an
environmental gradient.
Example
 Common Yarrow (Achilea
millefolium)
• Wild-flower that grows in various
North American habitats.
• Exhibits clinal variation in height
 Due to different climates at
different elevations/altitudes.

Could clinal variation result in a new
species?
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Review
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Balanced
Polymorphism
Genetic
Variation
Neutral
Variation
Heterozygote
Advantage
Frequency
Dependent
Selection
Geographic
variation
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