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Date:
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Notes: Patterns of Natural Selection
Learning Objectives
Students will be able to:
-identify graphs of directional, stabilizing, and disruptive selection
-describe the outcomes of directional, stabilizing, and disruptive selection
Today, we will learn about:
-natural selection in single-gene traits
-natural selection in polygenic traits, including
-directional selection
-stabilizing selection
-disruptive selection
-genetic equilibrium
Natural Selection on Single Gene Traits
Review:
•
A single gene trait is a trait that is controlled by only one
gene. Recall that organisms with two of each
chromosome will have two copies of each gene, but those
copies may be different _____________________________.
•
Evolution can be defined as changes in allele frequency in
a population
In a species of lizard, skin color is a ____________________________
____________________________ ____________________________. The diagram below shows
changes in lizard skin color over time due to natural selection.
Natural selection on single gene traits can lead to changes in __________________________
____________________________and thus to ____________________________.
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Natural Selection on Polygenic Traits
•
____________________________ traits are traits that
are controlled by more than one gene. In
many polygenic traits, there is a
____________________________of phenotypes
instead of just a few. One example of a
polygenic trait is skin color in humans
•
Natural selection can affect the
____________________________of
____________________________ phenotypes in three
ways: ____________________________selection,
____________________________selection, and
____________________________selection
•
Think of a ____________________________as the way phenotypes are spread out
over a population
Directional Selection
•
____________________________ selection occurs
on polygenic traits.
•
In directional selection, one
____________________________ trait is
____________________________ over the others,
meaning the trait one extreme causes the
organism to be more fit and therefore
have more offspring that survive.
•
An example of this is running speed in
rabbits. The faster rabbits can outrun
predators easier, so they are less likely to
get eaten, and more likely to survive and
produce offspring. Directional selection
favors the trait of fast running.
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Stabilizing Selection
•
In ____________________________ selection, the traits that are the most
average are selected for, and the ____________________________ are selected
____________________________.
•
One example of a trait that has experienced stabilizing selection is birth
weight. Babies that are very small are often not healthy enough to
survive, while babies that are too large may get stuck in
the birth canal, causing death of the baby and frequently
death of the mother as well.
•
The result of stabilizing selection is that most individuals
will have a phenotype that is within a narrow range.
Disruptive Selection
• In ____________________________ selection, the extreme
traits are selected for, and ____________________________
traits are selected against.
•
One example of this is beak sizes in birds. If the only
seeds available in an environment are
____________________________ seeds and
____________________________ seeds, natural selection
will favor birds with either small or large beaks.
The birds with medium sized beaks will not be
very effective at ____________________________ , so
medium beaks will be selected against.
Genetic Equilibrium
• When the allele frequencies in a population are not
changing, we say that the population has reached
____________________________ ____________________________ .
This can happen in populations of organisms that
are very well adapted to their environment AND
the environment is not changing
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