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 alleles
• Evolution can be defined as changes in allele frequency in a population
In a species of lizard, skin color is a single gene trait. 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 allele frequency
and thus to evolution
Natural Selection on Polygenic Traits
• Polygenic traits are traits that
are controlled by more than one
gene. In many polygenic traits,
there is a range of phenotypes
instead of just a few. One
example of a polygenic trait is
skin color in humans
• Natural selection can affect the
distribution of polygenic
phenotypes in three ways:
directional selection, stabilizing
selection, and disruptive
selection
• Think of a distribution as the
way phenotypes are spread out
over a population
Directional Selection
• Directional selection occurs
on polygenic traits.
• In directional selection, one
extreme trait is favored 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.
Stabilizing Selection
• In stabilizing selection, the traits
that are the most average are
selected for, and the extremes
are selected against.
• 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 disruptive selection, the
extreme traits are selected
for, and average traits are
selected against.
• One example of this is beak
sizes in birds. If the only
seeds available in an
environment are small seeds
and large seeds, natural
selection will favor birds with
either small or large beaks.
The birds with medium sized
beaks will not be very
effective at feeding, 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
genetic equilibrium. This
can happen in populations
of organisms that are very
well adapted to their
environment AND the
environment is not changing