3 Causes of Deviation from
Equilibrium

1. Natural Selection
○ Stabilizing selection
○ Disruptive selection
○ Directional selection

2. Genetic Drift
○ Founder Effect
○ Bottleneck Effect

3. Gene flow
1. Natural Selection

When natural selection occurs, variation
of traits in the populations change over
time

3 patterns of change due to natural
selection
 A. Stabilizing selection
 B. Disruptive selection
 C. Directional selection
Bell Curve
All of the patterns
are compared to
the standard bell
curve.
 Shows that most
members of the
population share
similar values for
a certain trait.

What do you observe?
A. Stabilizing Selection

Individuals with average (intermediate)
forms of a trait has the highest fitness
and the extreme traits have a lower
fitness
Extremes- selected against
 Intermediate- selected for

 Ex: Human birth weight
A. Stabilizing Selection
What do you observe?
What do you observe?
B. Disruptive Selection
Selection where both extreme traits are
considered more “fit” for the
environment than the intermediate trait
 Both Extremes- selected for
 Intermediate- selected against

 Ex: African Swallowtail
B. Disruptive Selection
What do you observe?
C. Directional Selection
C. Directional Selection
Individuals in a population with one
extreme trait have higher fitness than
the individuals with the average trait
 One extreme is selected for
 The average trait and the other extreme
trait are selected against

 Ex: Beak size in finches
 Guppy experiment
Guppy Experiment
To Summarize…
2. Genetic Drift

Causes deviation from equilibrium

Genetic drift- change in gene pool due
to chance
 The smaller the population, the greater the
impact it has on the population
○ Ex: Founder effect
○ Ex: Bottleneck effect
Founder Effect
When alleles occur at a higher frequency in a
population isolated from the general
population
 The founding individuals could only contain a
fraction of the total genetic diversity of the
original gene pool.
 So the new population has only the alleles that
the founders can provide

 Ex: The Amish
Founder Effect
Polydactyly in the Amish
•The Amish in Lancaster County
are an isolated population that
was begun by German founders.
•Some of the founders carried
the recessive allele which causes
polydactylism (extra fingers)
•Today, as many as 1 in 14
individuals carries that recessive
allele.
Bottleneck Effect
Occurs when a species is subjected to
near extinction due to natural disaster,
overhunting, disease, or habitat loss
 Only a few survivors are left from the
event
 Bottleneck effect prevents the majority
of the original genotypes from
participating in the next generation.

 Have much less genetic variation than the
original population
Bottleneck Effect
3. Gene Flow

Gene flow- gaining or losing alleles
when individuals move into or out of a
population
 Immigration- moving into
 Emigration- moving out of

Could be moving for new breeding
ground, place to hibernate, or more food
sources
 Ex: Salmon migration
Salmon Migration
Concluding Questions

What are the three instances where
populations will deviate from
equilibrium?


Natural selection, genetic drift and gene flow
What is the name for the changes in
gene pool due to chance?
 Genetic drift
What is this picture an example of?
Founder
Effect
What types of selection do each of
these curves represent?
Directional
Disruptive
Stabilizing