Download Population Bottlenecks

Do Now
Answer the following
questions in your notebooks:
1. What causes variation in
populations?
1. What is a gene pool?
What have we learned about
evolution so far?
• Evolution is change in the traits of a population
over time
• Natural selection is the mechanism of
evolution, and it acts on an organism’s
phenotype
• Evidence of evolution comes from the fossil
record, DNA and proteins, comparative
anatomy, and embryology
• Isolation can lead to speciation. The major
types of isolation are geographic, behavioral,
temporal, and reproductive
• Variation within a species increases the
probability that some members of the species
will survive
• A great diversity of species increases the
chance that at least some organisms will
survive great changes in the environment
Review: Mutations
Mutations are changes in DNA
sequence or chromosome structure
that can change the proteins produced
by an organism. Mutations can result in
different traits.
How do mutations occur? Do
mutations ever stop happening? Write
the answers in your notes now.
Mutations produce variation within
species.
Vocabulary
1. Alleles: different forms of a gene
2. Frequency: how often something happens
3. Allele frequency: the percentage of alleles for a
specific trait within a gene pool
3. Immigration: to enter into a new population
4. Emigration: to exit a population
5. Genetic drift: change in allele frequency in a
population
6. Founder effect: when allele frequencies change as
a result of the migration of a small subset of a
population out of the group
7. Population bottleneck: when a population
experiences a large decrease in the number of
individuals
Genetic Drift
In small populations, an allele can become more or
less common in the gene pool simply by random
chance, rather than by natural selection. The smaller
a population is, the greater the chance that this kind
of random change in allele frequency will happen.
This random change in allele frequency is called
genetic drift.
Genetic Drift: Key Point
In small populations, individuals that carry a particular allele may have
more offspring than other individuals just by random chance. Over
time, several random chance events like this can cause an allele to
become common in a population.
Answer in your notes now: After genetic drift occurs, will there be
more or less variation in the population? Why?
The Founder Effect
The founder effect is a
specific type of genetic
drift. The founder effect
can be seen when a few
individuals migrate out of
a population and start a
new population with a
different allele frequency
than the original
population.
The Founder Effect: Ellis-van Creveld
Syndrome
The founder effect can be seen
in the Amish population in
Eastern Pennsylvania. This
population was founded by
about 200 German immigrants.
One couple in this founding
population, Samuel King and
his wife, each carried an allele
for a rare recessive genetic
disorder called Ellis-van Creveld
syndrome and passed it onto
their children.
The Founder Effect: Ellis-van Creveld
Syndrome
This syndrome causes short
stature, polydactly, and several
other abnormalities. Since the
members of this population only
have children with other
members of the Amish
population, no new genetic
variation is being introduced.
Now, the percentage of people in
this Amish community with Ellisvan Creveld syndrome is much
higher than the percentage of
people with the disease in the rest
of the United States.
Population Bottlenecks
A population bottleneck is when
a population becomes much
smaller due to random events
(earthquakes, fires, floods,
droughts, etc) or due to human
activities such as excessive
hunting, habitat destruction, and
poaching. When a population
becomes much smaller, the
amount of genetic variation is
also reduced. This reduction in
variation also reduces the chances
that the population will survive.
Some populations that experience
bottlenecks go extinct.
Population Bottleneck in Cheetahs
About 10,000 years ago because of climate
changes - all but one
species of the cheetah
became extinct. With the
drastic reduction in their
numbers, close relatives
were forced to breed, and
the cheetah became
genetically inbred,
meaning all cheetahs are
closely related.
Population Bottleneck in Cheetahs
When geneticists looked at the amount of genetic
variation in cheetahs, they found that they have
much less variation than other mammals. The
inbreeding in cheetahs has led to low survival rates,
and greater susceptibility to disease. Inbred animals
suffer from low genetic diversity. This means
cheetahs do not have the ability to adjust to sudden
changes in the environment, such as disease
epidemics, and have unusually high susceptibility to
certain viruses. For example, if a virus gets into a
healthy population of leopards, not every animal
dies; only some do, because leopards are genetically
diverse. But if every animal is genetically the same,
like the cheetah, and one gets infected, all of them
may become infected and die. Because of their low
genetic diversity, a deadly virus could kill all of the
worlds' wild cheetahs instead of just the susceptible
animals. 
Question for YOU!
Write the answer in your notes now:
What is the major difference between the
founder effect and the bottleneck effect?
Directions for Genetic Drift Activity
In this activity, you will examine the
effects of genetic drift on the
phenotypes of a population of the
organism Colored pompomius. You
may work with a partner on the
activity, but each person must
complete their own handout and
answer questions in their own words.
Let’s read the instructions and the
questions together before we get
started.