Compiled by Jennifer Burcroff
Kingsley High School
Email: [email protected]

Genetic Drift Simulation

Goal: Explore the effects of genetic drift on a sample population.

HSSCE:
. B5.2c: Trace the relationship between environmental changes and
changes in the gene pool, such as genetic drift and isolation of
subpopulations.
. B5.3e: Explain how changes at the gene level are the foundation for
changes in populations and eventually the formation of a new species.
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Background:
In addition to natural selection, one of the mechanisms of evolution
is genetic drift. Genetic drift is the change of allelic frequencies in
the gene pool due to random sampling. This simulation will explore how
sample size can influence the effect on genetic drift on the gene pool.
-Note: The two types of genetic drift, population bottlenecks, and the
founder effect are not suggested vocabulary words in the HSSCE
companion document.

Prior Knowledge:
. Students should be familiar with the concept of random sampling and
sample size.
. Students should understand how changes in alleles frequencies lead to
changes in the gene pool of the population (they should also know what
an allele is).
. Students should be familiar with the basic concepts of evolution.
. Students should know the biological definition of a species and have a
basic grasp of methods of speciation.

An example of the basic idea using marbles...
Population bottlenecks occur when a population's size is reduced for
at least one generation. Because genetic drift acts more quickly to reduce
genetic variation in small populations, undergoing a bottleneck can reduce
a population's genetic variation by a lot, even if the bottleneck doesn't
last for very many generations.
This is illustrated by the bags of marbles shown below, where, in
generation 2, an unusually small draw creates a bottleneck.
What happened to the allelic frequency of the lightest phenotype
(yellow) between generation two and generation three? Due to the small
draw and random chance, the lightest phenotype was lost from the population
between generations 2 and 3. This created a major change in the gene pool.
Why genetic variation is important. Reduced genetic variation means
that the population may not be able to adapt to new selection pressures,
such as climatic change or a shift in available resources, because the
genetic variation that selection would act on may have already drifted out
of the population.


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Vocabulary

. Genetic drift
. Gene pool
. Allele (allelic frequency)
. Evolution
. Speciation
. Genetic Variation




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Materials

. Students will collect data in partners or small groups. Each group
will need 100 items representing individuals in a population (the
example above uses marbles). It is suggested that teachers provide 3
different "phenotypes" of the item. The initial population should
have about 30 of two phenotypes, and 40 of one phenotype. Some
suggested items include:
o Beads- three different colors.
o Beans- three different types of bean (kidney, pinto, black etc).


. One container per group (cups or beakers will work... as long as they
can hold a population of at least 100).
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Procedure

Safety: This lab has no safety concerns.

Set up and Data collection:
1. Each group needs to record the initial phenotype composition of their
original population (these populations can be pre-made by the teacher,
or the teacher can provide instructions to the students).
2. Students will draw random samples from the population. Students will
be doing two separate simulations with their beads (a large draw of
about 40 for simulation one, and a smaller sample of about 10 for the
second simulation).
a. Making the draw random: Students can close their eyes and count
out the correct sample size. We are not testing natural
selection, but genetic drift. If they are looking at the beads
we can not rule out selection as a force.
3. Students will record the phenotypic ratio from the draw and then
RETURN the sample back to the original population. Returning the
sample back to the original population is important so that each trial
has the same phenotype composition as the original.
4. Students will repeat steps 2 and 3 for at least three separate
trials. For each trial they will record their data and calculate the
phenotypic ratio for each trial.









Sample Charts:


Simulation I (Large sample):


| |Original |Trial #1 |Trial #2 |Trial #3 |
| |Population | | | |
|Phenotype |# Beads|Ratio(%)|# Beads|Ratio(%)|# Beads|Ratio(%)|# Beads|Ratio(%)|
| | |