Purpose: Explain how the allele frequency of a small population can be affected by genetic drift.
Background Information:
Genetic drift is defined as the random change in allele frequencies that occurs in small populations. It is not due to natural selection. Genetic drift is strongly tied to population size and becomes important in situations where a small segment of an original population splits off or becomes isolated from the parent population. A change in allele frequencies as a result of this migration of a small subgroup of a population is known as ‘founder effect’. Other times this process occurs when the original population undergoes a devastating population reduction (i.e. by natural disaster) and the allele frequencies are changed as a result. This is referred to as the ‘bottleneck effect’. Evolution at the population level is defined as a change in allele frequency over time. This lab will demonstrate how genetic drift can be a strong evolutionary force in certain situations. We will see that a smaller population size can show a much more dramatic shift in allele frequencies over time due to these two events.
Pre-lab Questions: Answer the following questions in complete sentences, restating the question in your answer.
1.
What is genetic drift? (399)
2.
What are the 2 types of genetic drift?
3.
Define gene pool. (394)
4.
What is meant by the term allele (relative) frequency? (394)
5.
In mice, black fur (B) is dominant over brown fur (b). Show the cross between a homozygous black fur mouse with a brown fur mouse. Using the Punnet square, calculate the allele frequency of the black fur gene (B) and the brown fur gene (b) in the offspring.
(allele frequency = # 𝑜𝑓 𝑔𝑒𝑛𝑒 𝑡𝑜𝑡𝑎𝑙 # 𝑜𝑓 𝑔𝑒𝑛𝑒𝑠
x 100%)
Materials: Small Island Cup, Big Island Cup, Bean Organisms: black, white, red, yellow
Procedures:
1.
Place fifteen (15) beans of each color into the Big Island Cup (Total = 60 beans).
2.
Place only three (3) beans of each color into the Small Island Cup (Total = 12 beans).
3.
Record your data in your data table under the “Year 0” column. For each bean color, calculate the allele frequency in the population. Remember allele frequency = # of gene/ total # genes.
4.
With your eyes closed (remember that Genetic Drift is random, and not like Natural Selection), sample 30 lucky members from the Big Island (50% of the population) and 6 lucky members from the Small Island (50% of that population) to reproduce.
5.
Keep the two groups of breeding beans in separate piles and empty both of the islands of all of the non-breeding beans.
6.
Let each breeding bean reproduce one individual of its own kind (so for example, if you picked 5 white beans it becomes 10 white beans) and place those parents and their progeny (kids) back into their appropriate islands. (The big island should again have 60 beans and the small island should have 12 beans.)
7.
Record the new number of each color of bean and allele frequency n your data table under "Year
1".
8.
Continue this procedure for three (3) more years, all the way to “Year 4”

Graph: Using the allele frequency data, graph how the allele frequency of each bean changed over the years. X-axis will be the “Year” and the y-axis will be the “Allele Frequency” You will need to create 2 graphs; one using the “Small Island” data and “Big Island” data.
40
25
20
15
10
35
30
Black Bean
White Bean
Red Bean
Yellow Bean
5
0
Year 0 Year 1 Year 2 Year 3 Year 4
Analysis Questions: Answer the following questions in complete sentences, restating the question in your answer.
1.
Describe how the proportion of alleles (for bean colors) changed over time on each of the islands.
2.
Did the proportion of alleles change more on one of the islands than on the other? If so, explain why.
3.
Did any alleles go extinct on either of the islands? If so, do you think that losing alleles will help or hurt the population (think about the importance of genetic variation in a population)? Can you propose any ways that an island could get extinct alleles back into its gene pool?
4.
Natural Selection and Genetic Drift can both cause populations to evolve over time, but they do it differently.
Explain how they're different.
Conclusion:
In a paragraph explain how genetic drift affects the allele frequency of a population. Use the following sentence frame below to construct your response (hint think about the simulations that was done in today's activity).
Genetic drift is the process where _______________________________________________________.
In today’s simulation we used different beans colors to represent ______________________________.
Genetic drift is a ______________ process, where events like __________________ and ________________ can cause ________________________________________. At the beginning of the simulation there was an equal representation of each allele. Over time the allele frequency changed; for example the black bean allele frequency started at _____ during year 0, than changed to _____ at year 1, then ____ at year 2, _____ at year
3, and finally to ______ at year 4. Than the white bean allele changed as well, starting at _____ at year 0, than
_____, than _____, than _____ and finally ____. The yellow bean allele changed as well, starting at _____ at year 0, than _____, than _____, than _____ and finally ____. Lastly the red bean allele changed as well, starting at _____ at year 0, than _____, than _____, than _____ and finally ____. From this simulation it is clearly seen that genetic drift affects __________________________.

Data Table:
Phenotype
Black
White
Yellow
Red
Black
White
Yellow
Red
#
Year 0
Allele frequency
#
Year 1
Allele frequency
#
Year 2
Allele frequency
#
Year 3
Allele frequency
#
Year 4
Allele frequency