Introduction
Evolution can be described as the change in the allele frequencies of a gene pool over time. Natural selection can place pressure upon specific phenotypes and cause a change in the frequency of the alleles that produce the phenotypes. For example, predators see prey more easily if they do not blend in with their natural surroundings. In a normal woodland environment, an eagle would probably be more likely to select a white (albino) rabbit than a brown one. The eagle represents a natural selection pressure that selects against the albino rabbit and the genes that produce the albino condition.
Assume that the pinto beans represent alleles that produce normal-colored brown rabbits and that brown fur is dominant to white fur. The white navy bean, then, represents a recessive allele that produces albino rabbits in the homozygous condition. The dark-colored pinto bean represents a dominant allele that, when present, produces brown rabbits. Selection against brown-colored rabbits (BB or Bb) occurs 25 percent of the time. That is, predators remove 25 percent of all the normal phenotypes each generation. Selection against albino rabbits (bb) is 100 percent. This means that predators eat all of the albino rabbits present in the population in each generation.
Procedure
1.
Place 50 brown pinto beans and 50 white navy beans into the paper bag.
2.
Shake up the beans. Without looking, reach into the bag and remove two beans. These represent an individual rabbit's genotype. Set these beans aside. Continue to remove beans, two at a time, until you have 50 "rabbits," each with two alleles for fur color.
3.
Arrange the beans on a flat surface in two columns. The two columns represent the two possible rabbit phenotypes, brown (BB and Bb) and albino (bb). NOTE: Do not separate the homozygous (BB) brown rabbits from the heterozygous (Bb) brown rabbits. You are only grouping by the color of the rabbits (brown in one column and white in the other)
4.
Examine your columns of "rabbits" and remove 100 percent of the albino individuals (bb).
5.
You must then remove 25 percent of the brown-colored rabbits (BB, BB). Use Table 1 to help you with these calculations. Start by recording the number of brown bunnies in the second column.
6.
To calculate the number of brown "rabbits" to be removed, divide the # brown bunnies by 4. If the number of rabbits you calculate is a fraction, remove a whole "rabbit." Assume that it was injured, but that it escaped and later died. For example 30 "rabbits" divided by 4 equals 7.5 "rabbits." You would then remove 8 "rabbits." Record the # brown bunnies killed in Table 1 and remove them.
7.
Once you have removed all of the bunnies that are killed, count the number of pinto beans (B) and navy beans ( b
) that remain, and record the numbers in Table 1.
8.
Calculate the frequency of each type of bean remaining and record your results in Table 1. To calculate frequency, divide the number of beans of one type by the total number of beans. For example, if there are 15 pinto beans and 10 navy beans remaining, the frequency of pinto beans is 15 divided by 25 (15 + 10), or 0.6. (NOTE: Frequencies will always be decimal fractions.)
9.
Begin the next generation by placing 100 beans into the paper bag. The proportions of pinto and navy beans should reflect the frequencies you calculated in the previous step. In the given example, you would place 60 pinto beans and 40 navy beans into the bag.
10.
Repeat steps 2 through 7 until you have generated data for five generations.
11.
As you calculate the frequencies of each allele and record them in Table 2.
Analysis
Graph the frequencies of each allele (2 nd
& 4 th
columns from Table 2) versus the Generation # (1 st
column from Table 2). Use a different color for each allele and include a legend.

Questions
1. Did allele frequencies change over time? Cite data to back up your answer.
2. Did either of the alleles totally disappear? Cite data to back up your answer.
3. If 100% of the white bunnies are killed every generation, explain how white alleles make it into the gene pool of the next generation?
4. What does shaking of the bag of beans simulate?
5. Why do you remove two beans for each individual?
6. Does natural selection act on an organism’s genotype or its phenotype? Explain your answer.
7. Predict how your results would have been different if white was the dominant allele, but all other parts of the simulation were the same. Explain your answer.
7. Predict what would happen to the B and b
allele frequencies if the number of eagles declined (if, for example, the eagles' eggs did not hatch because of humans' use of the pesticide DDT)? Explain your answer.

Data and Observations
Table 1: Results of Runs #1 – 5.
Run #
# Brown
Bunnies
# Brown Bunnies
Killed
(# Brown bunnies/4)
# Alleles in Surviving Brown Bunnies
B b
Total
Allele Frequency in
Survivor Gene Pool
(# each allele/total # alleles)
B b
1
2
3
4
5
Make sure you remove the brown bunnies that are killed before you count the alleles in the survivors. Transfer the frequency of each allele to the data chart in the lab (results for run #1 go in the generation 1 row, etc.).

Table 2: A Summary of Allele Frequencies Over Five Generations
B Allele
Generation #
Frequency
# brown beans to put in bag for next run
Start (Generation 0) 0.5 50
Generation 1
Generation 2
Generation 3
Generation 4
Generation 5
NOTE: # Beans to put in bag for next run = Frequency X 100
Frequency
0.5 b
Allele
# white beans to put in bag for next run
50