Download Natural Selection and Allelic Frequency

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Natural Selection and Allelic Frequency
Introduction: A population is said to evolve as the gene pool changes over time. If it
changes dramatically enough, new species result. Natural selection is the process by
which evolution works. Natural selection acts upon the phenotypes of a population and
can cause a change in the alleles (genotype). In this lab you will simulate the effects of
eagle predation on a population of rabbits that has two phenotypes for fur color: brown
and white. Brown fur (B) is dominant and white fur (b) is recessive.
BB = homozygous dominant for brown fur
Bb = heterozygous dominant for brown fur
bb = homozygous recessive for white fur
Materials: 2 colored pencils, paper bag, graph paper, pinto beans, white navy beans
Problem: How does natural selection affect allele frequency?
Objectives:
 Simulate natural selection by using beans of two different colors
 Calculate allele frequencies over 5 generations
 Demonstrate how natural selection can affect allelic frequencies over time
Procedure:
1. Place 50 pinto beans (brown) and 50 navy beans (white) into the paper bag.
2. Shake the bag. Blindly remove one pair of beans = one pair of alleles! This is your
rabbit’s genotype. Set the pair aside and remove 49 more pairs.
a. It helps to set the white rabbits (bb = 2 white navy beans) in one column
b. Place all the brown rabbits together (BB = 2 pinto beans, Bb = one pinto/one
navy bean) in another column as you pull them out…no need to sort because
they are all brown!
3. Examine your columns. Remove all of your white rabbits (yummy!). Count up your
brown rabbits and randomly remove 25% of them (leave no partial rabbits…round up).
4. Count the number of pinto beans and navy beans remaining AFTER predation! Record
these numbers in the NUMBER column under each allele (B and b, respectively).
5. Divide each allele number by the total number of beans left. For example: B/(B+b) or
b/(B+b). This will give you the FREQUENCY of each allele. Record this in your table.
6. Multiply your FREQUENCIES by 100 to get PERCENTAGE for each allele. This
lets you know how many pinto beans and navy beans go back into the bag for the
next generation.
7. Repeat steps 2-6 for 5 generations.
Allele B = brown (pinto bean)
Generation
Start
1
2
3
4
5
Number
50
Frequency
0.50
PERCENTAGE
50
Allele b = white (navy bean)
Number
50
Frequency
0.50
PERCENTAGE
50
Total Left
100
Remember:



Total Left = # of pinto and navy beans left AFTER predation (B + b)

Percentage = # of each bean that goes into bag for next trial (You always need 100 beans total!)
Number = # of pinto or navy beans left AFTER predation (B or b respectively)
Frequency = # of pinto or navy beans / total number of pinto and navy beans [B or b / (B + b)]
 Remember that the frequencies of B + b = 1 (Hardy-Weinberg)
8. Make a double line graph (1 for each allele) for the allele frequencies over the 5
generations. Plot the frequency (or percentage) of the allele on the Y-axis and the
generation number on the X-axis. Use a different colored pencil for the 2 alleles.
Analysis:
1. Did either allele disappear? Why or why not? (Hint: Consider the genotypes)
2. Is this population in equilibrium? Explain!!!
(Hint: Compare the allele frequencies of each generation)
3. What does your graph and data show about the effect of natural selection on allelic
frequencies?
4. Why do you think the eagles preferred the white rabbits?
5. What would happen to the allele frequencies if the number of eagles declined? (Be
specific to both allele frequencies!!!)