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Title: Fishy Frequencies—How Natural Selection Affects the Frequency of Alleles
Purpose: Understanding natural selection can be confusing. People often think that animals
consciously adapt to their environment or move towards “perfection.” For example, that the
peppered moth can change its color, that the giraffe can permanently stretch its neck, or the polar
bear can turn itself white, all so that they can better survive in their environment.
In this lab, you will use fish crackers to help further your understanding of natural selection as well
as apply the role of genetics and gene frequencies in evolution of a species.
Background: Facts about the ‘Fish’
1. These little fish are the natural prey of the terrible fish-eating sharks—YOU!
2. Due to incomplete dominance, there are three phenotypes: orange, purple, and green.
a. Orange: This is the homozygous recessive (ff) trait; these fish taste yummy and are
easy to catch. Their color is easy to see against the coral background colors (no
camouflage)
b. Purple: This fish is a heterozygote (Ff); these fish taste yummy too, but are harder to
catch. The purple color blends better with the coral background (some camouflage).
c. Green: This is the homozygous dominant (FF) trait; these fish taste salty, are sneaky
and hard to catch. The green color blends extremely well with the coral (max
camouflage)
3. You, the terrible fish-eating sharks, much prefer to eat the yummy orange fish; you eat ONLY
orange fish unless none are available in which case you resort to eating purple fish in order
to stay alive.
4. New fish are ‘born’ every year; the birth rate equals the death rate. You simulate births by
reaching into the ‘ocean of spare fish’ and selecting RANDOMLY.
Hardy-Weinberg Assumptions:
Hardy-Weinberg came up with five assumptions to maintain a STABLE POPULATION (no
evolution). They are the following:
1. Large population
2. No Migration
3. No Natural Selection
4. No Mutations
5. Random Mating
In this lab, you are assuming that the total population is large (all groups combined), that mating
is random—all fish colors choose each other equally (reach in the bag without looking), there is
no migration of fish in or out of the area, and there are no mutations in the gene controlling
color. What you are assuming is that natural selection IS occurring because YOU (the shark)
prefer certain fish colors.
In this population, there are only two alleles for color, F and f. If you counted all the alleles for
these traits, the fraction of ‘f’ alleles plus the fraction of ‘F’ alleles would add up to 1.
Procedure:
1. Get a random population of 10 fish from the ‘ocean.’
2. Count orange, purple, and green fish and record in your chart.
3. Eat 3 orange fish; if you do not have 3 orange fish, fill in the missing number by eating
purple fish.
4. Add 3 fish from the ‘ocean.’ (One fish for each one that died.) Be random. Do not use
artificial selection.
5. Record the number of orange, purple, and green fish.
6. Again eat 3 fish, all orange if possible.
7. Add 3 randomly selected fish, one for each death.
8. Count and record.
9. Repeat steps 6, 7, and 8 two more times.
10. Fill in the class results on your chart.
11. Calculate the frequency of alleles in each generation for group and class data.
12. Prepare your graph, and answer the questions.
Chart: Lab Group
Generation Orange(ff) Purple(Ff) Green(FF)
Allele F
Allele f
%F
%f
Allele f
%F
%f
1
2
3
4
5
Chart: Class
Generation Orange(ff) Purple(Ff) Green(FF)
1
2
3
4
5
Allele F
Analysis:
1. Prepare a graph of your data and the class results. On the ‘x’ axis put generations 1-5 and
on the ‘y’ axis put frequency (0-1). Plot both the F and f for your data and for the class
data. Use one color for your data and another color for class data. What generalizations
would you make about your results? How do they compare to the class results?
2. Look up the terms gene pool and relative frequency (page 394). Explain each term.
3. What conditions (Hardy-Weinberg assumptions) would have to exist for the relative
frequencies of genes to stay the same over time? What assumption(s) were not met in this
lab?
4. Why is a large population important for evolution NOT to occur? (HINT: Read about
genetic drift on page 400)
5. Why is it important to collect class data?
6. Explain which phenotype is NOT favorable to the fish and why?
7. What happens to the genotype frequencies from generation 1 to generation 5 in your
group data? In the class data?
The frequency of the homozygous dominant FF started out ___________ and ended ____________.
The frequency of the heterozygote Ff started out _____________ and ended ________________.
The frequency of the homozygous recessive ff started out _____________ and ended ______________.
8. What process is occurring when there is a change in genotype frequencies over a long
period of time?
9. What happened to the recessive alleles over successive generations and why?
10. Why doesn’t the recessive allele disappear from the population?
11. What is the source of all genetic variation? (page 394)
Conclusion:
 Did you support or reject your hypothesis? Explain using data.
o Discuss your data for your group and the class.
 Discuss the Hardy-Weinberg (page 402) assumptions and how they relate to natural
selection.
 Is there any error? Explain.
Extra Credit Questions:
1. How does the founder effect relate to genetic drift?
(page 400)
2. Explain the three different types of natural selection on
polygenic traits. If fish color was a polygenic trait,
which type of selection occurred in this lab? (page 397)
3. How can mutation affect a single gene trait? Describe
figure 16-5. (page 397)
4. What are three types of isolating mechanisms that can
form new species? Explain each type. (page 404-405)
5. How is natural selection being tested in nature?
Explain how speciation occurred in Darwin’s finches.
(pg 406-410)
6.