Pre-AP Biology: Hardy-Weinberg Equilibrium and Evolutionary Mechanisms
Purpose:
 Analyze how evolutionary mechanisms (e.g. genetic drift, gene flow [immigration & emigration], and
mutations) and their consequences provide a scientific explanation for the diversity and unity of past life
forms, as depicted in the fossil record, and present life forms.
 To study how generations in populations would differ if evolution was acting upon them (normal) vs. if
there was no evolution occurring (Hardy-Weinberg equilibrium).
Background:
You will be utilizing Gold Fish to determine gene frequencies within a population for two specific
circumstances: First, if Hardy-Weinberg Genetic Equilibrium is observed; and second, if evolutionary
mechanisms such as emigration, immigration, mutation and genetic drift had taken place. You may work in
pairs for this activity. Two types of fish are present in the Gold Fish population. The first type is Happy Fish,
who is ____________ (color). The second type is Sad Fish, who is _____________ (color). Happy Fish is
phenotypically dominant (homozygous dominant p2 or heterozygous 2pq) for the “happy gene”, Sad Fish is
homozygous recessive q2 for the “sad gene”.
Materials: 20 Gold Fish
Procedure:
Activity One:
You receive a randomly selected population of twenty Gold Fish.
1. Calculate what percentage of your population is composed of phenotypically Happy Fish.
2. Calculate what percentage is composed of phenotypically Sad Fish.
3. Steps 1 and 2 will give you the phenotypic percentages of the population. We will now calculate the
genotype percentages. First, determine the percentage of the q – recessive allele in your population.
Remember this recessive allele codes for Sad Fish. (Hint: remember, the q2 is the number of homozygous
recessive individuals [qq] so to find the value of q you will need to take the square root of q 2). You will always
do this step first when doing a Hardy-Weinberg problem if they don’t directly tell you the number of
each allele.
4. Now that we know the value of q, we can use the equation p + q = 1 to find the percentage of the p –
dominant allele in your population. Remember this dominant allele codes for Happy Fish.
5. Steps 3 and 4 have told you the allele frequencies in a population, but most eukaryotic organisms are diploid
(meaning they have 2 alleles for each gene). Remember we have homologous chromosomes (1 from mom
and 1 from dad) that have 1 allele a piece for each gene. These alleles were the letters you used when
making Punnett squares. Plug in the values for p and q into the equation p2 + 2pq + q2 = 1 to determine the
percentage of the population that is homozygous dominant, heterozygous, and homozygous recessive.
(Remember, p2 = homozygous dominant [pp], 2pq = heterozygous [pq], and q2 = homozygous recessive [qq].)
6. Assuming this population follows the rules of Hardy-Weinberg Genetic Equilibrium, how might you determine
the percentage of Happy Fish and Sad Fish found in generation two?
7. What would the gene frequencies be for these generation two offspring?
Activity Two:
1. Using the same randomly selected population of twenty Gold Fish, once again record the percentage of
Happy Fish and Sad Fish. This population of Fish is now subjected to a variety of evolutionary mechanisms:
a) Happy Fish taste sweet and are easy to catch. Sad Fish taste bitter and are sneaky and difficult to
catch. Half of your Happy Fish are consumed by a hungry predator (literally eat half the “Happy Fish”).
Do so now.
b) Happy Fish are very threatened by this new predator, so half of the remaining Happy Fish emigrate to a
new community (The new location can be your belly if you feel so inclined). Do so now.
c) Sad Fish love to mate with other Sad Fish, so they immigrate to your population and take the place of
all the Happy Fish. Retrieve these gold fish now. For instance, if you started out with 12 Happy Fish
and 8 Sad Fish, then 6 of your Happy Fish would have been eaten in step a. That would have left you
with 6 Happy Fish and 8 Sad Fish. In step b 3 more of your Happy Fish would have emigrated (leaving
you with 3 Happy Fish and 8 Sad Fish – total of 11). That means you imagine you were given 9 more
Sad Fish. (New totals = 3 Happy Fish and 17 Sad Fish)
d) Now, record the percentage of Happy Fish and Sad Fish within your population. Assuming that these
Fish practice non-random mating, so that Happy Fish only mate with Happy Fish and Sad Fish mate
with other Sad Fish, what would the percentage of each type be in generation two? Assume 1 person in
Generation 1 creates 1 offspring for generation 2 [so generation 2 will also have 20 Gold Fish].
e) What would the gene frequencies be for these generation two offspring? Hint: Part D only told you the
phenotypic frequencies (what you can look at and see). Use the Hardy-Weinberg equation to tell you
the actual allele frequencies (p & q) as well as the genotypic frequencies (p2, 2pq, and q2).
Analysis questions for activity 2:
1. Define the following terms:
Immigration –
Emigration-
Natural selection-
Genetic drift-
Mutation-
Gene Flow -
2. What affect do the above mechanisms have on gene frequencies in a population? List each one and
describe how it affects gene frequencies/evolution.
3. How did immigration, emigration, natural selection, and non-random mating affect the Happy Fish
population?
4. If all of these circumstances continued, what might happen to the Happy Fish population in this geographical
area?
5. Would the population of Sad Fish remain consistent throughout time? Why?
6. In order for Hardy-Weinberg Genetic Equilibrium to occur what must happen (Hint: there are 5
requirements)? List them.
7. Do these 5 steps happen often? Why or why not?
8. How does natural selection and evolutionary mechanisms such as immigration and mutation provide an
explanation for the diversity of life?