AP Biology: Unit 5: Evolution: AP Lab #10: Population Genetics and Evolution Lab
Africa's Lake Victoria formed around 100,000 years ago, and since that time as many as 400 species of a
particular group of fish called cichlids have evolved there from one or a very few ancestral species. Some
researchers suggest that the majority of them evolved even more recently because the lake was much
reduced or possibly even dry during a period of glaciation that ended around 15,000 years ago. The cichlid
populations are still evolving, responding to changes in the environment. Once speciation has already
occurred, it is usually easy to recognize the separate species, but how do scientists recognize whether a
population is evolving?
Background
Hardy-Weinberg Equilibrium
In the early 1900s, many biologists attempted to explain evolution in terms of the emerging science of
genetics. In Mendelian genetics, because the F, generation of a monohybrid cross shows a 3:1 ratio of
dominant to recessive phenotypes, many assumed that populations would evolve toward similar ratios of
phenotypes. Two mathematicians, Godfrey Hardy and Wilhelm Weinberg, realized that the frequency of
alleles in a population was independent of the alleles' inheritance pattern from individual parents to
offspring. They postulated an ideal breeding population with the following properties:
1.
The population size is infinite, or very large.
2.
Mating within the population is random. This means that there is no mating preference for any
specific phenotype over another.
3.
There is no mutation occurring in the population.
4.
There is no exchange of genetic information with other populations—no immigration or
emigration of individuals.
5.
There is no selection for one phenotype over another. All phenotypes have an equal chance of
survival and of having their genes passed on to future generations.
If these conditions are met, the population's allele and genotype frequencies will remain statistically constant
over time, a condition referred to as Hardy-Weinberg equilibrium. If we determine the frequency of a pair of
alleles in a population, we can sample that population over several generations to determine if the frequency
changes. If it does, we know that the population is evolving with respect to that pair of alleles. Several
mechanisms are known to disrupt the Hardy-Weinberg equilibrium in a population:
Genetic Drift
Genetic drift occurs when the allele frequencies in a population change between generations due to a
population's finite size. For example, a bottleneck effect results when an environmental change greatly
reduces the size of a population. Another type of genetic drift, the founder effect, occurs when a small
portion of a population becomes isolated. In these cases the allele frequencies in the smaller population
may be quite different from those across the original, large population.
Gene Flow
Allele frequencies change when genes move into a population through immigration of individuals or
out of it, by emigration.
Mutation
Mutation is a change in DNA, which alters alleles directly. Varieties of mutation include point mutations, in
which one nucleotide base pair changes, deletion mutations, in which base pairs are lost, insertion mutations
in which base pairs are added, and frameshift mutations, in which a number of nucleotides inserted or
deleted is not a multiple of three (throwing off the translation of the rest of the gene into a protein).
Mating Selection
If mating is not random, some allele combinations may produce individuals who are preferred as
mates. These individuals tend to breed more and thus produce more offspring, increasing the
frequency of those alleles in the population.
Natural Selection
Certain alleles code for traits that are more favorable for long-term survival, or, at least for
greater reproductive success. Others code for traits that are less favorable. Natural selection
increases the frequency of the beneficial alleles and decreases the frequency of the detrimental
ones by disproportionately removing maladapted phenotypes from the population.
Wisconsin Fast Plants
In Wisconsin Fast Plants, there are two possible stem colors, purple and non-purple (green). The purple color
is due to the pigment anthocyanin. In these plants, the gene coding for anthocyanin is dominant (ANL). Thus,
both homozygous genotypes (ANL/ANL) and heterozygous genotypes (ANLIan!) express the purple stem
phenotype, though the amount of anthocyanin may vary. Plants that show a green stem phenotype are
homozygous for the recessive form of the anthocyanin gene (an/) and do not produce the pigment.
Two variations in leaf color are also observed in Fast Plants, yellow-green and green. In these plants, the gene
coding for the yellow-green color is recessive (ygr). Only those plants that are homozygous for this trait (ygr/ygr)
show the yellow-green leaf phenotype. These plants show less chlorophyll production and slower development.
Plants that are heterozygous (YGRlygr) or homozygous dominant (YGRIYGR) express a green leaf phenotype.
Pre-laboratory Questions
1.
In a mythical species of dragons, a blue skin color (B) is dominant to a yellow skin color (b). If
two heterozygous blue dragons were bred together and produced 160 offspring, how many
offspring would you expect of each color? How many of each genotype?
2.
If the dragons' babies were examined by a population ecologist, would there be any way for the
ecologist to tell the difference between the heterozygous blue babies and the homozygous blue
babies without mating them? Explain.
3.
Why might it be important to know allele frequencies in a population?
Guided Activity
Materials
If you are using the 8-station kit, your group needs these items:
F2 Non-Purple Stem, Yellow-Green Leaf Wisconsin Fast Plants® Seeds
8-oz plastic container
24-oz plastic container
cotton wicks
fertilizer pellets
plant labels
soil
watering pipets
water
balance
Genetics Cards
Activity A: Simulation of a Population
Introduction
In this activity, your class will simulate a breeding population of diploid organisms, Wisconsin Fast
Plants®. You have four cards, which represent gametes produced during meiosis. The letter on the
card represents an allele that is inherited with the gamete. You will contribute one gamete to each of
your offspring. Everyone in the class will begin with the same four cards, two of ANL and two of aril.
Procedure
1.
Place your cards (two and cards and two ANL cards) face down and shuffle them until you do not know
which card is which. Record the initial frequency of p and q (Recall that if 60% are of a specific type,
that the frequency of that type is 0.60).
2.
Stack your cards, continuing to keep them face down.
3.
Pair with a classmate. Draw the top card from each stack. These cards represent the genotype of
your first offspring. One of you should record this as Generation 1 Genotype in Table 1.
4.
Each of you should take back your original card and reshuffle your stacks, keeping all of the
cards face down.
5.
Draw another card from each stack. These cards represent the genotype of your second offspring. The
person who has not yet recorded a Generation 1 Genotype should now record this as the Generation 1
Genotype on their Table 1.
6.
Now, assume the genotype of your recorded generation 1. This means that you should have four cards
total, two matching each allele recorded in generation 1. For example, if you recorded anllanl then
you should have four and cards now. Additional cards may be obtained from your instructor.
7.
Randomly pair with another student and repeat this process for five generations, recording your results
in Table 1. Before each new mating, assume the genotype of the newly recorded generation.
8.
Collect the class data for each generation and record it in Table 2.
9. Using the class data, determine the frequencies for each allele (anl or ANL) for each generation and
record it in Table 3. To determine frequency, divide the total number of an allele in a generation by
the total number of alleles in that generation.
10. Answer the questions for Activity A.
Activity B: Evaluation of a Population
Introduction
In this activity you will plant a population of Wisconsin Fast Plants® to simulate a real population. The plants
that you will be using are the F2 offspring from a cross between a purple stemmed, yellow-green leaf plant and a
non-purple stemmed, normal leaf plant. After allowing the plants to grow for a week, they will be scored. The
allele frequencies of this generation will be compared to the allele frequencies of the original generation.
Procedure
Planting (for the 8-station kit only)
1. Put approximately 55 g of soil into the large container.
2. Add 50 mL of water and mix thoroughly until all the soil is evenly moistened.
3. Push the wick through the center of the precut x in the bottom of the smaller container.
4. Pour approximately 25 g of the moistened soil into the smaller container.
5. Distribute 16 fertilizer pellets on top.
6. Spread about 50 g more of moistened soil on top of the fertilizer pellets.
7. Evenly space 10 Fast Plants seeds on the surface of the soil.
8. Add another 25 g of moistened soil evenly over the Fast Plants seeds.
9. Water gently with approximately two pipets full of water.
10.
Rinse the large container and then fill the container half full with water.
11.
Place the smaller container on top of the larger container so that the wick of the smaller container
is in the water and the smaller container is being supported by the larger container.
12.
Place these in the lighted space designated by your teacher.
Scoring
1. After a week, examine your plants for the two stem colors, purple and non-purple (green).
2. Count the number of plants showing each characteristic and record the data on Table 4.
3. Collect the class data and record it on Table 4 (if using the 8-station kit).
Data Analysis
1. Use the class data to calculate the frequency of each allele. Record it on the data sheet.
2. Use the class data to calculate the frequency of each genotype. Record it on the data sheet.
3. Use the class data to calculate the number of each genotype. Record it on the data sheet. Answer the
Laboratory Questions for Activity B.
Data Sheet and Laboratory Questions
Activity A
Table 1: Generation Genotypes
Genotype
allelle 1
alielle 2
Generation 1
Generation 2
Generation 3
Generation 4
Generation 5
Table 2: Class Data
Number of Students with Each Genotype
ANL/ANL
ANL/anl or aril/ANL
anl/anl
Generation 1
Generation 2
Generation 3
,
Generation 4
Generation 5
Table 3: Allele Frequencies
Allele Frequencies
ANL (p)
Generation 1
Generation 2
Generation 3
Generation 4
Generation 5
ANL (q)
Activity B
Table 4: Phenotype Numbers
Number of Plants Expressing the Phenotype
Purple Stemmed
Green Stemmed
Group Data
Class Data
Allele Frequency:
Frequency of ANL(p)
Frequency of anl (q)
Genotype Frequency:
Frequency of ANLIANL (p2)
Frequency of ANL/anl (2pq)
Frequency of anl/anl (q2)
Number of Individuals:
Homozygous dominant ANL/ANL = _______
Heterozygous (ANL/anl)
Homozygous recessive (ant/anl)
Laboratory Questions
Activity A
1.
2.
3.
4.
Are the generation 5 values for p and q different from your initial values?
Is this population at equilibrium? Explain.
Which conditions of Hardy-Weinberg equilibrium might not have been met in this simulation?
In this activity, you were examining individuals' genotypes. How might an actual population
study differ?
Activity B
1. Given that the parental generation had allele frequencies of ANL (p) at 0.50 and and (q) at 0.50,
use your experimental allele frequencies to explain whether this population is evolving or not
with respect to these alleles.
2. Imagine that a population of herbivorous animals who dislike the taste of anthocyanin is
introduced into the environment of the Fast Plants. How would this affect the allele frequencies
in the Fast Plants population? Over time, what do you think would happen?
3. Explain what you would expect to see in allele frequencies if a population were evolving.
4. Consider your activity with Fast Plants. Explain why natural selection acts on phenotypes rather
than on genotypes?
5. Imagine that you are a member of a committee assigned to evaluate a report of neurofibromatosis, a
disorder inherited through a dominant allele. Neurofibromatosis is a condition in which noncancerous
tumors grow along nerves. The report concludes that because three-quarters of the offspring of parents
who are heterozygous for neurofibromatosis will have the disorder, eventually, 75% of the population
will have neurofibromatosis. Do you agree or disagree with the report's conclusion? Explain your
reasoning.
Inquiry Activity
In the previous activities you examined a simulated population as well as an actual population of
Wisconsin Fast Plants®. Based on the activities that you just completed, develop a question that tests
the conditions of Hardy-Weinberg equilibrium or an evolutionary process. When developing an
experimental question, consider the materials and equipment available to you. Consult your instructor
for the availability of additional supplies.
Materials
F2 Non-Purple Stem, Yellow-Green Leaf Wisconsin Fast
Plants®
Purple Stem, Hairy Wisconsin Fast Plants®
Non-Purple Stem, Yellow-Green Leaf Wisconsin Fast Plants®
8-oz plastic container
plant stakes
dried bees
plant labels
soil
watering pipets
Genetics Cards
24-oz plastic container
cotton wicks
fertilizer pellets
1. As
a group,
collaborate
come upConsult
with a question
about the conditions of Hardy-Weinberg equilibrium
Other
materials
may betoavailable.
your teacher.
or any evolutionary process that may be tested. If you have trouble, ask your teacher for guidance.
Procedure
2. Design an experiment to test your question. Consider the following as you frame your experiment:

Question - What are you testing in your experiment? What are you trying to find out?

Hypothesis - What do you think will happen? Why do you think so? What do you already
know that helps support your hypothesis?

Materials - What materials, tools, or instruments are you going to use to find the answer to the question?

Procedure - What are you going to do? How are you going to do it? What are you measuring? How can
you make sure the data you collect are accurate? What are the independent and dependent variables in
this experiment? What is/are your control(s)? What safety practices do you need to use?

Data Collection - What data will you record, and how will you collect and present it? Show and
explain any data tables and graphs that you plan to use.
3. Have your teacher approve the experimental procedure before you begin the exercise.
4. After you perform the experiment, analyze your data:

Data Analysis - What happened? Did you observe anything that surprised you? Show and explain
any
tables and graphs that support your data.

Conclusion - What conclusions can you draw from the results of your experiment? How does this
compare with your initial hypothesis? Identify some possible sources of error in your
experiment. If given the opportunity, how might you conduct the experiment differently?
5.
Be prepared to present the findings of your experiment to the class according to your
instructor's specification.
Experimental Design Template
Part A: To be completed and approved before beginning the investigation
What question will you explore?
On the basis of your previous laboratory exercise, background knowledge, and research, what is the
hypothesis that you will test?
What will be the independent and dependent variables?
What will be the control group(s)?
What equipment and materials will you need (list items and quantity)?
What procedure (step-by-step) will you follow?
What safety steps will you follow (equipment and procedures)?
How will you collect data?
How will you analyze data?
Teacher approval to begin your investigation:
Part B: To be completed during or after your investigation
What changes or modifications have you made to the investigation?
Attach any data collection or analysis as instructed by your teacher.
What results did you see in the experiment?
Was the hypothesis accepted or rejected? What conclusions can you draw on the basis of the data and
analysis?
What sources of error may have existed, and how might the experiment have been conducted
differently? What additional questions arose from the experiment?
Big Idea Assessments
1.
In addition to the designation of an A, B, AB, or 0 blood type, a positive or negative is also typically
denoted. This symbol corresponds to the Rh factor, specifically the D Rh factor. People are said to be Rh+
if they have a dominant D allele, whereas people are said to be Rh— if they are homozygous recessive.
Given the following data about a population of 6000 individuals and that the population is in HardyWeinberg equilibrium, determine the number of individuals who are heterozygous for this allele.
Number of individuals
2.
Rh+
Rh-
4680
1320
Sickle-cell trait is a blood disorder in which a recessive allele results in the production of an abnormal
hemoglobin molecule. People who are homozygous recessive for this trait have the disease sickle-cell
anemia; however, individuals who are heterozygous for this trait have an increased resistance to malaria
over people with normal hemoglobin. Given that 2% of individuals in a population are affected with
sickle-cell anemia, what percentage of individuals would have higher malaria resistance?