Download Biology 2108 Laboratory Exercises: Variation in

Biology 2108
Laboratory Exercises: Variation in Natural Systems
Ed Bostick
Don Davis
Marcus C. Davis
Joe Dirnberger
Bill Ensign
Ben Golden
Lynelle Golden
Paula Jackson
Ron Matson
R.C. Paul
Pam Rhyne
Gail Schiffer
Heather Sutton
Kennesaw State University
Department of Biology and Physics
LABORATORY 1
Evolution: Genetic Variation within Species - Part 1
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OVERVIEW OF LAB
Over the next two weeks you will investigate factors that influence genetic variation within species. You will examine
traits known to be determined by genes. You will test the same hypothesis in two very different ways. Testing a single
hypothesis in different ways can greatly increase the level of confidence in the conclusions. The general hypothesis
to be investigated is:
Environmental conditions cause changes in gene frequencies.
This week you will analyze a laboratory experiment that tests the effect of a specific environmental change on a single
set of traits (fruit fly wing shape). In this lab, the experiment has already been performed and you will tabulate flies
based on wing shape, then analyze these results.
BACKGROUND FOR LAB
The above hypothesis is a test of causes for evolution. Evolution is quite simply a change in frequency of alleles in a
population over time. An allele is one of two or more states of a gene. We will consider four major mechanisms that
may result in evolution. More than one mechanism may influence a population at one time.
Natural Selection - Differential survival and reproduction of genotypes.
Migration - The arrival and incorporation of new individuals from another population.
Genetic Drift - Random fluctuations in allelic frequencies due to chance (expected to have significant impact only
when the population is small).
Mutation - The creation of an allele due to a random change in DNA structure (mutation rates are usually very low for
a given gene, though can increase upon exposure to radiation and certain molecules).
INTRODUCTION
The Introduction should state the general hypothesis to be addressed (see above), make a brief statement
summarizing the experiment to indicate how the hypothesis will be tested, and formulate specific predictions of
possible results. The Introduction should set the context for your experiment by briefly providing background
information from previous studies, and then state what additional information your report may provide. Be sure to give
proper citations when you state facts or ideas from outside sources. Specifically state your hypothesis at the end of
the Introduction. The following questions should always be addressed in the Introduction:
1. Why was this study performed?
2. What knowledge already exists about this subject?
3. What is the specific purpose of the study?
Portions of the Introduction for this lab are completed for you below and must be included in your submitted
document.
1. “Darwin (1859) noted that individuals in natural populations must compete for survival and the chance to
reproduce. He proposed that those individuals better suited for the environment will survive, reproduce, and
pass their genes to their offspring while those less suited will not do as well.”
2. “In this lab we tested the general hypothesis that… “
3. “In order to test this hypothesis we used…”
4. “The specific predictions tested were that…”
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METHODS AND MATERIALS
The Methods section should be written so that other researchers could duplicate your experiment.
In this report, the Methods section is written for you. The experiment has already been set up and initiated (described
in the first paragraph of this section). You will tabulate flies based on wing shape and analyze the results (described in
the remaining paragraphs of this section). Note that the entire section is written in past tense, as it would appear in a
lab report or published paper. You may copy the following text into your submitted report:
To test our prediction, we used the fruit fly, Drosophila melanogaster. Our test populations have two types of wings -normal and vestigial. Vestigial wings are tiny and do not allow flight. These phenotypic variations are determined
genetically from both alleles of one autosomal gene. To change the independent variable in this test of the hypothesis,
we introduced sticky paper as the environmental factor which could give one phenotype an advantage over the other.
The two cases of the independent variable are: population cage A without sticky paper and population cage B with
sticky paper.
To measure the dependent variable, phenotypic frequencies were determined by examining samples of flies from
each population cage four generations after the population cages were established. The number of normal wing flies
and the number of vestigial wing flies were tabulated for each cage. Between 100 and 125 flies were tabulated in
each cage.
Both cages were started with parents that were heterozygotes for normal wings and vestigial wings. Normal wings
are dominant to vestigial wings. Let "p" represent the frequency of the dominant allele (in this case, normal wings)
and let "q" represent the frequency of the recessive allele (in this case, vestigial wings). The genotype frequency of
the heterozygous parents for these populations was "2pq". Fifty heterozygous flies were added to each cage at the
beginning of the experiment.
We tested the specific hypothesis by comparing the phenotypic frequencies observed in each cage after four
generation to the frequencies expected if the populations were in Hardy-Weinberg equilibrium. A population is in
Hardy-Weinberg equilibrium when gene frequencies remain constant generation after generation (i.e. no evolution)
and equilibrium is expected to occur if there are no evolutionary processes acting on the populations (e.g. mutation,
migration, natural selection, and genetic drift). In order to compare observed phenotypic numbers to expected
phenotypic numbers, we used the following equations to calculate expected phenotypic frequencies after four
generations from the known allelic frequencies of the flies at the beginning of the experiment. Because we are dealing
with a population with only two alleles for this trait:
p+q=1
By squaring each side of the equation, the following equation was derived based on random mating in order to
calculate expected genotypic frequency under conditions for Hardy-Weinberg equilibrium:
p2 + 2pq + q2 = 1.0
where:
p2 = the frequency of the homozygous dominant individuals
2pq = the frequency of the heterozygous individuals
q2 = the frequency of the homozygous recessive individuals
Because the original population in this experiment was composed of only heterozygous individuals, one-half (0.5) of
the genes are "p" and the other half (0.5) are "q". The "p" allele is dominant, so that the phenotypic frequency of
normal wing flies is the sum of the first two terms in the above equation (p2 + 2pq). The "q" allele is recessive, so
that the phenotypic frequency of vestigial wing flies is the third term in the above equation (q2).
A chi-square test was used to compare observed and expected number of flies. To compare the expected frequencies
to the observed number of flies tabulated from the cages, expected frequencies were converted to expected number
of flies. This was done for each phenotype in each cage by using the following equation:
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expected number of
flies of a given genotype
= the expected frequency of
a given genotype
from that cage
X
the total number of
flies tabulated
By comparing the observed frequencies after four generations to those expected under Hardy-Weinberg equilibrium in
Cage B, we test whether sticky paper is causing genetic change within the population. By comparing the observed
frequencies after four generation to those expected under Hardy-Weinberg equilibrium in Cage A, we test whether
variables other than the sticky paper are influencing genetic change within the population. Other variables were
controlled in this investigation by keeping the amount of food, amounts of egg-laying space provided, temperature,
and light the same in each cage. Also, cages were designed to prevent flies from entering or leaving the cage.
RESULTS
Table 1. Comparison of Observed and Expected Wing Phenotypes in Population Cage A
Calculated Expected
Calculated Expected
Actual Observed Number
Wing Phenotype
Frequency based on H-W Number
based on H-W
Normal wing
Vestigial wing
Total
Chi-square p-value =
Table 2. Comparison of Observed and Expected Wing Phenotypes in Population Cage B
Calculated Expected
Calculated Expected
Actual Observed Number
Wing Phenotype
Frequency based on H-W Number
based on H-W
Normal wing
Vestigial wing
Total
Chi-square p-value =
Your Results section should contain the following bar graphs:
Fig. 1. Comparison of Observed and Expected Wing Phenotypes in Population Cage A
Fig. 2. Comparison of Observed and Expected Wing Phenotypes in Population Cage B
Finally, your Results section should contain a short paragraph that describes the results summarized in your
tables and graph.
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DISCUSSION
Refer to your previous lab reports and A Short Guide to Writing About Biology as you write this section. Specifically,
you should use the results from the chi-square test to address whether the data from each cage deviate from the
frequencies expected under Hardy-Weinberg equilibrium. If so, be sure to identify the evolutionary mechanism which
is likely to explain the difference between observed and expected, and clearly explain how this mechanism could have
caused this difference. Discuss why you believe other evolutionary processes are not acting on these populations.
LITERATURE CITED
List any literature/references you have specifically cited within the report. Consult Chapter 4 of A Short Guide to
Writing About Biology by Jan A. Pechenik for more information on proper citation formats.
You must have the following reference in your submitted report. You are welcome to add additional references as
long as they are relevant to the report and cited within the text
Darwin, C. 1859. On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in
the Struggle for Life. John Murray, London.
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