Population Genetics and Evolution
Introduction
Population genetics is the study of how genetic principles apply to entire populations. Population
genetics involves studying the frequency with which a particular gene or genotype occurs in a
population. The frequency of a gene or genotype may change in a population due to selective pressure
acting on the population. Selective pressure may involve predators, food supply, and space available for
the population.
Goals
1. Determine the gene and genotype frequencies in a population
2. Demonstrate the effect of selective pressure on the evolution of a species population
Questions
1. In part I, you have no selection occurring. You start off with 40 of each type of bean. What do
you think will be your resulting genotypes and gene frequencies. My Hypothesis is
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2. In part II, you have predators eating all the dd individuals. What do you think will be your
resulting genotypes and gene frequencies. . My Hypothesis is
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3. In part III, you have predators eating all the dd individuals and having some of the DD individuals
die. What do you think will be your resulting genotypes and gene frequencies. My Hypothesis is
_____________________________________________________________________
Materials
40 black beans
40 red beans
Paper cup
Data sheet
Population sheet
Coin
Procedure
Beans will be used to represent color genes in a population of moth’s gene pool. Black beans represent
the dominant gene (D) and red beans represent the recessive gene (d).
1. Start with a population of
10 Homozygous dominant individuals (DD)
20 Heterozygous dominant individuals (Dd)
10 Homozygous recessive individuals (dd)
2. Use the appropriate genes (beans) to establish this population on your population sheetremember you must have two genes for each individual.
3. Determine the frequency for the D gene and the d gene. To do this, count the number of dark
beans (D) and the number of light beans (d). Place these numbers in the appropriate columns on
the data sheet and also total the number of genes.
4. Convert the number of each gene to a decimal from a fraction to figure the gene frequency.
5. Determine the phenotype ration by adding the number of homozygous dominant genotypes and
the number of heterozygous genotypes. This will equal the number of dark moths in the
population. The homozygous recessive genotype will equal the light moths.
6. Assume that no selective pressure is acting on the population of moths and all phenotypes have
an equal chance of surviving and reproducing. Further assume that members of the moth
population mate at random with each other, and reproduce another generation of 40
individuals. To simulate reproduction, put all the genes in the gene pool in a paper cup.
Randomly pick out two genes at a time to represent offspring. Put each pair of genes in the
appropriate column on your data sheet. When you have selected all the pairs of genes, then
record the data for generation #2 on the data sheet. Picked genes are not returned to the cup
until the next generation.
7. Repeat the above for generations #3 and #4 and record the results on the data sheet. Leave the
genes for generation #4 on the population sheet and follow the directions below.
8. Assume now that a predator, such as birds, is introduced to the area. All the trees on which the
moths rest most of the time are colored the same as the dark colored moths. Remove all the
individuals that are homozygous recessive (dd) from generation #4. The remaining individuals
are then able to reproduce in the same manner as above. Record the results for generation #5a.
9. Repeat for generations #6a-#8a by removing all of the homozygous recessive individuals from
the population.
10. Return to frequencies in generation #4. Follow the same procedure from step 8. In addition to
that, when you pick a homozygous dominant individual flip a coin. If the coin lands on a head
then you will record that individual as surviving and if the coin lands on a tail, you will record
that individual as dying before it was able to pass on its genes (do not include these individuals
for the next generation. Record the results for generation #5b.
11. Repeat for generations #6b-8b.
Questions (analysis)
1. Did the gene frequency change from generation #1 to #2?
2. Did the gene frequency change from generation #2 to #3?
3. When there is no selective pressure, what happens to the gene frequencies from generation to
generation?
4. What happened to the homozygous recessive light colored moths after the predator was
introduced to the area after generation #4? Why were the homozygous light colored moths
removed from the population?
5. What effect does selection have on gene frequencies in a population?
6. Predict what would happen to the frequencies of the genes if there was a change in the
environment that caused light colored moths to be at an advantage?
Graph
You will graph your data using Excel. First enter your data into Excel to make a data table like
the one provided or you may open the data table from the website. On graph 1, you will graph your data
on generation 1 – 8a (not 4b-8b) and the genotype. You will have three lines on this graph: one line for
DD, one line for Dd, and one line for dd. On graph 2, you will graph your data for generation 1 – 8a (not
4b-8b) and the alleles. You will have two lines on this graph: one for D and one for d. On graph 3, you
will graph your data on generation 1 – 8b (not 4a-8a) and the genotype. You will have three lines on this
graph: one line for DD, one line for Dd, and one line for dd. On graph 4, you will graph your data for
generation 1 – 8b (not 4a-8a) and the alleles. You will have two lines on this graph: one for D and one for
d. Hints for graphing- remember a title, a key, labels for each axis, and to have your graph oriented
correctly.
Lab Report
Use the attached lab report template to type a lab report (one for your group).