Download Bio 475 Evolutionary Biology

Bio 475 Evolutionary Biology
Population Genetics Lab
Team Assignment #4: Modes of Selection Team Assignment
Background:
It is possible for natural selection to affect allele and genotype frequencies in several ways depending
on the mode of natural selection occurring within the population. Different modes of natural selection
can select for or against a particular trait, thus affecting the balance of phenotypes in the population. For
example, in a mode of natural selection called directional selection, selection favors individuals with
one of the extreme phenotypes--a phenotype at either end of the range of phenotypes. For example, all
black or all white moths are at the extremes while brown moths represent an intermediate phenotype. In
stabilizing selection, extreme phenotypes are selected against and intermediate phenotypes have higher
rates of survival and reproduction. Conversely, diversifying (disruptive) selection favors individuals
with a range of extreme phenotypes over individuals with an intermediate phenotype.
In the following experiment you will investigate how fitness--the probability that a particular phenotype
will survive and produce offspring --affects changes in allele frequency in the population.
When you have completed this exercise you should have a good understanding of what the different
modes of selection are, how and why each occurs and how allele and genotype frequencies are
affected in a population.
Before beginning this investigation be sure to read about modes of selection in your textbook,
section 9.6 on pages 350-355. You will also want to look up information on the founder effect.
Investigation into the modes of selection
Using the instructions given on the next page, investigate the effects of different modes of
selection. As you investigate the various outcomes of your experiments, keep in mind the questions
listed below. It is expected that you will become experts on the answers to these questions. The
examination questions that you write and turn in as part of this assignment should be based on the
answers to these questions.
You will need to do some research in your textbook on modes of selection so that in your portion
of the lab report you will be able to address what occurs in the different modes of selection and how they
come about. You also need to address how modes of selection are important in evolution. When you
teach the class you will need to address BOTH the textbook and other outside material as well as the
results of your experimentation in the population genetics lab.
Questions which you must address in your investigation and which you will use to prepare your inclass presentation are:
 What happens to allele frequency, genotype frequency and heterozygosity in the case
of directional selection? Does directional selection for dark color produce the same
result as directional selection for light color?
 What happens to allele frequency, genotype frequency and heterozygosity in the case
of balancing or stabilizing selection?
 What happens to allele frequency, genotype frequency and heterozygosity in the case
of diversifying or disruptive selection?
Modes of Selection Team Assignment: PopGen Lab
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Bio 475
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Are small differences in fitness effective in changing allele frequencies? (Shown
through experiments with varying proportions of tree types )
Under what mode of selection might genetic variation be maintained? Under what
mode of selection might there be a "founder effect," where the final allele frequency
depends on the initial allele frequency? Use examples from your experiment to show
this in your presentation.
Include any other information in your report that you feel will help you explain modes of selection to
your reader.
Instructions for lab activity:
Begin an experiment with default conditions for Hardy-Weinberg equilibrium for all parameters
except the number of tree stands and genotype frequencies. You will be changing the distribution
of tree types in the stand to set up conditions for directional, stabilizing and disruptive selection.
Set the number of stands to 100 for all experiments that you run.
1. a) First, set the number of stands at 100 and use the default values for all other
parameters except distribution of tree types.
Run the experiment and review the results of a population in equilibrium.
b) Choose a new distribution of tree types that you believe would lead to directional
selection (In this experiment you are essentially changing the fitness level of the moths).
Run the experiment and examine what happens to population size, allele and genotype
frequencies and heterozygosity.
Make sure to test for directional selection for both white and black moths.
Take careful notes on what you find.
Be sure to check what is happening with the individual stands as well as the average.
c)Now changing only the original genotype frequencies run the same experiment again.
Try several different beginning genotype frequency combinations and compare to what
happened when you used the original default values.
Finally, run the experiments again as you fiddle with the distribution of tree types that
are available. Make sure the distribution you select is still likely to cause directional
selection but see if you can find how much of a change in tree type distribution is actually
needed to lead to directional selection.
2. Do exactly as you did in number 1 a, b, and c above but set the experiment up to test for
stabilizing selection.
3. Again repeat the experiment as above but this time set up for disruptive selection.
Use the results of these experiments and the textbook information to prepare answers to the questions on the
previous page. You will be teaching what you learned to your classmates.
One last word: When you teach your classmates they will need to understand what you have learned just as
well as you do. Remember that each of you is responsible for learning about all parts of the Population
Genetics Lab. You are relying on each other to you for the exam on this material. Teach your section and do
not just report some facts facts that you discovered. Be prepared to illustrate your points using the PopGen Lab.
Modes of Selection Team Assignment: PopGen Lab
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Bio 475