Download Mechanisms of Evolution

Tutorial: Mechanisms of Evolution
Introduction
There are four mechanisms that can alter the gene pool over time: mutation, selection,
genetic drift, and gene flow. In this tutorial you will use mathematical models of these
mechanisms to explore the effect that each has on allele and/or genotype frequencies.
Requirements
To use this tutorial you will need to have Microsoft® Excel® available on your computer.
You will need to download the Excel® workbook from the previous page.
Some equations used in the workbook are on the diagram sheets for lectures 23 and 24.
Instructions
The workbook consists of four worksheets, one for each of the four mechanisms
(mutation, selection, genetic drift, and gene flow) that you will be exploring. You can
switch between the worksheets by clicking on the tabs at the bottom of the screen.
For each mechanism there are several parameters that you can change to see how they
affect allele and genotype frequencies over time. The cells with these parameters are
highlighted in green. If you try to change any of the other cells in the worksheet you will
see a message saying that the cell is “protected”. This is to prevent you from accidently
changing the formulas that the worksheet uses for its calculations.
The tutorial will take you through each of the four mechanisms in turn.
Mutation
There are three parameters that you can adjust in this worksheet.
• Allele Frequency is the initial frequency of the “A” allele. It will be a number between
zero and one.
• Creation Rate is the rate at which mutation forms the “A” allele. Typically this would
be between 1 x 10-6 and 1 x 10-8. Note that the worksheet takes the number you type in
here and divides it by one million before using it in the calculations. That means that
you can type “1” instead of “0.000001”.
• Elimination Rate is the rate at which mutation converts the “A” allele into something
else. Since there is only one way to create the “A” allele and numerous ways to change
it into something else, this number will typically be larger that the Creation Rate.
Mutation as a means of change
Set the Allele Frequency to 0.5, the Creation Rate to 1, and the Elimination Rate to some
other value.
Does mutation change allele frequencies rapidly or slowly?
Try other values for the Creation Rate.
Mutation as a means of generating alleles
Set the Allele Frequency to 0, the Creation Rate to 1, and the Elimination Rate to 10.
Does the “A” allele frequency change?
Try other values for the Creation Rate and the Elimination Rate.
Does the “A” allele frequency rise steadily or does it reach an equilibrium value? Does
this happen rapidly or slowly? What determines the final value?
Selection
There are four parameters that you can adjust in this worksheet.
• Allele Frequency is the initial frequency of the “A” allele. It will be a number between
zero and one. The worksheet calculates the frequency of the “a” allele and the genotype
frequencies expected from Hardy-Weinberg equilibrium.
• Relative Fitness allows you to enter the relative fitness values for the “AA” genotype
(WAA), the “Aa” genotype (WAa), and the “aa” genotype (Waa). These will be
numbers between zero and one. The most fit genotype must have a value of one.
Results of different fitness scenarios
Set the Allele Frequency to 0.5. Set the Relative Fitness values to answer the following
questions.
Assume that “A” is dominant and “a” is recessive.
What happens if the “a” allele is least fit (i.e., the “aa” genotype is least fit)? Do the allele
frequencies reach an equilibrium or is one allele lost?
What happens if the “A” allele is less fit (i.e., the “aa” genotype is most fit)? Do the allele
frequencies reach an equilibrium or is one allele lost?
Under which of these two circumstances do allele frequencies change more rapidly?
What happens if the “Aa” genotype is most fit? Does it matter whether the “AA” and
“aa” genotypes have the same or different fitnesses? Do the allele frequencies reach an
equilibrium or is one allele lost? What determines the final values?
What happens if the “Aa” genotype is least fit? Does it matter whether the “AA” and “aa”
genotypes have the same or different fitnesses? Do the allele frequencies reach an
equilibrium or is one allele lost? What determines the final values?
Do the changes observed occur more or less rapidly than changes by mutation?
Set the Allele Frequency to zero or one. Set the Relative Fitness values as you wish.
What happens to the allele frequency? What mechanism would change this pattern in real
life?
Try other values for the Allele Frequency and Relative Fitnesses.
Genetic Drift
There are three parameters that you can adjust in this worksheet.
• Allele Frequency is the initial frequency of the “A” allele. It will be a number between
zero and one.
• Population Size allows you to enter sizes for two different populations. The minimum
population size is five.
Randomness of genetic drift
Set the Allele Frequency to 0.5. Set the Population Size to 10 for both populations.
Do the two populations behave in the same way? Have the worksheet recalculate (press
Command-= in Mac OS or F9 on Windows). Did the result change?
Effect of population size
Set the Allele Frequency to 0.5. Set the Population Size so that one population is small
(between 5 and 20) and the other population is large (at least 100).
Which population showed more dramatic variation? Was one of the two alleles lost?
Have the worksheet recalculate ten to twenty times, recording each time whether one of
the alleles was lost. Which population (small or large) is more likely to lose an allele by
genetic drift?
Effect of initial allele frequency
Set the Allele Frequency to 0.1. Leave the Population Size values unchanged.
Have the worksheet recalculate ten to twenty times, recording each time whether one of
the alleles was lost. Compare your results with your previous data set. Are alleles more or
less likely to be lost when the initial allele frequency is high or low?
What does this say about new alleles arising by mutation in small populations?
Try other values for the Allele Frequency and Population Sizes.
Gene Flow
There are three parameters that you can adjust in this worksheet.
• Allele Frequency is the initial frequency of the “A” allele. It will be a number between
zero and one.
• % Migrants is the percentage of migrants in the population each generation. This sets
the rate of gene flow. It will be a number between 0 and 100.
• Migrant Frequency is the frequency of the “A” allele in the population from which the
migrants came. It will be a number between zero and one.
Effect of difference in allele frequencies
Set the Allele Frequency to 0.6. Set the % Migrants to 10. Set the Migrant Frequency to
various values.
What happens to the allele frequency? How does the initial difference between the allele
frequency in the population and the allele frequency in the population from which the
migrants came affect the rate of change and the end result?
Effect of rate of gene flow
Set the Allele Frequency to 0.6 and the Migrant Frequency to 0.8. Set the % Migrants to
various values between 0 and 100.
What happens to the allele frequency? How does the percentage of migrants in the
population each generation affect the rate of change and the end result?
Try other values for the Allele Frequency % Migrants, and Migrant Frequency
parameters.