Chapter 2. EVOLUTION II—SELECTION

Learning Objectives

General Purpose

Conceptual

• Define the term gene pool and explain how it relates to the concept of evolution.
• Gain an understanding of the concept of genetic equilibrium and the conditions required for it to occur.
• Gain an understanding of the relationship between genetic equilibrium and evolution.
• Gain an understanding of the Hardy-Weinberg equation and how it relates to population evolution.
• Gain an understanding of the impact of selection, mutation, and population size.
• Gain an understanding of how and why replica plating can be used to test a population against different selective pressures.

Procedural

• Be able to use the Hardy-Weinberg equation to calculate allele frequencies and the percentages of genotypes in a population.
• Be able to use replica plating technique to transfer identical populations to Petri plates.

2.1 Exercise 1

Use the PopG computer program to model the following scenario for the impact on the allele frequency in a population.

Simulation Instruction

1. Double-click on the PopG icon on the desktop.
2. Once the program opens, go to the “Run” menu and pull down to “New Run.”
3. Type in the settings for this simulation from the cases in the lab manual.
4. Make sure that the “number of simultaneously evolving populations” is set to “1.”
5. Click OK.
6. To rerun the simulation using the same settings, go to the “Run” menu and pull down to “Restart.”
7. To run a simulation using different settings go to the “Run” menu and pull down to “New Run.”

Case 1. Genetic Equilibrium

This simulation is meant to mimic the conditions required for genetic equilibrium. It establishes the baseline to which the other simulations can be compared.

PROCEDURE

Set the simulation controls to the following settings:

In the last lab, using the beans was not a case of genetic equilibrium because the population was small. Here, however, the population is large and you’ll use the same parameters you used with the beans in the last lab. Recall that those conditions were as follows:

Run the PopG simulation five times, recording the final value of P after each run in a table you create in your lab notebook, using Table 2-1 as a template.

Table 2-1. Final P value from five separate simulation runs.

The simulation graphs the expected frequency of the dominant allele (heavy line) and the actual frequency (thin line) based on random processes within the simulation.

In your laboratory notebook, answer the following questions:

Based on the results of your trials did this population exhibit genetic equilibrium?

On what do you base the answer for the previous question?

Case 2. Selection

Now that you have established a baseline you can begin to modify the simulation, making it more realistic. In this case study, the individuals with the homozygous recessive phenotype will be selected against, but only slightly, as 5% of the individuals with this genotype die before reaching reproductive maturity. These homozygous recessive individuals will be selected against and will have a fitness of 0.95 instead of 1.0. All of the homozygous dominant and heterozygous individuals will survive to maturity. These phenotypes will have a fitness of 1.0.

PROCEDURE

Set the simulation controls to the following settings:

Run the PopG simulation.

In your laboratory notebook, answer the following questions:

What happened to the frequency of the dominant allele?

Did the dominant allele reach a frequency of 1.0?

Case 3. Heterozygote Advantage

In case 2 the impact of a recessive gene with decrease fitness was examined. What happens if both of the homozygous conditions are selected against? In this case study, the individuals with both the homozygous genotypes will be selected against, but at varying rates. 5% of the individuals with the homozygous dominant genotype die before reaching reproductive maturity, and 10% of the individuals with the homozygous recessive genotype die before reaching reproductive maturity. These homozygous individuals will be selected against and will have a fitness of 0.95 and 0.9 respectively instead of 1.0. All of the heterozygous individuals will survive to reproductive age. These phenotypes will have a fitness of 1.0.

PROCEDURE

Set the simulation controls to the following settings:

Run the PopG simulation.

In your laboratory notebook, answer the following question:

How did the frequency of the dominant allele change?

Case 4. Genetic Drift

Genetic drift is a change in a gene pool that occurs purely as a result of chance. In this case study, the fitness level of all genotypes will be back at 1.0. The impact of a population that is 1/10th the size of the baseline in case 1 will be examined. Because this smaller population can exhibit quicker changes in allele frequency the number of generations will be reduced to 500.

PROCEDURE

Set the simulation controls to the following settings:

Run the PopG simulation five times. Record the number of generations until fixation and which allele became fixed in the population in a table you create in your lab notebook using Table 2-2 as a template.

Table 2-2. Data from five separate runs to allele fixation.

In your laboratory notebook, answer the following question:

How does genetic drift impact the genetic variability within a population?

Exercise 2

Replica Plating

Last lab you plated a dilute suspension of bacterial cells onto tryptic soy agar growth media in a Petri plate. Those bacterial cells produced clones by binary fission, so that all the bacteria in a colony are genetically identical to a single bacterium which was transferred onto that location on the Petri plate. Optimally, this origin stock plate will have many (20–40) bacterial colonies (and no contaminants). Replica plating will create plates that have the same bacteria as the stock plate, with the identical orientation.

PROCEDURE

1. Disinfect your workspace with 10% bleach.
2. Label your four Petri plates (control and three antibiotic concentrations) on the bottom with your section number and group identifier.
3. Prepare your replica plating device.
4. Make a single vertical mark on the rim of each Petri plate bottom. This mark will later serve as an orientation marker.
5. Obtain your original culture and make an orientation mark on the rim of the bottom half. Open the plate and align the orientation mark on the Petri plate directly toward you. Press the Petri plate gently but firmly onto the replica plating stand so that the cloth picks up cells from all colonies.
6. Carefully lift off the Petri plate and immediately close it. Open the lid of the Petri plate with the highest antibiotic concentration. Align the orientation mark on the Petri plate directly toward you and firmly press the Petri plate onto the replica plating stand. Carefully lift the Petri plate away, and close the lid immediately.
7. Proceed directly to the plate containing the intermediate concentration of antibiotic. Repeat step 6. Proceed directly to the plate containing the lowest antibiotic concentration. Repeat step 6. Proceed directly to the control plate, containing no antibiotics.
8. Seal all four plates with Parafilm. Return your original plate to your lab instructor, who will store it upside down in the lab refrigerator. The four replica plates should be returned to the lab instructor who will place them in the incubator.