Bottlenecks and Founder Effects



Compiled by Jennifer Burcroff

Modified by Keith Ives

Genetic Drift Simulation

Goal: Explore the effects of genetic drift on a sample population.

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Background:

In addition to natural selection, one of the mechanisms of evolution is genetic drift. Genetic drift is the change of allelic frequencies in the gene pool due to random sampling. Genetic drift is strongly tied to population size and becomes important in situations where a small segment of an original population splits off or becomes isolated from the parent population. A change in allele frequencies as a result of this migration of a small subgroup of a population is known as ‘founder effect’. Other times this process occurs when the original population undergoes a devastating population reduction (i.e. by natural disaster) and the allele frequencies are changed as a result. This is referred to as the ‘bottleneck effect’. Evolution at the population level is defined as a change in allele frequency over time. This lab will demonstrate how genetic drift can be a strong evolutionary force in certain situations. We will see that a smaller population size can show a much more dramatic shift in allele frequencies over time due to these two events.

Prior Knowledge:

• Students should be familiar with the concept of random sampling and sample size.

• Students should understand how changes in alleles frequencies lead to changes in the gene pool of the population (they should also know what an allele is).

• Students should be familiar with the basic concepts of evolution.

• Students should know the biological definition of a species and have a basic grasp of methods of speciation.

An example of the basic idea using marbles…

Population bottlenecks occur when a population’s size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population’s genetic variation by a lot, even if the bottleneck doesn’t last for very many generations.

This is illustrated by the bags of marbles shown below, where, in generation 2, an unusually small draw creates a bottleneck.

What happened to the allelic frequency of the lightest phenotype (yellow) between generation two and generation three? Due to the small draw and random chance, the lightest phenotype was lost from the population between generations 2 and 3. This created a major change in the gene pool.

Why genetic variation is important. Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources, because the genetic variation that selection would act on may have already drifted out of the population.

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Vocabulary

• Genetic drift

• Gene pool

• Allele (allelic frequency)

• Evolution

• Speciation

• Genetic Variation

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Materials

• Students will collect data in partners or small groups. Each group will need 100 items representing individuals in a population (the example above uses marbles). It is suggested that teachers provide 3 different “phenotypes” of the item. The initial population should have about 30 of two phenotypes, and 40 of one phenotype. [pic]

Procedure

Safety: This lab has no safety concerns.

Set up and Data collection:

1. Each group needs to record the initial phenotype composition of their original population (these populations can be pre-made by the teacher, or the teacher can provide instructions to the students).

2. Students will draw random samples from the population. Students will be doing two separate simulations with their material (a smaller sample of about 10 for simulation one, and a large draw of about 40 for the second simulation).

a. Making the draw random: Students can close their eyes and count out the correct sample size. We are not testing natural selection, but genetic drift. If they are looking at the beads we can not rule out selection as a force.

3. Students will record the phenotypic ratio from the draw and then RETURN the sample back to the original population. Returning the sample back to the original population is important so that each trial has the same phenotype composition as the original.

4. Students will repeat steps 2 and 3 for at least three separate trials. For each trial they will record their data and calculate the phenotypic ratio for each trial.

Data Tables: 14pts.

|Simulation 1: Small Founder |Original Population |Trial #1 |Trial #2 |Trial #3 |

|Effect | | | | |

|Phenotype |# Individuals |(%) |# Individuals |(%) |# Individuals |(%) |# Individuals |(%) |

| | | | | | | | | |

| | | | | | | | | |

|Totals = | | | | |

|Simulation 2: Large Bottleneck|Original Population |Trial #1 |Trial #2 |Trial #3 |

|Effect | | | | |

|Phenotype |# Individuals |(%) |# Individuals |(%) |

1. Count out the numbers of each phenotype in your large population (container received from the teacher). Determine your phenotypic percentage.

2. Randomly take a sample from the population (roughly 10-15 for Founder Effect roughly 40-50 for Bottleneck Effect).

3. Determine the ratio in the sample- did any of the allelic frequencies change?

4. Repeat three times (after returning the sample back to your main population to ‘restock’ each time).

5. On the back of your data table create 2 bar graphs (for all three trials for bottleneck and founder effect) of the change in each allele. Have one bar for before and one bar for after the effect. Analyze the change. Be sure to include a proper title, labels, and units.

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Analysis Questions: 16pts

Pre-lab

1. Define genetic drift, founder effect, and bottleneck effect in your own words.

2. How can genetic drift lead to evolution of a population?

3. Predict: Will the effects of genetic drift on the gene pool differ with sample size?

4. Look over the procedures: Why does the teacher want you to take random samples? Would you be illustrating genetic drift if you selected the individuals non-randomly?

Post-Lab

5. Types of genetic drift: Bottlenecks and the Founder Effect.

a. What type of genetic drift are you simulating if all the other beads die out? Explain.

b. What type of genetic drift are you simulating if your sample leaves the main population and becomes reproductively isolated? Explain.

6. Compare your results from simulation I and II. Did genetic drift have more of an effect on the allelic frequencies in the larger or smaller samples?

7. An environmental change gave one phenotype an adaptive advantage over the others, increasing its fitness. This would cause a shift in allelic frequencies as the fittest beads are best able to survive and reproduce. Would this be an example of genetic drift? Why or why not?

8. Could genetic drift eventually lead to speciation? Explain why or why not.

Graphs: 20pts.

[pic]

Answers:

1. See below

a. Genetic drift is the change of allelic frequencies in the gene pool due to random sampling.

b. Founder effect is…

c. Bottleneck effect is…

2. Evolution is decent with modification. This genetic drift can lead to changes in allelic frequency in a gene pool (modification) which will then be passed on to future generations (decent). Changing the allelic frequencies of a population is evolution.

3. Students should predict that the smaller sample size will have a greater influence.

4. Genetic drift is defined as the effect of random sampling. Some students may link non-random sampling with natural selection.

5. See below

a. Founder effect

b. Bottle neck.

6. Students should see a greater change in phenotypic ratios when smaller samples are taken.

7. This is a description of natural selection because one trait has an advantage over another. In order to be genetic drift, the selection must be random.

8. If genetic drift lead to enough changes in a populations gene pool that it could no longer reproduce with the original population than speciation would occur. Speciation is the formation of a new species when one population can no longer reproduce and create fertile offspring with the original population.

Teacher notes:

I used M&Ms the first time. For 70 students, groups of 3-4, 19 groups, with 100 M&Ms per group I went through 6-7 bags. Also, need new cups and gloves to maintain sanitation.

Ways in which this lab could be modified

|Test Natural Selection instead of |This lab could be modified to simulate natural selection instead of genetic drift. Selection would not be random (students |

|genetic drift. |could look at the population). |

| | |

| |Example: Students could simulate selection of predators on bean populations living on brown paper or white paper (different |

| |environments). |

| |Students would work in partners- one student would be the predator while the other would be the time keeper. |

| |The time keeper would lay out the bean population on one of two environments (brown or white paper). They would then time the |

| |predatory student for about 15 seconds as the predator tries to select as many beans as they can within that time frame. |

| |Students should do multiple trials for each environment- returning their beans to the original population between each trial. |

| |Alternately, students could test across generations by not returning their selected beans to the population (perhaps each bean |

| |could double between each generation to simulate reproduction). How does the phenotypic ratio change over time? |

|Inquiry |This lab could be made more inquiry based by making some or all of the following changes. |

| |Have students create their own labs or tables. |

| |Provide the materials section and the table, but have students write out the procedure. |

| |Have students generate a hypothesis involving the effects of genetic drift on the gene pool and have them develop their own |

| |simulation to test that hypothesis (perhaps provide a suggested materials list). |

| |Have students share their proposed simulation with the class, discuss as a class the merits and problems with each design, and |

| |then come up with a set of procedures and data table as a class. Then each group could perform the class experiment and share |

| |data. |

| |Alternately each group can get their simulation approved by the teacher before they do their own trials. |

|Mixed Inquiry |This could be a two part lab. Students could do either the natural selection or genetic drift simulation with teacher guidance|

| |(or as a cookie cutter lab). For the second part of the lab students could design their own experiment to test the remaining |

| |mechanism of evolution (either natural selection, or genetic drift). |

Student Handout: Genetic Drift Simulation

Background:

In addition to natural selection, one of the mechanisms of evolution is genetic drift. Genetic drift is the change of allelic frequencies in the gene pool due to random sampling. This simulation will explore how sample size can influence the effect on genetic drift on the gene pool using different types of beans to represent individuals in the population.

Vocabulary

• Genetic drift

• Gene pool

• Allele (allelic frequency)

• Evolution

• Speciation

• Genetic Variation

Materials

• 100 beans; 30 kidney, 40 black, and 30 pinto

• One cup (this will be used to hold your beans.

Safety

• Choking hazard: Do not put the beans in your mouth! Do not eat the beans!

Pre-lab Questions: Read the procedures before you answer the pre-lab questions.

1. Define evolution.

2. Define genetic drift.

3. What do I mean when I say that I saw a change in “allelic frequencies”?

4. Could genetic drift lead to evolution of a population? Explain.

5. Predict: Will the effects of genetic drift on the gene pool differ with sample size? Explain.

6. Look over the procedures: Why does the teacher want you to take random samples? Would you be illustrating genetic drift if you selected the individuals non-randomly?

[pic]

Procedure

• You will be working in small groups.

• Each group will do two simulations each with three separate trials.

• You will need a cup filled with your bean population. This population will consist of 100 beans as described in the materials section.

• Read the procedure carefully before starting your trials.

Simulation I (Large sample):

| |Original Population |Trial #1 |Trial #2 |Trial #3 |

|Phenotype |# Beads |Ratio(%) |# Beads |Ratio(%) |# Beads |Ratio(%) |# Beads |Ratio(%) |

| | | | | | | | | |

| | | | | | | | | |

| Bead Totals = | | | | |

1. Count out the numbers of each phenotype in your large population (container received from the teacher). Determine your phenotypic ratio (depict your ratios of each phenotype as percentages).

2. Randomly take a sample from the population (roughly 40-50).

3. Determine the ratio in the sample- did any of the allelic frequencies change?

4. Repeat three times (after returning the sample back to your main population to ‘restock’ each time).

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Simulation II (small sample):

• For simulation II follow the same procedures as in simulation I above, but use a smaller sample size of 10 beans.

• Record your data in the table below.

| |Original Population |Trial #1 |Trial #2 |Trial #3 |

|Phenotype |# Beads |Ratio(%) |# Beads |Ratio(%) |# Beads |Ratio(%) |# Beads |Ratio(%) |

| | | | | | | | | |

| | | | | | | | | |

| Bead Totals = | | | | |

[pic]

Post-lab Questions

1. Compare your results from simulation I and II. Did genetic drift have more of an effect on the allelic frequencies in the larger or smaller samples?

2. An environmental change gave one phenotype an adaptive advantage over the others, increasing its fitness. This would cause a shift in allelic frequencies as the fittest beads are best able to survive and reproduce. Would this be an example of genetic drift? Why or why not?

3. Could genetic drift eventually lead to speciation? Explain why or why not.

Student Handout: Natural Selection Inquiry Simulation

Background:

In addition to natural selection, one of the mechanisms of evolution is genetic drift. Genetic drift is the change of allelic frequencies in the gene pool due to random sampling. This simulation will explore how sample size can influence the effect on genetic drift on the gene pool using different types of beans to represent individuals in the population.

Vocabulary

• Natural Selection

• Gene pool

• Allele (allelic frequency)

• Evolution

• Speciation

• Genetic Variation

Suggested Materials

• Beans

• Paper

• Timer

• Cups

• Spoons

• Tweezers

•

Safety

• Choking hazard: Do not put the beans in your mouth! Do not eat the beans!

Pre-lab Questions: Read the procedures before you answer the pre-lab questions.

1. Define evolution.

2. Define natural selection.

3. What four pre-conditions must be present in order for evolution by natural selection to occur?

Procedures

Your group will be creating your own simulation to illustrate the concept of natural selection. You will be testing this idea in bean populations (but you are not limited to the phenotypes we used in the last simulation).

1. On a separate sheet of paper, write out your planned procedures for this simulation. Be sure to consider all the characteristics that make good science. Draw diagrams if needed.

2. Get teacher approval before you do the simulation.

3. Create your own data tables to keep track of your results.

4. Run the simulation according to your procedures.

Conclusion

Answer the following questions after you perform your simulation.

1. Did your simulation produce results that supported the idea of natural selection? Why or why not?

2. Do you think your simulation could have been improved in any way? What would you do differently?

3. Compare your simulation with other groups in the class. Describe how one group’s simulation was similar or different from yours. Compare and contrast the two simulations.

4. What did you learn from this simulation?

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