4.1: Natural Selection Lab

Natural Selection

Format: In-person or online

Authors: Julie Wieczkowski, Susan Maguire, Melanie M. Mayberry, and Lisa Marie Anselmi

Time needed: 30-40 minutes

Supplies Needed

• Calculator

Readings

• Alveshere, Andrea J. 2019. Chapter 4: Forces of Evolution. Explorations.
•  
• Coty-Barker, Valencia. How to Form a Hypothesis.

Introduction

This lab allows students to see the impact of a selective agent, an insectivorous primate, on a population of ants. There are green and brown ants present in the population. In the rainy season, the primate eats the ants from green leaves. In the dry season, the primate eats the ants from brown branches. Calculating allele frequencies for several generations of ants in both seasons, the students will see how allele frequencies change in response to the selective pressure.

Steps

• Review dependent and independent variables, as well as key content from chapter four including natural selection, selective pressures, allele frequencies, and microevolution.
• Distribute student handouts. Students can work through the handout independently or in small groups.

Conclusion

There are things in the environment (selective pressures or agents) that select for or against certain phenotypes. These selective agents can cause allele frequencies to change, thereby causing evolution (microevolution in this population) to occur.

If the environment changes, the phenotype that is selected for may now be selected against. This illustrates that evolution does not proceed in only a one-way direction. This example also illustrates an important point about natural selection: there must be individual-level variation present in the population in order for natural selection to work.

Adapting for Online Learning

1 Not adaptable 2 Possible to adapt 3 Easy to adapt

Students can work through this activity individually as an assignment or in small groups (synchronously online). Students will need a clear introduction to the concepts ahead of time, and time to ask questions if students have them.

References

Alveshere, Andrea J. 2019. “Chapter 4: Forces of Evolution.” In Explorations: An Open Invitation to Biological Anthropology, edited by Beth Shook, Katie Nelson, Kelsie Aguilera, and Lara Braff. Arlington, VA: American Anthropological Association. http://explorations.americananthro.org/

Helmenstine, Anne Marie. 2019. What are Independent and Dependent Variables? Thought Co. https://www.thoughtco.com/independen...xamples-606828

Coty-Barker, Valencia. July 10, 2013. “How to Form a Hypothesis.” YouTube video. https://www.youtube.com/watch?v=bp2f...ature=youtu.be

Image Attributions

Marmoset eating a grasshopper by Tambako the Jaguar is designated under a CC BY-ND 2.0 license.

Natural Selection: Worksheet

Imagine your anthropology professor studies a primate that eats insects. The primate’s favorite food is a type of ant. She asks if you are interested in helping her analyze the data. You, of course, say yes!

Your professor tells you more about the ants, the primates, and the habitat. Ant color is determined by one gene. There are two alleles for this color gene: the brown allele and the green allele. The primates eat the ants year-round, but your professor knows that the way they eat the ants differs between the rainy season and the dry season. During the rainy season, when there are green leaves on the trees, the primates eat the ants off the green leaves. During the dry season, when the trees lose their leaves, the primates eat the ants off the brown tree branches.

Your professor has collected data to investigate if the primate acts as a selective pressure on the ant population, possibly changing the allele frequencies over time.

Step One: Hypothesis

You need to start with a hypothesis. The following questions will help you to frame your hypothesis.

1. How do the ants vary?
2. What is the selective pressure?
3. What is the dependent variable?
4. What is the independent variable?
5. Now, write a hypothesis for the following question: During the rainy season, which allele frequency (brown or green) will increase over time?

Step Two: The Rainy Season Data

Your professor is happy with the hypothesis that you have written. She gives you the data that she collected on the ant population. Because of the fast rate of reproduction among ants, she was able to collect a number of ants of each color over four generations. The data are in the table below.

Table 1: Number of ants collected during the rainy season

Generation of ants	Brown ants	Green ants	Total ants
Generation 1	100	100	200
Generation 2	90	120	210
Generation 3	75	150	225
Generation 4	65	170	235

Calculate the frequency of the brown allele and of the green allele in each of the generations. We assume that each ant is a homozygote. Round to the nearest thousandths.

To calculate allele frequencies within each generation:

1. Calculate the number of brown alleles by multiplying the number of brown ants (from Table 1) by 2. Calculate the number of green alleles by multiplying the number of green ants (from Table 1) by 2. Write in Table 2.
2. Calculate the total number of alleles by adding the number of brown alleles and the number of green alleles. Write in Table 2.
3. Calculate the brown allele frequency by dividing the number of brown alleles by the total number of alleles. Round to the nearest thousandths. Write in Table 2.
4. Calculate the green allele frequency by dividing the number of green alleles by the total number of alleles. Round to the nearest thousandths. Write in Table 2.
5. Check your math by calculating the total allele frequency. Write in Table 2.

Table 2: Allele frequencies in the rainy season

	Generation 1		Generation 2		Generation 3		Generation 4
	brown	green	brown	green	brown	green	brown	green
Number of alleles
Total # alleles in generation (brown + green)
Allele frequency
Total allele frequency in generation (brown + green)

Answer the following questions based on the above (rainy season) data.

1. What was the general trend (comparing Generation 1 to Generation 4) of the brown allele frequency over the four generations? Did the brown allele frequency increase or decrease? Write the actual numbers for each generation here in support of your answer.
2. What was the general trend (comparing Generation 1 to Generation 4) in the green allele frequency over the four generations? Did the green allele frequency increase or decrease? Write the actual numbers for each generation here in support of your answer.
3. Was your hypothesis in Question 1 supported? Explain why or why not.

Step Three: The Dry Season Data

During the dry season, the trees lose their leaves. The primates now eat the ants off of the brown tree branches.

1. In this environment, which allele frequency (brown or green) do you hypothesize will increase over time?

Table 3: Number of ants collected during the dry season

Generation of ants	Brown ants	Green ants	Total ants
Generation 1	65	170	235
Generation 2	90	150	240
Generation 3	125	120	245
Generation 4	150	100	250

Calculate the frequency of the brown allele and of the green allele in each of the generations. We assume that each ant is a homozygote. Round to the nearest thousandths.

To calculate allele frequencies: Within each generation,

1. Calculate the number of brown alleles by multiplying the number of brown ants (from Table 3) by 2. Calculate the number of green alleles by multiplying the number of green ants (from Table 3) by 2. Write in Table 4.
2. Calculate the total number of alleles by adding the number of brown alleles and the number of green alleles. Write in Table 4.
3. Calculate the brown allele frequency by dividing the number of brown alleles by the total number of alleles. Round to the nearest thousandths. Write in Table 4.
4. Calculate the green allele frequency by dividing the number of green alleles by the total number of alleles. Round to the nearest thousandths. Write in Table 4.
5. Check your math by calculating the total allele frequency. Write in Table 4.

Table 4: Allele frequencies in the dry season

	Generation 1		Generation 2		Generation 3		Generation 4
	brown	green	brown	green	brown	green	brown	green
Number of alleles
Total # alleles in generation (brown + green)
Allele frequency
Total allele frequency in generation (brown + green)

Answer the following questions based on the above (dry season) data.

1. What was the general trend (comparing Generation 1 to Generation 4) in allele frequency that you observed for the brown allele over the four generations? Did the brown allele increase or decrease in allele frequency? Write the actual numbers for each generation here in support of your answer.
2. What was the general trend (comparing Generation 1 to Generation 4) in allele frequency you observed for the green allele over the four generations? Did the green allele increase or decrease in allele frequency? Write the actual numbers for each generation here in support of your answer.
3. Was your hypothesis at the beginning of Step Three supported? Explain why or why not.

Step Four: Wrap Up

The following questions refer to the whole lab.

1. How did the ants in this population vary? In other words, what is the main difference between them?
2. Why is variation necessary for natural selection to work?
3. Explain selective pressure. What was the selective pressure in this simulation?
4. What is the definition of allele frequency? How are allele frequencies related to the idea of evolution?
5. Because some individuals in a population are more fit than others, the ultimate result of natural selection is a population that is better adapted to its environment. a. What trait was adaptive in the rainy season?
   b. What trait was adaptive in the dry season?
   c. Are your answers for a) and b) the same or different? If different, what changed between the two seasons to cause this difference?
6. Define microevolution. Define macroevolution. Which did you observe in this ant population? Explain why?