Selective Pressure Happens!

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Students will apply laws of Mendelian inheritance to predict the genotype and phenotype of progeny. Students will also determine the effects of selective pressure on progeny.

Big Idea

Genetics isn't the only thing that can affect what offspring live to reproduce.

What Students will Learn in this Lesson

1 minutes

In today's lesson, students review the principles of Mendelian inheritance by creating model of arthropods. We focus on the difference between genotype and phenotype because I have found that even in high school, students still struggle with the implications of these two concepts. My anecdotal evidence is supported by educational research. As a class, we determine what are the dominant and recessive traits in the phylum Arthropoda. Then students construct imaginary arthropods using materials provided. Next, students predict the possible gene combinations from a cross of two arthopods. Finally, students determine the phenotypes of those progeny and make sample specimens. Students also review the impact of selective pressures on the population of insects by examining the results of predation, competition, habitat destruction, and drought on the resulting progeny. (Note: This lesson is designed for a ninety minute class period. If your class periods are shorter, it will need to take two days.) This is day one of a two day lessonHere is an overview of what students will learn today.  

Hook/Check for Understanding

5 minutes

Give student the Fur Color Assessment Probe and allow student several minutes to consider the scenario. Using this assessment probe, students should determine the possibility of a certain type of offspring.  

Once students have had a chance to ponder the scenario, ask for volunteers to explain their thinking. As a class, discuss any additional information one might need to determine the fur color of any offspring.  

Next, ask the class if the same rules that apply to the inheritance of fur color in rabbits, apply to other living things.  

Explain to students that they will be using insects as a model organisms to review the rules of Mendelian inheritance today. Have students list the Laws of Mendelian inheritance in their notebook. As a class, summarize what each of the laws states.  

Give students the traits table for today's student activity and briefly explain the dominant and recessive genes for each traits. Have students predict the genotype for each of the traits. Have students explain why there can be more than one dominant or recessive trait. (Note: Here is an example of a student completed trait table.

Student Activity: Arthropod Construction

7 minutes

Place the following supplies at each station:  

  • For the body: egg cartons, toilet paper rolls, paper towel rolls, styrofoam cups, and plastic cups
  • For the legs: straws, popsicle sticks, milk straws, pipe cleaners, plastic bendy straws
  • For the eyes: buttons, stickers, beads
  • For the antennae: twist ties, Q-tips, toothpicks, paper clips, short pipe cleaners
  • For the wings: waxed paper, coffee filters, foil, Saran wrap
  • scissors
  • glue sticks
  • tape
  • pins

At each station, place a construction checklist. Randomly select the type of arthropod that individual students will construct. Each students should construct an arthropods. (Note: Make sure that an equal number of arthropod types are selected so that each arthropod will be able to produce progeny.) Students should build their assigned arthropod by selecting one material for each characteristic (body segments, legs, eyes, and antennae). Remind them not to mix white and colored materials. They should follow the typical body plan of their assigned arthropods, but they can use some creativity as well in the design of their specimen. Move about the room checking student models with the rubric on the reverse side of the checklist.  

This activity is modified from Zumwalde, Sharon. 2000.  Arthropod Genetics. Science Scope.

Partner Activity: Mapping the Family Genes

20 minutes

Students should pair with another student that has constructed a similar arthropod. Using the student worksheet, they should determine the phenotype and genotype for each of their organism's traits. Next, using a Punnett square as a model, they should predict the probability of the offspring's genotype and phenotype.  

Students should randomly choose two offspring from the second generation. They should perform a genetic cross between the two offspring and determine the phenotype and genotype for each of their organism's traits in the third generation.   

Then have students construct models one of the offspring from generation two and generation three using the supplies provided in the original construction activity.

Class Discussion: Punnett Square Review

12 minutes

Bring the class back together and ask for several volunteers to share the results of the generation one's genetic crosses. Review the Laws of Mendellian inheritance and point out the application of those laws in the construction of the Punnett square.  

Ask students the following questions:

  • Where on the Punnett square are we applying the Law of Independent Assortment? Have students highlight that portion of the Punnett square in pink highlighter.  
  • Where on the Punnett square are we applying the Law of Segregation?  Have students highlight that portion of the Punnett square in yellow highlighter.
  • Where on the Punnett square are we applying the Law of Dominance? Have students highlight that portion of the worksheet in blue highlighter.  


Stress to students that the top and the side of the Punnett square represent the possible gametes produced during meiosis. Then, ask for several volunteers to share the results of the generation two's genetic crosses.  

Ask students to consider any differences between the three generations:

  • What traits were seen in all three generations?
  • Did any traits "disappear" from one generations to the next and then "reappear."  What can explain that phenomena?


Finally, ask students to brainstorm possible selective pressure that might affect if organisms might be able to survive to reproduce.  

Exploring Further: Selective Pressure Happens!

15 minutes

Assign each student group one of the following scenarios: a drought occurs, an invasive species that shares the same food source is introduced into the ecosystem, a predator is introduced into the ecosystem, or the habitat is destroyed because of the development of a mall.  

Using the results from the genetic crosses from generation two, consider which offspring will reach sexual maturity and have progeny of their own.  

Advise students of the following caveats:

  • In a drought, arthropods with tough exoskeletons are better able to conserve water.  All arthropods with styrofoam body parts live to sexual maturity and are able to reproduce.
  • In the competition scenario, arthropods with colorful legs produce a protein that allow it to better utilize their food source.  This protein also make them run faster so they can outcompete each other and the competitor for the food source.  All arthropods with colorful appendages live to sexual maturity and are able to reproduce.
  • In the predation scenario, arthropods with white body parts and wings are better able to hide than their colorful or wingless counterparts.  All arthropods with wings or white body parts live to sexual maturity and are able to reproduce.
  • In the habitat destruction scenario, only small bodied arthropods will live to adulthood.  All arthropods that have large bodies, legs, or antennae die before reaching sexual maturity and will not be able to reproduce.  


Perform several random genetic crosses using Punnett squares as models. Determine what the possible offspring of generation three because of the selective pressure. 


Student Presentations: Results of Selective Pressure

20 minutes

Bring the class back together. Have partners share the results of their genetic crosses with and without selective pressures. Ask students to consider the following questions:

  • How were your genetic crosses with and without selective pressures the same?

Possible student answer: There was still a lot of variation with the selective pressures. I was surprised about that there was still so much variety within the species. 

  • How were your genetic crosses with and without selective pressures different?

Possible student answer: Genetic crosses with selective pressures could be different than those without selective pressures depending on what types of traits an living thing has. If the trait will help the living thing, then they will survive and be able to reproduce. Otherwise, there is a chance they might not survive to reproduce.

  • Were there any results that surprised you?

Possible student answer: I was surprised that selective pressure took such a long time to change how a species looks. I look it would happen a lot more quickly. 

  • If the selective pressure continue for two or more generations, how might this change how the arthropod populations look? (Note: Encourage students to give specific examples.)

Possible student answer: You would see those traits become more common in the populations.  However, the other traits would not completely disappear from the populations because of hybrids. 


Students should record their answers in their lab notebooks.

Homework: Students should read the following articles about hox genes (Hox genes and animal body plans, A brief overview of Hox genes, and Homeotic Genes and Body Patterns). They should complete a Cornell notes template for each article.