Conflicting Selection Pressures

The Concord Consortium
Type Category
Instructional Materials
Interactive Simulation , Lesson/Lesson Plan
This resource, vetted by NSTA curators, is provided to teachers along with suggested modifications to make it more in line with the vision of the NGSS. While not considered to be “fully aligned,” the resources and expert recommendations provide teachers with concrete examples and expert guidance using the EQuIP rubric to adapted existing resources. Read more here.



Conflicting Selection Pressures is the second lesson from Innovative Technology in Science Inquiry, in a three lesson series that leads students to make sense of evolution.  This lesson is meant to be used after A Selection Pressure which contains the anchoring phenomena and a set of questions (located in the teacher guide), but can also be used as a stand alone activity.  This lesson examines what can happen to a population when a trait can help an organism in one way, but hinder it in another.  Students then engage in discussion that leads them to gather data on a population of sheep (also used in lesson one).  Students investigate population growth under different conditions, using an interactive simulation, to gather data in order to create an explanation for evolution.  This lesson is designed to be done individually, however it could be done in groups depending on device availability. The teacher has the ability to set up a class and assign the lesson through a portal on the ITSI website (requires teacher to sign up for the website for free).   


Intended Audience

Educational Level
- none -
Access Restrictions

Free access with user action - The right to view and/or download material without financial barriers but users are required to register or experience some other low-barrier to use.

Performance Expectations

HS-LS4-4 Construct an explanation based on evidence for how natural selection leads to adaptation of populations.

Clarification Statement: Emphasis is on using data to provide evidence for how specific biotic and abiotic differences in ecosystems (such as ranges of seasonal temperature, long-term climate change, acidity, light, geographic barriers, or evolution of other organisms) contribute to a change in gene frequency over time, leading to adaptation of populations.

Assessment Boundary: none

This resource is explicitly designed to build towards this performance expectation.

Comments about Including the Performance Expectation
The activity guides students through a series of simulations that allow students to experience the concept of adaptation through natural selection. This simulation uses a population of sheep with different quality of teeth to model what can happen to populations when there is conflicting selective pressures. In these simulations, student identify patterns in sheep population when there is conflict to select for better teeth quality. The sheep with better teeth quality get more energy from their food, but the farmer also gets a better market price for sheep with better teeth quality. The student activities model the concept of adaptation through natural selection but with a focus of equilibrium. Students try to figure out the right amount of sheep with better quality teeth that can be taken out of the population without having it die out. During the lesson, students are asked to investigate the how a population could change when they have conflicting selection pressures. The lesson does offer some other scenarios, however the teacher could include a class discussion about the other animals used in the lesson. The discussion could be furthered by including local animals.

Science and Engineering Practices

This resource is explicitly designed to build towards this science and engineering practice.

Comments about Including the Science and Engineering Practice
Throughout the lesson students participate in a variety of simulations. Students run the simulations several times in order to collect data on the sheep population. As evidence is collected, students are required to save and analyze their data. This data is used to help students create an explanation about conflicting selection pressures. Student use the data to try to figure out a way to keep the population of sheep with better teeth in the farmers population and still allow for some to be sold, as better teeth are favored. The unit addresses the practice in full, however the teacher may want to expand on the other examples used within the lesson. Students give their thoughts about moose size and the tails of the male peacocks, but there is no discussion about their ideas or if their ideas are on the right track. Including a whole group discussion on these conflicting selection pressures would help students make deeper connections to the concept of competing selection pressures. In order to include the HS element (Construct and revise an explanation…) a suggestion would be to have students go back and continually evaluate and revise their explanations after each simulation.

Disciplinary Core Ideas

This resource is explicitly designed to build towards this disciplinary core idea.

Comments about Including the Disciplinary Core Idea
All of the areas of this DCI are addressed in the unit. The simulation helps explain how populations could change or not change over time due to competing selection pressures. This activity gives students a visual on how populations could fluctuate under different conditions (how many of the sheep with better teeth are given away) and how it may or may not result in adaptation within the population. This lesson also helps students create an explanation for advantageous traits, and what it means to be advantageous. To expand this lesson a teacher could include a discussion about what makes the other examples in the lesson a competing selective pressure and how that trait was advantageous or not depending on environmental factors. An extension activity based on the STEM career -Wildlife Biologist could also be included by having students investigate a local animal population (many state Department of Natural Resources websites have population data on local animals that can be used).

Crosscutting Concepts

This resource appears to be designed to build towards this crosscutting concept, though the resource developer has not explicitly stated so.

Comments about Including the Crosscutting Concept
This lesson addresses cause and effect relationships by specifically looking at what happens to the sheep population when you remove some of the ones with better teeth. Cause and effect is studied across the lesson and students are asked to relate the cause/effect relationship after each simulation. Students run the simulations with the competing selection pressure off and on in order to look for patterns that help explain the fluctuation within the populations. One suggestion for improvement could be to make the use of the CCC more explicit throughout the lesson, as it is currently only explicit in one question. The teacher could also include a discussion about what a cause/effect relationship is, and what it is not. It is important to note that the element identified in the lesson is from the middle school grade band, however it could be expanded to include the high school element. In order to include the high school element of cause and effect, a suggestion would be to include probing questions about what could happen to the population if the environment shifted (heavy rainfall vs. low rainfall)and/or questions about artificial versus natural selection.

Resource Quality

  • Alignment to the Dimensions of the NGSS: This lesson introduces students to the concept of competing selection pressures. Although the students do not ever go back to the anchoring phenomena from lesson one, A Selection Pressure, the lesson does guide students in building an understanding about how a trait can be favorable in one situation and a hindrance in another. Using the three-dimensions, students build an understanding of how adaptations could occur within a population if there are competing selection pressures. Although the lesson uses the three-dimensions in concert with each other throughout the unit, the CCC could be more explicit. Students are asked to analyze and interpret their data in order to identify patterns in cause and effect relationships in order to explain how natural selection could lead to adaptations within populations. Students will also identify where population changes may be more random and not directly related to a cause. Tips for improvement could include: Make the CCCs more explicit throughout the lesson by referring to them consistently in the questions. Using examples and non-examples could also help students build an explanation of adaptation. Go back to the anchoring phenomenon (the bug population) and revisit the initial questions to see if you still agree with your predictions. Have a discussion on the other animal populations used in the lesson (moose and peacocks). Have students share their ideas and any questions they have about why moose size and male peacock tails are competing selection pressures.

  • Instructional Supports: This lesson does have some instructional supports including an optional activity that could expand on student understanding. The phenomena is relatable and it is built on accurate science. However, the opportunities for differentiation are not included. In order to make this lesson more accessible to a more diverse student population a teacher could create differentiation options for English language learners, students with special needs, and other diverse learners. Tips for including instructional supports include: Scaffold in reading supports this could be a text to speech option on the computer or a print copy if students need additional one-to-one help. Use a voice to type program or scribe for students that have difficulty with writing or typing. Create a graphic organizer for students that need written language support.

  • Monitoring Student Progress: This computer based simulation lesson has opportunities for monitoring student progress in several ways. The online version can be monitored by the teacher as students work their way through the lesson. If the print version is used the teacher can monitor progress while walking around the room. There are also natural breaks within the lesson to check for understanding. At the end of the simulations, when students need to share data and discuss their ideas with others, a teacher could use this as a formative assessment opportunity. There is a teacher guide included that contains front matter, guidance questions, and an answer key. Tips for including progress monitoring include: The teacher could create a rubric designed to be used with the student explanation sections. identify some student understandings to look for when formatively assessing students throughout the lesson. An example of some ‘look fors’ could include: traits are not always an advantage or disadvantage, sometimes the environment dictates whether a trait is desired or not, and sometime human interference will cause a population to change.

  • Quality of Technological Interactivity: The simulations provide different results to each student or groups of students as they progress through the lesson. Students are asked to store their simulation results in order to share out to the larger groups, as well as to document their observations. As data is shared a broader class discussion is used so students can compare data sets. The simulations are fairly easy to use as long as the directions are followed. Each of the simulations allow students to further their understanding about natural selection and adaptation by collecting and analyzing data in order to develop explanations.