Evolution is often challenging for high school students to understand because it encompasses an array of interlinked processes that occur across a broad swath of biological scales. To help address this, we have developed a set of freely accessible, online, and interactive lessons that focus on the evolution of sweet garden peas from their starchy tasting ancestors. Gregor Mendel first explored the genetics of garden peas in the mid-1800s; our materials help students explore the basis of the R and r alleles from genetic, protein, cell function, artificial selection, and population genetics perspectives. These Next Generation Science Standards (NGSS)–linked lessons integrate concepts across scales and are designed to be used in a flexible order, with support provided to teachers on how to choose a sequence that meets their students’ needs. Throughout, students act as scientists as they uncover how multiple processes at disparate scales all worked together in the evolution of sweet and wrinkled peas from ancestors that were starchy and round.
Evolution is foundational to biology but a holistic understanding of its mechanisms can be a challenge to teach, and for students to learn. One of the reasons for this is that evolution involves processes that operate at disparate biological scales (Catley et al., 2005). Nucleotide mutations happen within the microscopic realm in the nucleus; natural selection is an interaction between an individual and the environment, compounding in a species across landscapes and over time. In addition, a host of processes happen between and beyond these two levels, from protein formation and function, to biogeographic events. When these biological processes occurring at different scales are taught in isolation from one another, as it is often presented in textbooks (e.g., Nehm et al., 2009), students can struggle to connect them (Lazarowitz & Penso, 1992; Yarden et al., 2004). In an attempt to connect biological processes across scales, White et al. (2013b) developed curricular materials that presented cases of trait evolution where students could explore a phenomenon from the micro to the macro scale; students were guided to investigate a single example of trait evolution, from DNA mutation, to protein function, to phenotype expression, to natural selection, to population-level processes. When doing this, students tended to develop a more complete understanding of the biological underpinnings of evolution (White et al., 2013a). While the approaches of White et al. described above were intended for introductory undergraduate classes, Ellis et al. (2021) recently redesigned some of the materials (exploring the evolution of light fur color in deer mice) to make them more appropriate for high school biology students, including Next Generation Science Standards (NGSS; NGSS Lead States, 2013) performance expectations in an online user-friendly format.
In this paper, we describe new curricular materials we have developed, based on a second example of trait evolution: the evolution of sweet garden peas (Pisum sativum ssp. sativum). The materials examine how sweet peas arose due to a mutation in a gene responsible for a key starch-forming enzyme, and then how those peas were propagated by artificial selection by early human farmers. The lessons we present here are loosely based on the curricular materials developed by White et al. (2013b). However, our work is unique in that (1) it comprises of a set of unique lessons targeted for high school biology, (2) it involves interactive online simulations that we designed for this project, (3) it includes a teacher portal and teacher support materials, and (4) it expressly connects with NGSS learning expectations. The lessons that we describe below are freely available online at http://learn.concord.org/cbio-peas.
Overview of the Curricular Materials
Today, garden peas come in both starchy and sweet varieties. Historically, wild peas were starchy. This was due primarily to the activity of the starch branching enzyme (SBE1) that converts simple starch (amylose) into complex starch (amylopectin). The starch branching enzyme, SBE1, is transcribed and translated from the starch branching enzyme gene (sbe1). Starchy peas, in addition to tasting starchy, also have an interesting biophysical property where they maintain a round shape upon drying down in the pod. In the alternate phenotype—the sweet pea—an approximately 800 base pair insertion into the sbe1 gene (Bhattacharyya et al., 1990) makes the associated SBE1 enzyme nonfunctional. In individuals homozygous for this mutation, the amount of complex starch that is made is greatly reduced, with the excess sugars being converted into sucrose, resulting in a sweeter-tasting pea.
Across our curricular materials (Figure 1: Lesson Map), students investigate the mechanisms and processes that underpin the evolution of the sweet/wrinkled pea phenotype from the ancestral starchy/round state. Students explore and compare DNA sequences, synthesize and compare the proteins, breed pea plants, and more. Along the way, they uncover both the individual and interconnected biological processes involved in the evolution of the garden pea sweet/wrinkled trait. The lessons we have developed can be used in a variety of permutations, as teachers often differ in the order with which they tackle biological topics within the school year. After a recommended Introduction, teachers can work through the materials with their students in the order they see fit. Open structure allows for customization of the curriculum to meet the needs of teachers and students in various instructional contexts.
The curricular materials are divided into four sets of lessons. The first set (lessons 1–5) make up the core lessons for the evolution of garden peas. Students are introduced to the biology of pea plants and pea plant reproduction, then explore the genetic and protein basis for the shape of pea seeds. After the Introduction (lesson 1), the subsequent four lessons can be completed in any order, as preferred by the teacher. The second set (lessons 6–8) are linked extension lessons. These lessons are intended as enrichment learning opportunities that can be added on to specific core lessons, at the discretion of the teacher. The third set (lessons 9–11) are unlinked extension lessons, meaning that they can be completed at any point in the lesson sequence, irrespective of whether or not any of the core lessons or linked extension lessons have been completed. The final set (lessons 12–13) are broader connection lessons that focus on building connections between the topics explored in previous lessons.
Guiding question: What are the characteristics and history of the plants that Mendel used to study genetics?
NGSS alignment: HS-LS4-2, HS-LS4-4
This Introduction provides students with baseline knowledge about peas, focusing on seed shape and taste. The lesson begins with a brief history on the domestication of peas, where students compare and contrast ancient peas with modern peas. They first analyze the structure of the pea pod; ancient pea pods opened easily, while modern peas stay closed much longer. This lesson introduces the concept of natural versus artificial selection, which plays a role in later lessons. Students then transition to focusing on the trait of round versus wrinkled seeds, with a brief explanation for how the pea shape trait (round vs. wrinkled) is linked to the taste trait (starchy vs. sweet).
Next, students engage in an online simulation called Selective Farmer (Figure 2). Students play the role of the farmer and go through repeated seasons of planting and harvesting a field of round and/or wrinkled peas. As they plant and harvest the peas over many generations, they discover how the pea population changes as a result of their selection choices.
The lesson concludes with an overview of plant reproduction to reinforce for students that plants reproduce sexually and that peas are the “offspring” of pea plants. Students examine diagrams of pea flowers to review anatomical features, watch a time-lapse video of a growing pea plant, and answer targeted questions that call attention to the sexual nature of plants.
2. Sequences of the sbe1 Gene
Guiding question: How do the sequences of DNA influence the proteins formed in peas?
NGSS alignment: HS-LS1 -1
Lesson 2 begins with an overview of two of the different types of starches found in plant cells—amylose and amylopectin—and how the starch branching enzyme (SBE1) catalyzes the conversion of one (amylose) into the other (amylopectin) (Figure 3A). Students then investigate the two versions (alleles) of the sbe1 gene that result in the two versions of the SBE1 protein. These two alleles are the well-known R and r associated with Mendel’s peas. Students discover that the DNA sequence for one of the alleles is longer than the other, due to an 800 base pair insertion mutation (Figure 3B). After predicting what this insertion might do, students use a protein synthesis simulator (Concord Consortium, 2021) to transcribe and translate targeted sections of both sequences and examine the resulting proteins (Figure 3C). Students discover a stop codon within the insertion mutation, which truncates the SBE1 protein and makes it nonfunctional. Students are guided to tie this information back to the amylose and amylopectin starches and to the question of how differences might change the taste of the pea. Combining what they have learned here with the material in the Introduction (above), students use an online model building tool (Sage Modeler; Concord Consortium, 2020) to develop a conceptual model to show how different DNA sequences relate to different protein shapes, to different protein functions, and finally to differences in expressed phenotypes (Figure 3D).
3. Allele Frequency
Guiding question: How can allele frequencies be used as an indicator of evolutionary change in pea plant populations?
NGSS alignment: HS-LS2-2, HS-LS4-3, HS-LS4-4, HS-LS4-5
In this lesson, students are introduced to the concept of random sampling within a population, and the difference between frequency and relative frequency. Students arrange a series of images taken from a very small population of harvested peas over time, and they propose an explanation for how the phenotypes within the population change over time. Next, the students are given definitions for frequency and relative frequency and discuss when each measure is useful to use. They also compare how these measures differ over time when the measurement is in an entire population versus a smaller sample from that population. Given a sample from a population from three points in time, students calculate the relative frequencies of the phenotypes, and the relative frequencies of the two alleles R and r (Figure 4). As they model the allele frequencies, students present their data in pie charts, as this gives a visual representation to their numerical results. Students also explore the utility of taking a sample from a population rather than needing to measure a parameter across all individuals.
In the final part of the lesson, students brainstorm ideas for what may cause a population’s allele frequency to change over time, connecting their calculations to other concepts they have learned in prior lessons. The lesson concludes with a discussion of how relative frequency of alleles can be a good measure for evolution.
4. Genetic Inheritance
Guiding question: How well does knowing the phenotype of a plant help determine its genotype?
NGSS alignment: HS-LS3 -1, HS-LS3-2, HS-LS3-3, HS-LS4-2
This lesson focuses on how parental genotype is used to determine offspring ratios and explores the molecular and cell biological basis of trait dominance. The lesson begins with a scenario where the students examine three pea plants (Figure 5A, 4B). First, students use a breeding simulator to breed different combinations of flowers from the three plants. The results of the breeding experiments are shown as peas in a pod, and the cumulative totals of peas (they can cross each parent combination multiple times) are displayed in pie charts (Figure 5C). As students examine the phenotypes and phenotype ratios of the offspring, they realize that even though two of the plants grew from round seeds, the differences in offspring phenotypes suggest that the genotypes of the two seeds must have been different (i.e., homozygous dominant vs. heterozygous dominant). Students then examine the genotypes of the offspring and reverse engineer Punnett squares to determine the genotype of the seed that gave rise to each plant (Figure 5D). Once the genotypes of the original three plants have been uncovered, the lesson transitions to an open investigation where students develop their own testable question about pea plants that they can investigate using the embedded simulation.
5. The Physical Structure of a Pea
Guiding question: Why do only some peas wrinkle when dried?
NGSS alignment: HS-PS1-6, HS-LS1-6
This lesson explores the biochemical and physical differences between round and wrinkled peas. It begins with an anecdote about the eighteenth-century French queen Marie Antoinette and her documented preference for sweet peas. Students are prompted to investigate why both sweet and starchy peas are round when freshly picked, but when dried, only the sweet peas wrinkle. Students watch a time-lapse video of peas drying and then use an osmosis simulation to better understand the process. Next, students examine the structures of the molecules in the osmosis simulation (sugar, amylose, and amylopectin) to better understand why only starchy peas maintain their shape after they have dried down (Figure 6). Students are then given the opportunity to critique a video that simplifies the process of peas wrinkling. To complete the lesson, students synthesize the lesson material by developing a conceptual model in the online lesson portal (Concord Consortium, 2020).
Linked Extension Lessons
6. sbe1 Extended
Guiding question: How do insertion mutations impact associated protein structure?
NGSS alignment: HS-LS1 -1
This linked extension lesson builds from the Sequences of the sbe1 Gene lesson (lesson 2) and examines how the 800 base pair insertion influences the shape of the SBE1 protein. Students hypothesize what would happen to the end of the SBE1 protein if they manually removed the stop codon from the DNA sequence so that the full sequence could be synthesized. Students then simulate this using the protein synthesis simulator (Concord Consortium, 2021). The purpose of this activity is to draw attention to a frameshift mutation associated with the nucleotide insertion, and the consequences thereof. Students discover that, even in the absence of a stop codon within the 800 base pair insertion, the SBE1 protein would contain different amino acids because of a change in the translation reading frame.
7. Hardy-Weinberg in Peas
Guiding question: How can we predict the offspring from a set of pea seeds?
NGSS alignment: HS-LS3-3
This lesson connects to the Allele Frequency lesson and the Genetic Inheritance lesson (lessons 3 and 4). In this lesson, students “visit” the fields of four pea seed suppliers that each use a different planting strategy. Students use an interactive simulator that includes the alleles from the parent generation as they simulate the pollination process that produces the offspring. The lesson transitions to the assumptions needed to use the Hardy-Weinberg equilibrium formulas, and students apply them to the supplier’s fields. The students then calculate the likely allele frequencies of the subsequent generation and compare their calculations to the simulated results. This activity is followed by a thought experiment for how finding a population that is not in Hardy-Weinberg equilibrium can be an indicator of evolution. The lesson concludes with a brief summary of what the students have learned and what they still want to know.
8. Membrane Eggs-periment
Guiding question: How does the movement of water across a membrane influence an object’s shape?
NGSS alignment: HS-PS1-6, SEP4, SEP5
This lesson is linked to the Physical Structure of the Pea lesson (lesson 5). To support student understanding of osmosis, we provide a lab activity to mimic the change in pea seed shape using chicken eggs. In this system, students examine what happens to eggs when placed into different solutions. Here, the egg plays the part of a pea plant seed, where the direction of water flowing through the membrane is based on the different solutes in the pea cells. This exercise flips the script and uses a constant “cell” (i.e., an egg) and instead changes the solute concentrations in its surroundings. Students can either perform the experiment live in their classroom or analyze prerecorded data that are provided within the lesson. Students graph the data and come to conclusions about the direction of water flow. The activity concludes by linking back to the pea with a claim-evidence-reasoning question.
Unlinked Extension Lessons
9. Complete Harvest Competition
Guiding question: How does the proportion of dominant and recessive alleles change as artificial selection is enacted over time?
NGSS alignment: HS-LS4-5, HS-LS4-2, HS-LS4-4, HS-LS3-3
In this activity, students are presented with two neighboring farmers who have different goals. The first farmer wants to produce only round peas while the second wants to produce only wrinkled peas. As students evaluate the two goals, they are asked to predict who will achieve their goal first. Students are encouraged to think about how this might apply in real-world scenarios and the impact of a heterozygous genotype on offspring phenotypes. This activity can be used as a review, as a quiz, or as a warm-up.
10. Beyond the Pea
Guiding question: What parts of the sweet peas system can be applied to other biological systems?
NGSS alignment: SEP3, CCC1
This activity encourages students to apply what they learned in the pea lessons to other phenomena. Students first brainstorm the concepts they have explored using peas, such as artificial selection or dehydration. Then, they hypothesize where else in nature these processes might occur. For example, students may suggest that the same mechanism that causes a wrinkled pea also causes a grape to wrinkle into a raisin. Finally, students brainstorm how they could investigate their hypotheses. This lesson is open-ended to allow for student creativity.
11. Thinking Forward
Guiding question: What other biological factors might influence pea taste?
NGSS alignment: SEP1
This short lesson stimulates student thinking about other factors that might influence the evolution of pea taste and how they could be investigated. This activity can be used as a bridge between lessons, as the basis for developing a “driving question board,” or as a transition from the peas lessons back to other curricular material.
12. Synthesizing the Evolution in Garden Peas
Guiding question: How do the concepts explored in the Introduction and other core lessons work together to help explain why some peas are sweet while others are starchy?
NGSS alignment: SEP2, SEP6, SEP8, CCC1, CCC2, CCC3, CCC4
In this lesson, students use a table to summarize the main points from each lesson and then consider how the materials from pairs of lessons interact to influence pea shape. While many of the connections between lessons have been implied throughout, this is an opportunity to explore evolution as an integrative concept, requiring knowledge of biological principles from a number of topic areas and across different levels of biological organization. Students tie their ideas together using the integrated model building tool (SageModeler; Concord Consortium, 2020) and summarize their work to describe how wrinkled peas evolved over time. Finally, students distill their understanding of evolution by creating an “elevator pitch.”
13. Making Connections: Peas Discussion
Guiding question: How does knowledge across different biological scales integrate to influence pea shape and taste?
NGSS alignment: SEP6, SEP8, CCC2, CCC3
This short activity helps students make connections between lessons. Like the Synthesizing the Evolution in Garden Peas lesson (lesson 12), this is an opportunity for students to be explicit in describing how the process of evolution involves a series of interwoven processes occurring at disparate scales. Students note which lessons they have completed, discuss with peers how the content of these lesson fits together, and then write a brief summary of their discussion. This activity can be revisited throughout the implementation of the peas case, as it helps students be deliberate as they revise and expand their understanding of the evolution of garden peas.
Accessing the Curriculum
The lessons are publicly available at both https://connectedbio.org and https://learn.concord.org/cbio-peas. In addition, educators can sign up for a free teacher account on learn.concord.org, to be able to assign lessons to their students and to access a real-time dashboard that populates with student responses as they answer each question or complete each activity. From this dashboard, the teacher can also provide personalized student feedback and optionally assign a grade. Furthermore, teachers have access to additional resources to help them prepare for implementing the lessons, in which we provide an overview of the phenomenon, give summaries of each lesson, indicate NGSS alignments, and support teachers in anticipating common misconceptions. Teachers can also view a teacher edition of the lesson materials that provides teaching tips, discussion prompts, background knowledge, and exemplar answers to questions.
Support for this work was provided by the National Science Foundation’s Discovery Research DRK12 program under Award Nos. DRL-1620746 and DRL-1620910. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.