Constructing Explanations and Arguments: Inheritance, Variation, and Frog Ponds | EL Education Curriculum

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LS G3:M2

Constructing Explanations and Arguments: Inheritance, Variation, and Frog Ponds

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ARKive website

On February 15, 2019, the website was discontinued. Two pages from this website were used as resources in this module, in Unit 1 Lesson 5 (bullfrog traits) and Unit 2 Lesson 1 (frog habitats).

To address this, we have recreated the content from these pages on our website; please see the links below. We have also refreshed the lesson plans and supporting materials on the site to include these links. The only difference is that we were not able to bring over the "Common Frog" video used in Unit 2 Lesson 1, so we have created a "Frog Habitats" slideshow that serves the same purpose in the lesson. For the adjusted teaching guidance, see Opening B of Unit 2 Lesson 1.

The Life Science Module represents three additional hours per week of instruction during the eight to nine weeks covered by Module 2 of our Grades 3-5 Language Arts Curriculum. Although the Life Science Modules can stand alone, each one connects with and complements Module 2 of the grade-level language arts module lessons. For the complete materials list or to learn more about how the Life Science Modules are aligned with the Language Arts curriculum, read the K-5 Language Arts Guidance Document found on the Tools page.

In the Grade 3 Life Science Module, students build a basic understanding of the principles of evolutionary biology, including life cycles, inheritance of traits, and how an environment can influence an organism's survival. The study of heredity is confined to two generations, and the influence of the environment on the survival of an organism is confined to one generation. (Note: Students do not learn about adaptations over time or "survival of the fittest.") This module is divided into two units to help students work with two separate yet interrelated ideas: how physical traits are influence by heredity and how physical traits are influenced by the environment.

In Unit 1, students learn that organisms inherit traits from their parents. Because these traits come from male and female parents and can combine in many ways, the combination of these traits varies from sibling to sibling. The variation of these traits can sometimes provide an advantage to a particular organism and help it survive in a specific environment. Students begin the unit by looking at inheritance and variation in many organisms, including humans. They then focus on inheritance and variation in frogs.

First, students gather data about the physical traits of parents and offspring, in order to look for patterns of inheritance and variation. Next, they learn about a variety of life cycles, including plant life cycles, and how reproduction is the link between parents and offspring. Then students apply their understanding of inheritance by creating a model of a frog offspring that exhibits characteristics of both the male and female parents. Finally, students learn that variation can affect the survival of an organism and then use this information to construct an evidence-based explanation about the cause and effect relationship between traits and survival.

In Unit 2, students learn that the traits an organism inherits from its parents can be influenced by the environment (e.g., a plant's growth can be stunted if it does not get enough water). Thus, the environment in which an organism lives is very important. An environment has both internal and external factors that affect it. Students learn that one external factor that has the ability to positively or negatively affect an environment is humans. This unit focuses on how humans can help make environments in which many organisms can survive and thrive. As in Unit 1, students explore these ideas by focusing specifically on frogs and a pond habitat.

First, students are introduced to the issue of habitat loss for amphibians. They are challenged to design a frog pond that supports a frog's survival needs throughout its life cycle. They use a bullfrog simulation and an original investigation with duckweed to examine ways in which the environment influences the traits of an organism. Students then learn more about pond habitats and the needs of frogs. They use the Engineering Design Cycle to create a model that explains how their pond provides a healthy habitat for a frog throughout its life cycle. Finally, students use this explanatory model as evidence to argue that they have designed a pond that will support a frog's survival throughout its life cycle.

Throughout the module, students engage in the Science and Engineering Practices (things that scientists and engineers do) by making explanatory models, constructing explanations, and engaging in arguments. Students also consistently use Crosscutting Concepts (concepts that link across various scientific disciplines)--especially patterns and systems--to deepen their understanding of content. Routinely, they track their learning in a student science notebook and practice articulating, questioning, and refining their understanding in Scientists Meetings.

Although this Grade 3 Life Science Module was designed to work in concert with EL Education Language Arts Grade 3 Module 2, it can also stand alone. The content of the Language Arts module complements the student learning about frog life cycles and frog habitats in the Life Science Module, and in both the Language Arts and Life Science Modules, students engage in similar protocols and do close reading.

Download this module below to access Getting Started materials, full NGSS Standards descriptions, Week-at-a-Glance charts, materials lists, the Student Science Notebook, and all lesson supporting materials.

Guiding Questions and Big Ideas

Unit 1: Why does an organism look the way it does, and why does it matter?

  • Organisms look the way they do primarily because of the traits they inherit from their parents.
  • The environment also affects an organism's traits (this learning is the focus of Unit 2).
  • The cycle of life--birth, growth, reproduction, and death--drives the phenomena of inheritance (this learning is introduced in Unit 1 but assessed in Unit 2).
  • The traits that organisms inherit from their parents will vary from sibling to sibling.
  • Variation of traits (such as differences in color or size) can affect an organism's likelihood of surviving, finding mates, and reproducing.

Unit 2: What are necessary parts of a frog habitat, and how do they interact to support the survival of frogs throughout their life cycle? How can we build that (in the schoolyard or in the community or in a local park)?

  • Frog habitats must meet the needs of food, water, shelter, space, and air for the frog at all stages of its life cycle.
  • Frogs have very distinct phases in their life cycle; each phase has unique needs.
    • A frog's eggs do not need food, but they do need to be sheltered in a safe place that keeps them wet at all times. This is typically the shallow-water edge of a pond, with leaf litter and twigs to provide protection from possible predators.
    • When frogs are tadpoles, the food that is consumed is algae found along the edge or on the bottom of the pond.
    • When the tadpoles become froglets, their diet includes small insects in the water.
    • As adults, frogs will live on both the shore of the pond as well as throughout the water of the pond. The adult frog consumes insects and minnows--any animal small enough to fit in its mouth.
  • Frog ponds can be built by finding a suitable site and making a plan for the shape and depth of the pond, as well as what structures and features will meet the needs of the amphibians that will inhabit the pond.

The Four Ts

  • Topic: Constructing Explanations and Arguments: Inheritance, Variation, and Frog Ponds
  • Task: Frog pond explanatory model (with optional extension to build a local frog pond)
  • Targets:
    • NGSS Performance Expectations fully and explicitly taught and formally assessed: 3-LS3-1, 3-LS4-2
    • NGSS Performance Expectations explicitly taught but only partially assessed: 3-LS1-1, 3-LS3-2, 3-LS4-3, 3-LS4-4
    • CCSS ELA reinforced throughout the module: RI.3.1, RI.3.2, RI.3.3, W.3.1, W.3.2, W.3.8, W.3.9, SL.3.1
  • Texts:
    • Bullfrog at Magnolia Circle
    • a series of Life Cycle Stories of Animals
    • "Why Do I Look Like This?"
    • "Life in a Pond"
    • "Pollination: Bugs and Flowers Work Together"
    • "Wheat Germinating March" (video)
    • "From Seed to Flower" (video)
    • "Time Lapse Dandelion Flower to Seed Head" (video)
    • "Pear Flower to Fruit Swelling Time Lapse Filmed Over 8 Weeks" (video)
    • "Frog Habitats" (website)

Content Connections

CCSS ELA Connections

This module is designed to address NGSS standards. But the module intentionally incorporates content, protocols, and skills that align with EL Education's ELA Grade 3 Module 2. Both the ELA and Life Science Modules focus on frogs and shared texts, including Bullfrog at Magnolia Circle and Everything You Need to Know about Frogs and Other Slippery Creatures. In both the ELA and Life Science Modules, students also use similar protocols, including Back-to-Back and Face-to-Face and close readings.

In addition, in the Grade 3 Life Science Module, students routinely have opportunities to read informational texts (CCSS ELA RI.3.1, RI.3.2, and RI.3.3) and write arguments and explanations (CCSS ELA W.3.1 and W.3.2). The student science notebook gives students an additional opportunity to practice informative writing and gathering evidence (CCSS ELA W.3.2 and W.3.8). The Scientists Meetings, which students participate in throughout the Life Science Module, provide students the opportunity to formally practice their speaking and listening skills while collaborating in whole group discussion (CCSS ELA SL.3.1).


Unit 1: Explanation on Inheritance, Variation, and Survivability

Students look closely at pictures of two adult bullfrogs to identify their physical traits. After considering some of the possible combination of traits, they create a paper bullfrog offspring model that exhibits characteristics of both the male and female parents. They then construct two on-demand explanations. In the first explanation, students explain why organisms look the way they do and use the paper bullfrog offspring model, along with information gathered in their student science notebook, as evidence. In the second explanation, they explain how an organism's appearance can affect its survival. They use the paper bullfrog offspring model, along with information gathered in their student science notebook, as evidence. This assessment aligns with NGSS Performance Expectations 3-LS3-1 and 3-LS4-2.

Unit 2: My Frog Pond Is a Good Solution Argument

In conjunction with the performance task, students construct an on-demand argument in response to the following question: "What are necessary parts of a frog habitat, and how do they interact to support the survival of frogs throughout their life cycle? How can we build that?" Students use their designed frog pond explanatory model as evidence, as well as information gathered in their student science notebook, to help them provide scientific reasoning to justify their argument. This assessment aligns with NGSS Performance Expectations 3-LS3-2, 3-LS4-3, and 3-LS4-4. (Note: This task fully addresses 3-LS4-4 but only partially addresses 3-LS4-3 and 3-LS3-2.)

Original Student Investigations: Duckweed and Habitat

Students plan and carry out an original investigation to answer the question "Under what conditions in a pond does duckweed grow well?" First, students learn about the different places in a pond habitat and identify conditions that vary by location in the pond (i.e., duration of direct sunlight, water depth, water quality). They then create a test to recreate those conditions and observe how well duckweed grows in each condition. They continue to the test and make observations in their student science notebook for one week. This original student investigation centers on NGSS Performance Expectations 3-LS3-2 and 3-LS4-3, as well as the Science and Engineering Practice of Planning and Carrying Out Investigations.

Performance Task

Designed Frog Pond Explanatory Model

This performance task gives students the opportunity to showcase their deepened understanding of life cycles and how an environment can influence an organism's survival through an explanatory model. For this task, students design a frog pond that can help provide a solution to the problem of frog habitat loss. They create an explanatory model to show that their idea includes all the necessary parts of a frog habitat and will support the survival of frogs throughout their life cycle. This task aligns with NGSS Performance Expectations 3-LS1-1, 3-LS4-3, and 3-LS4-4. (Note: This task fully addresses 3-LS1-1 and 3-LS4-3, but only partially addresses 3-LS4-4, which is fully assessed in the Unit 2 Summative Assessment.)


For basic lesson preparation, refer to the materials list and Teaching Notes in each lesson sequence. The following are science-specific materials that will require significant advance preparation. More information on quantities and specific instruction is in the materials list in each lesson sequence.

Before beginning the Grade 3 Life Science Module

  • Consider purchasing aquatic plants to grow in your classroom, including water lilies.
  • Consider purchasing or acquiring aquatic animals so students can observe the animals throughout their life cycle. Work with a local wildlife expert to find and raise a native frog or other amphibian species. If you have acquired the animals locally and are implementing one of the frog ponds that your students are designing in Unit 2, consider letting the adult animals go in the new home. If you have ordered them online, do not let animals loose in the local habitat.

Week 1

Unit 1: Lesson Sequence 1

  • Create a teacher science notebook.
  • Copy and assemble the student science notebooks. (Not needed if your school has purchased the bound Student Science Notebooks.)
  • Create the Diversity of Organisms slideshow.

Unit 1: Lesson Sequence 2

  • Bring in three sibling water lilies, if available.
  • Create Frog Sibling Poster Session and Family Pass-around Cards.

Week 2

Unit 1: Lesson Sequence 3

  • Plan Life Cycle Expert Groups.
  • Gather materials for creating Animal Life Cycle models.
  • Make copies of expert texts: Life Cycle Stories of Animals.
  • Create General Plant Life Cycle picture cards.
  • Prepare technology to play time-lapse videos.
  • Create Plant Life Cycle model cards.

Week 3

Unit 1: Lesson Sequence 4

  • Gather materials for frog call shakers such as: film canisters, prescription bottles, or plastic eggs and materials to put in each canister: sand, paper clips, beads, dry beans, popcorn, rice, small and/or dry pasta noodles.

Unit 1: Lesson Sequence 5

  • None

Week 4

Unit 1: Lesson Sequence 6

  • Create a class lily pad scene where students can test the survivability of their paper bullfrog.

Week 5

Unit 2: Lesson Sequence 1

  • Obtain a copy of Bullfrog at Magnolia Circle.
  • Create Before and After Habitat slideshow.
  • Create Pond Success Stories photo cards.
  • Prepare the Planning a Frog Pond anchor chart.

Unit 2: Lesson Sequence 2

  • Gather materials for Hungry Bullfrog simulation including: sack for collecting tokens (such as a baby's sock or small bag), tokens (such as larger dried beans or marbles), scale, Hungry Bullfrog Simulation cards.
  • Gather materials for Duckweed Investigation, including: duckweed plants, plastic cups, distilled water, trays, various materials for testing, permanent marker, ice cube.

Week 6

Unit 2: Lesson Sequence 3

  • Gather materials for creating cross-section diagrams.

Week 7

Unit 2: Lesson Sequence 4

  • Create Habitat Stations.
  • Gather materials to augment the Habitat Stations, such as: jar of pond water, live specimens of common pond plants, live specimens of common bugs found in and around a pond, and abiotic features of ponds (such as a cup of mud, rocks, small fallen tree branches, or logs).

Week 8

Unit 2: Lesson Sequence 5

  • Gather materials for pond explanatory model.

Student Science Notebooks

The student science notebook plays a central role in the science classroom. This notebook is a place for students to track their learning and organize their evidence. Encourage students to take ownership of the notebook and use it to record all of their ideas and questions throughout the module, in addition to writing in response to the formal prompts.

The science notebook is patterned after an "interactive notebook." When opened flat, the left-hand side of the notebook is primarily for instructions and prompts; the right-hand side is primarily for student responses and ideas. When copying, and creating the notebook, be sure to staple correctly.

Students will use the notebook during every science class and return to it several times throughout the block. Consider the classroom systems and structures already in place to help students easily access and store their notebook.

Encourage students to use pencils, because they often will create detailed drawings and diagrams. As students return to and revise their ideas, have them lightly cross out changed thinking (rather than erasing) so their changes in thinking can be documented. Periodically, students may need to attach something to their science notebook. Use tape or staples (glue can make the pages stick together).

Periodically (once a week or so), collect the notebook to formatively assess students' understanding of the Disciplinary Core Ideas and Crosscutting Concepts as well as their ability to apply the Science and Engineering Practices. In each lesson sequence, the ongoing assessment box suggests parts of the notebook to focus on. Remember that the science notebook should not be used as a summative assessment. Rather, the notebook is a place where students are encouraged to try out new ideas, revise old ideas, and take risks.

For more information about the student science notebook, see the California Academy of Science, Teacher Perspectives: The Value of Science Notebooking.

Living Organisms in the Classroom

Science comes alive for students when there are real plants and animals in the classroom. EL Education encourages teachers to use living organisms in their classroom, and each life science module includes both formal and informal learning opportunities that incorporate live plants and animals. When done with careful thought and preparation, the close observation and study of living organisms not only teaches students about science and nature, but fosters an attitude of respect and kindness toward all living things.

To ensure the best learning experience with living organisms, it's important to plan ahead. First, familiarize yourself with the NSTA guidelines for the responsible use of live animals in the classroom. Then check up on local and state laws and regulations concerning the handling and transportation of animals, particularly non-native species. Most importantly, learn as much as you can about the particular plant or animal that you want to study. Take the time to have your students help you build a clean, safe and attractive habitat for the organism. The more you and your students learn about the safe handling of the organism, the better you will treat and care for the classroom visitor.

When planning classroom activities with a live organism, remember that the highest purpose is to promote observation and scientific curiosity, and instill an appreciation for the value of life. Under no circumstances should an activity cause an animal pain, deprive it of food or comfort, or expose it to harmful substances. Instead of "experimenting" with living things, help student discover ways to improve the organism's life by learning what it needs to thrive. It's not always necessary to have a formal research question. Close observation of an animal -- taking notes, asking questions, making hypotheses -- can be a powerful learning experience all its own.

A critical part of the planning process is deciding what to do with an animal after it leaves your classroom. If it's a native species, you could send the animal home with a student or release it into the wild. Non-native species require more forethought. If you buy the animal from a biological supply company, ask if they will take the animal back when you are done. If that's not possible, ask the supply company exactly where the animal was raised or collected. Contact a school in that area and see if a local science teacher would be willing to release the animal for you. The safe return of an animal to its home is an important lesson for your students to learn.

Science Background Information for Teachers

Below is science background information about life cycles, heredity, and variation of traits. Also included is information about frogs and duckweed. Use this information to help you effectively teach the science content of the Grade 3 Life Science Module. Refer to the sources and additional resources listed below for more information.

This module focuses on plants and animals that inhabit aquatic environments, specifically ponds. Through these animals, specifically frogs, and aquatic plants, students will learn why plants and animals look the way they do. Students also will learn about aquatic habitats and how they meet the needs of their inhabitants as well as influence their development.

Life Cycles

All organisms change over a lifetime in predictable ways. They progress through life cycles that follow similar patterns of birth (sprouting for plants), growth, maturation to adulthood, reproduction, and eventually death. Within this cycle, additional patterns are evident (e.g., adults reproduce and transfer their genetic information to their offspring). Animals also tend to exhibit those patterns of behavior that enhance their chances of survival, and therefore opportunities to reproduce. Plants, too, have developed a variety of specialized structures to disperse their seeds, sometimes with the help of a passing animal. Whether animal or plant, survival of the species is the purpose of the life cycle.


Heredity is a set of "instructions" that specifies the traits of an organism; it is the passage of these instructions from one generation to another. Organisms transmit genetic information through either sexual or asexual reproduction. Regardless of the type of reproduction, some traits are inherited while others result from interaction with the environment.

Sexual reproduction requires both the male (sperm) and the female (egg) to produce offspring. The offspring acquire a mix of traits from their biological parents. This mixture accounts for offspring resembling their parents yet never looking exactly like either parent. The transmission of genetics to offspring also accounts for traits being shared among siblings. Over many generations, these differences can accumulate so that organisms may be very different from their ancestors.

By contrast, asexual reproduction involves a single organism, and traits are inherited from a single parent. For instance, some plants can develop from a fragment of the parent plant and a potato can be cut into many parts and new organisms with the same genetic information will grow. Other examples of asexual reproduction include rhizomes (horizontal underground stems that send shoots upward) and bulbs (like tulips). The advantage of asexual reproduction is the creation of numerous offspring in a relatively short amount of time. The disadvantage is a lack of variation among the species. If there is a disease that affects one plant, it can potentially decimate the whole population.

Variation of Traits

The world is constantly changing. Some changes occur quickly, while others take place over extended periods of time. When there is a significant change in an environment, some organisms will be more likely to survive than others. An example of this is the peppered moth of England. Before the Industrial Revolution, the majority of peppered moths were mostly light-colored. But when pollution and coal smoke coated some of the trees in England, the number of dark peppered moths increased dramatically as they were able to hide from predators more easily than the light-colored moths. Differences in traits that are favorable to environmental conditions and allow organisms to survive get passed onto offspring. Organisms without those traits may be eliminated from the gene pool.

Variation among organisms of the same species--organisms that can mate and reproduce--can be influenced by the environment as well as genetics. Environmental factors can affect an organism's development, appearance, behavior, and likelihood of producing offspring. Salamanders in California provide a good example. Over millions of years, the same species of salamanders became separated geographically and followed two very different migratory routes. Those that traveled through forests survived by camouflage, while their relatives who took the coastal route adopted bright color patterns and behaviors of dangerously poisonous newts. Over millions of years, this same species adapted in very different ways to its environments.

Differences in where a plant grows or the food an animal consumes can cause organisms with the same genetic background to look and act quite differently. For example, arrow frogs that are poisonous in the wild are not poisonous in captivity. Scientists think this may be because these frogs may gain their poison from a certain arthropod as well as other insects they eat in their native habitat that frogs in captivity do not have access to.

Frogs: Integral Parts of Ecosystems

Frog habitat loss is an increasingly serious issue as land that was once wetland is gradually destroyed or damaged due to development. Frogs and other amphibians also face challenges caused by pollution, climate change, invasive species, and harvesting for pet and food trades. Frogs contribute to the biodiversity of an area. They are considered an indicator species that can provide information about the health of an ecosystem (because their skin is porous and extremely sensitive to changes in their habitat). A change in frog populations can be a warning that pollutants are invading an area.

Frogs have a unique life cycle that begins in the form of an egg. About four days after eggs are fertilized, tadpoles with gills and tails emerge. Within a few months, tadpoles transform into froglets (tiny frogs) and then, within one to three years depending on the species, froglets grow into adult frogs. They typically hatch from eggs laid in or near water. As adults, frogs live primarily on land but return to the water to breed and hibernate, so their habitat requires access to both land and water.

Frog populations have been declining at alarming rates. This is a concern because frogs are integral parts of the food web. Tadpoles help clean waterways by feeding on algae. Adult frogs are predators that eat large quantities of insects, including mosquitoes, yet also are an important food source for birds, snakes, and other animals. The loss of one part of the food web creates an imbalance that can result in negative impacts across the ecosystem.

Duckweed: Nuisance or Beneficial to Ecosystems?

Duckweed is an interesting plant to study because it has no stems or leaves and is the smallest flowering plant. Duckweed may have small roots but is primarily a green sphere or pair of spheres (often called fronds, although their proper name is thallus). These plants create a bright green, floating cover on the surface of water.

Rather than flowering and producing seeds, duckweed typically reproduces asexually by budding on the edge or base of the "fronds." Each frond can reproduce a number of times before turning yellow and dying of old age. This reproduction process under the right conditions can result in very high growth rates that can quickly cover an entire pond surface. Duckweed then becomes a nuisance that can block sunlight, constrain oxygen exchange, and lower dissolved oxygen levels as plants die and decay.

In areas with cold winters, duckweed produces buds that sink to the bottom of a pond in order to survive harsh temperatures. Although duckweed is found in many different climates, it typically grows in warm topical or temperate regions. Duckweed can survive under a variety of growing conditions. It thrives in nutrient rich, calm water and can grow in either sunlight or shade.

When growth rates are under control, duckweed is an important component in an ecosystem because it provides nutrient-rich food for waterfowl and fish. Smaller creatures also eat duckweed; these are then eaten by larger animals. Duckweed helps control the growth of algae by absorbing nutrients from the water and blocking out sunlight. The shade produced by duckweed also keeps the water cool, therefore having a positive impact on dissolved oxygen levels as well as minimizing water loss through evaporation.


Additional Resources

For more science background information:

For information on vendors:

Living organisms can be purchased at:

For more information on the NGSS Performance Expectations, including the evidence statements:

For more information on science instruction in the elementary classroom:

  • National Research Council, Ready, Set SCIENCE! Putting Research to Work in K-8 Science Classrooms describes the kinds of learning experiences and instructional practices that are necessary for students to develop a deep understanding of science.
  • The Inquiry Project--Talk Science Primer provides guidance for developing a culture of productive talk in classrooms.
  • Tools for Ambitious Science Teaching provides a constantly evolving set of tools aimed at improving student participation and learning.
  • NGSS@NSTA provides NGSS curriculum planning resources as well as professional learning materials.
  • NGSS Resources contains a variety of materials to support implementation of NGSS, including links to the Evidence Statements that help clarify the standards.

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