Phenomenon
General Phenomenon: Traits in populations change over time.
Phenomenon Specific Case:
We explore three different stories of change over time: wing color in the peppered moth, antibiotic resistance in bacteria and beak depth in Darwin's finches of the Galapagos Islands. The backbone of the learning sequence builds on a data set for Darwin's finches. There are a variety of possible choices and other interesting data sets/phenomena can fit into the general framework.
Question
Why do traits in populations change over time?
Specific Questions:
This will be specific to the phenomenon:
- Why did the average beak size of Galapagos ground finches increase over time?
- Why did bacteria develop antibiotic resistance?
- Why did the predominant wing color in peppered moths change?
Model Ideas
The following ideas are the basis of Darwin's model of natural selection:
- Populations have the potential to grow exponentially but they typically don't because of limited resources or environmental factors.
- Variation in traits naturally exists in populations.
- Depending on the environment, traits can be advantageous or disadvantageous.
- Individuals with more advantageous traits are more likely to survive have the opportunity to reproduce.
- Many traits are heritable so offspring tend to resemble their parents.
- In each generation the number born with the advantageous trait will increase and the number with the disadvantageous trait will decrease.
- Over time the distribution of traits in the population will change.
Overview
Transition in from Population Dynamics: After observing biodiversity changing over time, we ask more about “Why?” We've seen how population size of several species fluctuate and know that biodiversity has changed over time. In this model we will investigate how fluctuations in population size because of birth/death rate may affect the distribution of traits in a population.
We know that life on Earth has changed over time, but why? Here we'll guide students in developing a model for explaining how and why traits in populations change over time. First, students will explore three phenomena of change over short periods of time: wing coloration in peppered moths, bacterial antibiotic resistance, and beak size in the Galapagos finches. Students will have ideas about how these changes came about. Allow them to propose their initial model ideas. Students will then focus on a case study of the change in the distribution of beak sizes in Darwin’s finches on the Galapagos Islands. They will analyze data and use evidence from the case to refine their model ideas. They will then explore each idea in their model in more depth through a series of activities and along the way continue to refine their model for how traits in populations change over time. Next, they will compare their model to two other models proposed more than 100 years ago. Finally, they will revisit the phenomenon of the peppered moths and apply the model they just developed to explain why the wing color changed over time
Natural selection is a foundational model in the MBER-Bio curriculum and we will return to it throughout the year.
Transition out to Chemical Reactions: From our natural selection model we understand that species traits are shaped by which individuals survive to reproduce. This leads us to ask the question: What do individual organisms need in order to survive?
Overall Time: 9 to 13 days
Advanced Planning
MATERIALS (other than basic lab supplies) needed for this model include:
For the Variation lab:
- Variation in English peas: approximately 1# per class. These are easiest to find in July and early August - best source, farmers markets. Sugar snap peas can be substituted but are smaller and harder to work with. They are cheapest at COSTCO.
- Variation in femur length in grasshoppers: Preserved grasshoppers can be ordered from Carolina Biological. Pull off the legs and allow them to dry out before students measure them. They can be reused each year. Alternatively, students can measure their own eye widths.
For Worm-eaters:
- Dixie cups, masking tape, rubber bands,
- Plastic forks, spoons and sporks. Modify them as described in the teacher notes. You can start "collecting" sporks at fastfood restaurants or purchase them at a place like Cash and Carry. We have only been able to find them in boxes of 1000 however.
1. Present the phenomenon of species’ traits changing over time. In this learning segment students will observe the phenomenon that traits change over time. The slides contain a brief overview of three stories of population change: one about changes in the frequency of certain colors variants in peppered moths over time, another about antibacterial resistance, and another about an increase in average beak depth in Galapagos Ground Finches (which students will return to in segment 3).
MBER teaching tools used in this segment: doodle sheet, think-pair-share, making thinking visible
We explored the idea of changes within a population over relatively short periods of time and identified this as a phenomenon that we want to try and explain. We generated a shared way to describe this phenomenon.
2. Students will have opportunities to generate some questions they have about their observations of the three stories. The goal is for students to identify a question that allows them to explore the underlying mechanism causing the change in traits. The question can be similar to ”how do traits/species/characteristics change over time?” The final question should be posted somewhere in the classroom and written on the students’ doodle sheet so that they have a constant reminder of what they are investigating.
After examining the the phenomena in learning segment 1 we spent some time generating a question that will guide our work as we move through the triangle.
3. Students brainstorm initial ideas to answer the driving question. Students should begin brainstorming initial explanations for what might have led to the observed changes in the three stories. We hope to tap into students' curiosity and prior knowledge. This will be a very speculative process, some ideas will be great, some not that great, some will be well developed others not. At this point the purpose is to surface their thinking and not to evaluate the ideas. In the subsequent learning segments, we will explore the ideas in more depth and decide if they should stay, go, or be revised.
We explored our initial ideas about how traits change in populations over time and put our ideas in writing so we can refer to them as we learn more
4. Investigation of the Galapagos Finches. With the driving question in mind, students look more closely at the Galapagos finches. After reading background information individually, student groups will review graphs on weather, food, and finch beak size (depth). [note: these resources are in a separate zip file below from other NS resources].
We explored a comprehensive dataset related to the change in the distribution of finch beak size on the Galapagos in the 1970s. We organized the data so that we could see what happened with regard to rainfall, distribution of the trait, seed availability, etc. Next we will discuss what all this might mean and we continue to refine our ideas about trait change over time.
5. Students work together to construct an explanation (or “story”) accounting for what happened with the finches. They will make those ideas public and utilize evidence to support their claims, compare their ideas to those of other groups and then return to the initial model they developed in Segment 3. The class will work together to refine general model ideas based on the specific case of the finches.
We used our detailed exploration of the finches on Daphne Major to first develop a causal account for what happened to them over time and then we used these ideas to revise our initial model for how populations change over time.
6. Students have a range of opportunities to further investigate specific ideas in their model in service of evaluating and revising their model. This segment consists of a series of activities that will help students to explore and deepen their ideas about trait change to see if they are also seen across other species.
We continued to refine our model for trait change over time in populations by using some simulations and investigations. We now have a complete model that we are ready to apply and refine by returning to the finches.
7. Students use their model to create a new and improved model-based explanation to answer their question about the finches.
Originally we wrote our stories about what happened to the finches as a way to generate some model ideas. Then we spent some time testing and revising those ideas and in this learning segment we took our more formalized model and used it in a more explicit way to re-visit our finch stories. Our goal was to make sure we addressed all the elements of our model in our finch explanation. Next we’ll take a look at some other ideas and see if they seem to be present in our explanations and do one final round of revisions to our explanations.
8. Comparison of the class model with two other models. After generating their own model for change over time, students are ready to compare it to two other models proposed more than 100 years ago. Why do traits in populations change over time, has been and still is the “driving question” for many researchers. We provide two different models developed in the 1800s for students to compare with their own model. They can use the text version of the models (Models of Species Change Over Time) to underlie differences and similarities. One was proposed by Lamarck, the other by Darwin and Wallace separately.
Before comparing the models it is a good idea to give students the historical background of when and why these ideas were proposed.
Why did it take Darwin so long to come up with this model if it took students just few weeks/hours? These two short videos will provide some answers and are an optional resource for you to use:
- Who was Charles Darwin (6:30min)
- The life of Alfred Russel Wallace (7min)
We compared historical models for trait change to our model and to each other. We can now use our refined ideas to take one more pass at our finch explanations.
9. Students will revisit their finch explanations one more time to remove any statements or ideas that may be Lamarckian and/or not consistent with Darwin’s theory of natural selection. We will also return to our general model statements and revise them, if necessary.
Just like scientists, we revisited our explanations to tune them and make sure everything we said is consistent with a Darwinian viewpoint. Next we see if our model is useful when explaining phenomena other than the finches.
10. Application of the model. Students will apply their model to explain a new phenomenon. Now that students have experience in applying their model to a phenomenon (the finches beaks), choose between two scenarios about different phenomena for students to explain using their model of natural selection. Or, this would be a good opportunity to add in a socially relevant scenario that is geared toward your students in more specific ways.
We had an opportunity to apply our model of natural selection to at least one other scenario.
Epilogue: The Beak of the Finch
If time permits, you can show students the HHMI video The Beak of the Finch (16min). It is a nice summary of the Grant’s work and contribution. It will allow students to realize that they practiced biology! You can also use the interactive version that has quiz questions included. The quiz is not in MBERized framework.
Download Resources
| Attachment | Size |
|---|---|
| Natural Selection vBio.zip | 73.11 MB |