Phenomenon
In a population of organisms sometimes we see the distribution of traits changing over time. Specifically, we consider how bacteria have changed with regard to antibiotic resistance, how the frequency of certain peppered moths changed in pre/post industrial England, and how beak size changed in a population of finches on the Galapagos Islands.
Question
How do traits in a population change over time?
Model Ideas
The following ideas are the basis of Darwin's model of natural selection:
1. Populations have the potential to grow exponentially but they typically don't because of limited resources or environmental factors.
2. Variation in traits naturally exists in populations.
3. Depending on the environment, traits can be advantageous or disadvantageous.
4. Individuals with more advantageous traits are more likely to survive, and therefore, can reproduce.
5. Many traits are heritable, so offspring tend to resemble their parents.
6. In each generation the number born with the advantageous trait will increase and the number with the disadvantageous trait will decrease.
7. Over time the distribution of traits in the population will change.
Overview
In Unity & Diversity (UD), students generated some ideas about the breadth of diversity of life on Earth and the remarkable similarities that we see across species. They also observed how biodiversity has changed over eons.
In this triangle, natural selection (NS), students will begin to bring some of the ideas from UD to wonder about how traits change over time. Through observation and identification of a phenomenon–changes in traits across three unrelated species–and a data-driven activity focused on Galapagos Ground Finches, students will develop a model of natural selection that is similar to models synthesized by Charles Darwin and Ernst Mayr. They will then go on to apply and refine their model.
1:
Overview: 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).
What we figured out...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:
Overview: 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.
What we figured out...After examining 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:
Overview: 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.
What we figured out...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:
Overview: 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].
What we figured out...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 will continue to refine our ideas about trait change over time.
5:
Overview: 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.
What we figured out...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:
Overview: 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.
What we figured out...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:
Overview: Students use their model to create a new and improved model-based explanation to answer their question about the finches.
What we figured out...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 compare them to our current model.
8:
Overview: Comparison of the class model with two other models. After generating their own model for change over time, we provide two contrasting models developed in the 1800s. Students compare these to the class's model and highlight 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, which you can do through two short videos.)
What we figured out...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:
Overview: 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.
What we figured out...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:
Overview: 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.
What we figured out...We had an opportunity to apply our model of natural selection to at least one other scenario.