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MUSE | Natural Selection | Unit Overview and Materials | Section 2: Comparing Explanatory Models | 2C: Understanding Lamarck's Model | Instructional Notes

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INSTRUCTIONAL NOTES


Intended Learning Outcomes

Lamarck's model:

  • Lamarck's model invokes the needs of organisms to account for species change
  • Lamarck believed that inheritance could be affected by the use or disuse of particular body parts

Learning outcomes related to modeling:

  • Models are judged to be acceptable or not based on how well they explain the data, how consistent they are with other knowledge, and how well they can be used to predict new data

  • Models are ideas that scientists use to explain patterns they see in the world

  • Models can be compared based on data, the inferences made in the model, and the prior knowledge and beliefs upon which they are based

Supplies
  • Edited reading from Zoological Philosophy by Jean Baptiste de Lamarck
  • Reading questions: Lamarck and the Inheritance of Acquired Characteristics
  • Questions: Lamarck's Prior Knowledge and Beliefs

Time Frame and Sequence

This activity will take two class periods if students do the reading and answer the questions at home.

Assign students the Lamarck reading and the reading questions (Lamarck and the Inheritance of Acquired Characteristics) for homework one or two nights before the discussion of the Lamarck reading.

Day One

Begin class by giving students an opportunity to bring up their own questions about the reading or vocabulary that they found confusing. Like Paley, the writing of Lamarck can be difficult for students. As students bring up questions, encourage other students in the room to help answer them. This will take the focus off of you and transfer the responsibility for developing understanding to the class as a whole.

Once students have had a chance to get their more general questions answered, move on to a discussion of the Reading Questions. Have students take turns offering their answers to the rest of their classmates. Facilitate a discussion around the questions. It may be difficult for students to discern the exact mechanism that Lamarck proposed from this reading. It should, however, be clear to students that he did believe that organisms had changed over time and he begins the chapter by objecting to those who do not think this happened. The data that provided the foundation for some of his inferences came from the fossil record. In this excerpt there is also a lot of attention paid to the problem of defining a species. He argues that the fossil organisms that we no longer see on Earth have not become extinct, but rather have changed to such an extent that we no longer recognize them as being the same.

For the purposes of this unit we have chosen to focus on a simplified version of Lamarck's ideas. He did quite a bit of writing about origin and teleologic progress, but we have not included those parts of his writing in our unit. Instead, this excerpt provides some references to the idea that environmental changes directly alter individual organisms and that those new "acquired" characteristics are then passed on to the offspring (this is on the first page of the reading). The gradual accumulation of these changes over time accounts for the changes that are evident in the fossil record.

Day Two

Use this day to give students an opportunity to examine and discuss the prior knowledge and beliefs of Lamarck. You may want to begin the class period with a brief discussion of the fossil record. Provide students with real fossils or pictures of fossil organisms that no longer exist on earth. This will help them appreciate the observations made by Lamarck. Pass out the second set of questions (Lamarck's Prior Knowledge and Beliefs) and have students work in groups to answer them. Once students have answered the questions in their groups, call the whole class together and discuss their answers. They should be able to determine that Lamarck saw species as malleable and that he proposed a naturalistic mechanism that relied on the perceived needs of the organisms. The role of variation in Lamarck's argument may be difficult for students to understand. He does discuss variation between generations of organisms-essentially the idea that gradual changes accumulate-but he does not place any emphasis on variation that exists among organisms of the same generation.


Teaching Strategies and Student Ideas

Part of the reason for having students look at the arguments of Paley and Lamarck in addition to that of Darwin is to give them opportunities to examine arguments and use the framework developed during the Cartoon Activity. It is important for students to have practice doing this so that later they will be more skilled in critiquing arguments, an important part of the unit.

The distinction between phenomena and observations and the explanations for them is difficult for students to make at first. This difficulty often appears in the answers to the Lamarck reading questions. In the excerpt provided here, the major phenomenon that Lamarck was trying to explain is that species have changed over time. He used evidence from the fossil record to support his claim that the species we see today have arisen by the evolution of ancient organisms. The mechanism he proposes for this change is that during an organism's lifetime it will undergo changes in response to its environment. These changes will then be inherited by the offspring and over time all the organisms that have been subjected to that particular environment will have changed. Implicit in this assertion is that species change as the result of the "needs" of individuals.

Information about Lamarck is found in most standard biology textbooks. Usually, there will be a brief description of his ideas and then a paragraph or two explaining why he was wrong. Often, students think that models which are no longer accepted must have been poorly developed (a notion supported by many textbooks). Our intention is not that students come away thinking that Lamarck or Paley were inferior scientists. Instead, the purpose is to make the point that as the available data changes and the divisions between supernatural and naturalistic mechanisms became more distinct, what was considered acceptable by the scientific community also changed. The writings of both of these men made important contributions to evolutionary biology. Discussing these models provides a forum for talking about how scientific ideas change over time.

Lamarck's ideas also map onto many students' intuitive ideas about how evolution may occur. There have been a number of studies that have documented students' tendency to think in Lamarckian terms. That is, students often invoke the needs of organisms when accounting for change over time. They also often believe that evolution is goal-directed or teleological. Discussion of Lamarck's ideas will provide a context in which a clear distinction can be made between these ideas and those of Darwin. At this point in the unit, however, the purpose is not to begin to make comparisons (that will come in material 2E), but simply to make certain students understand the argument put forth by Lamarck so that later comparisons can be more fruitful.

 

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