back to front page of natural selection

click to print this page from Adobe Acrobat

MUSE | Natural Selection | Unit Overview and Materials | Section 2: Comparing Explanatory Models | 2E: Comparison of the Models | Instructional Notes


Student handouts


site map



Learning Outcomes

Darwin's Model of Natural Selection:

  • Populations are made up of individual organisms whose traits (structural, behavioral, biochemical) exhibit variation from one individual to the next.
  • Many trait variations are inherited
  • Some trait variations may be advantageous while others may not be
  • Whether or not a variation is advantageous depends on the environment within which a population lives
  • The frequency of particular trait variations in a population may change over time due to differential survival and reproduction
  • Evolutionary changes occur in populations or species, not individuals
  • Offspring are not exactly like their parents-they also vary, but the variations of their traits are likely to be similar to those of their parents
  • Variability in a population can be represented graphically
  • Organisms produce more offspring than can survive
  • There is competition between organisms for resources

Models of Paley and Lamarck:

  • Paley's model invokes an intelligent designer to account for the apparent perfection of organisms' adaptations to their environments
  • Paley believed species to be fixed and unchanging, each having an "essential" type that set it apart from all others
  • 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 can be compared based on data, the inferences made in the model, and the prior knowledge and beliefs upon which they are based

  • Phenomenon Handouts:

    • Pigeon Varieties
    • Origin of the Eye
    • Existence of Fossils

  • Paley, Lamarck, and Darwin Summary Page

Time Frame and Sequence

This lesson will take two 45 minute class periods.

Day One

As a result of lessons 2B, 2C, and 2D, the students should have a reasonable grasp of the ideas of Paley, Lamarck and Darwin. Although, they may have been tempted to compare the ideas all along, this activity will provide an opportunity for explicit comparisons between the models of these three men.

Begin by reminding students that the explanatory power of a model is one way that it can be judged against others (as discussed during lesson 2A). During this activity, students will have an opportunity to use each of the models to explain some phenomenon. Each group of students will think about one particular phenomenon from the perspective of all three men. There are three phenomena to be considered: the origin of the eye, the diversity among pigeon breeds and the existence of fossils. Each of these phenomena was addressed by one of the authors, but not the others. The students task is to imagine, based on what they understand about each model, how the model could be used to explain the phenomenon. For example, one group will consider how Paley, Lamarck, and Darwin would explain the existence of fossils, another group will consider how each would explain the origin of the eye, and yet another will explain the existence of fossils from each perspective. Depending on the size of your groups and the class as a whole, you will probably have multiple groups working on each phenomenon.

Divide the class up into their respective groups and give each group a phenomenon handout (either eye, fossils, or pigeons). Allow them about 20-30 minutes to discuss how each of the models could be used to explain their phenomenon and to write down their ideas on the page provided. With any remaining time in the class period have them pair up with another group that worked on the same phenomenon to compare answers.

Day Two

Begin the class period by discussing the explanations students developed yesterday. Depending on time, you may not want to discuss all three explanations for each phenomenon, rather ask for a sampling from various groups making sure to discuss each model at some point.

As this discussion progresses, begin to ask students which model they think has the broadest explanatory power. Often students will jump immediately to saying that Darwin's model can be used to explain all three phenomena without difficulty. One reason they probably say this is that they realize that Darwin's ideas are the accepted ones in science today. Try to push them on this point a bit. In many ways, the ideas of Paley and Lamarck also have a great deal of explanatory power if you accept their underlying prior knowledge and beliefs. In the case of Paley's model, it can be used to explain the eye and pigeon varieties by invoking the supernatural creator that is central to Paley's belief system. It may be a bit more challenging to use his ideas to explain the presence of fossils however. The point here is that although explanatory power is one criterion used to judge models, there are others as well. Examining the underlying prior knowledge and beliefs of a scientific argument is another legitimate avenue of critique. There is a handout called Paley, Lamarck, and Darwin Summary Page that can be passed out to students at this point.

Toward the end of the period tell students that the remainder of the unit will be spent learning more about and using the Darwinian model. The discussion at this point should center on why that is the case and also on why the models of Paley and Lamarck were discussed at all. Ask students for their ideas on these points. Explain to students, if they do not bring it up, that examining these historical ideas helps bring Darwin's model into clearer focus by providing some obvious points of contrast.

Teaching Strategies and Student Ideas

The goal of this activity is to allow students an opportunity to extend their understanding of the three models by using them to explain phenomena. A second goal is to provide a time for students to explicitly compare the three models. As students begin to work, they will probably find it difficult to use each of the models to explain their phenomenon. Suggest that they start with the person who brought up the phenomenon originally as a way to warm up. For example, with the pigeon group, suggest that they start with Darwin and then move through the others.

Frequently, this exercise uncovers some of the confusion students may have about the models. One such area involves the distinction between Lamarck and Darwin. Although these models are quite different, students often focus on the similarities between them. Both Lamarck and Darwin discuss the concept of evolution: change over time. They also both put a great deal of emphasis on the environment in their writing. Where they differ markedly, however, is in the mechanism for species change. Remind students who are having trouble that Lamarck focused on the changing of individuals while Darwin discussed the changes in populations over time. Lamarck also postulated that the needs of individuals resulted in change whereas Darwin pointed to existing variation as the raw material for change.

The discussion on the second day is an important turning point in the unit. Up until now the focus has been on the three different models and on the task of trying to analyze and critique arguments based on the framework developed during the Cartoon Activity (Lesson 1B). From this point on, the unit will concentrate solely on Darwin's ideas. It is important to discuss with students the reasons for studying the others so they don't feel their time was wasted. In asking students why they think Paley and Lamarck were discussed, we have found they often give insightful answers.

There are two points you should be sure to cover during this discussion. The first is that examining all three arguments provided students with valuable practice in analyzing and critiquing arguments. The second, and more important point, is that studying the other two helps bring Darwin's ideas into clearer focus. The goal is that by looking carefully at the prior knowledge and beliefs of Paley and Lamarck students will avoid some of the common problems encountered in learning about natural selection. Specifically, the distinction between a natural and supernatural explanation are highlighted by learning about Paley. Explain to students that modern science is based on a commitment to naturalistic explanations. There is a significant body of research showing that students are often led astray in their studies of natural selection by their own intuitive Lamarckian ideas. By studying his model explicitly, it may be possible to circumvent these difficulties. For example, students often invoke the needs of organisms when accounting for change over time. By having studied Lamarck's model, it will be possible to label such ideas as Lamarckian and discuss how they are inconsistent with Darwinian ideas rather than simply telling students they are wrong. We have found that having these types of discussions early on facilitates deeper understanding of Darwin's model of natural selection in the long run.


NEXT PAGES: Student handouts