Darwin's Model of Natural Selection:
· Excerpt from Origin of Species by Charles Darwin
· Reading Questions: Darwin's Natural Selection Model
· Questions: Darwin's Prior Knowledge and Beliefs
Time Frame and Sequence
This activity will take three class periods to complete.
Prior to Day One
Have students read the condensed Excerpt From Origin of Species and answer the first set of Reading Questions.
Begin class with a general discussion of the reading. Ask students what interesting things they noticed or if there were any parts that surprised them. Give them an opportunity to ask clarification questions on any areas that were confusing. Once all the general questions have been answered, move on to a more direct discussion of the reading questions. Students sometimes have difficulty distinguishing between the phenomenon that Darwin was trying to explain and the data that he used to back up his explanation. They also sometimes have trouble understanding how Darwin used the artificial selection example to help make his argument.
The longest part of the discussion will likely revolve around the mechanism Darwin proposed. This excerpt of Origin was chosen because all of the components of the natural selection model are laid out within it. Have students point to particular passages when making statements about the mechanism. They should be able to find information on superfecundity, the importance of variation, differences in reproductive success, and heritability. Your role is to help them put the pieces together to form a complete picture.
After students have read and discussed the Darwin reading, we usually have a pigeon breeder visit the class. She brings several different varieties of pigeons that have been artificially selected for various traits. She spends the class period talking to students about her breeding program and answering questions about how she is able to achieve such exaggerated variations of traits (such as leg length or crop size). This is a wonderful opportunity for students to see why Darwin chose this as an example in his own writing. The particular breeder we invite is active in a local pigeon breeding club and tells us she loves to have the opportunity to share her hobby with young people. If you are interested in contacting a breeder you may be able to find one through local pigeon breeding organizations. Alternatively, you may want to invite a dog breeder or any one who uses artificial selection. If no speakers are available, have students examine photographs of animals or plants that have been artificially selected. We have included in this site a collection of Pictures of Dog Skulls that could be used by students. We have also included several Images of the Pigeons that the breeder has brought to class.
After students have had the opportunity to see the power of artificial selection by examining organisms that have been exposed to strong selective pressures, spend a few minutes discussing how the process is similar to and different from Darwin's model of natural selection. Next, pass out the second set of questions (Lamarck's Prior Knowledge and Beliefs). Have students break up into groups to discuss these questions. This discussion should only take about 10 minutes because the questions are now familiar from the discussions of Paley and Lamarck. Usually, students find this task easier with Darwin than they did with the others. This may be because they are more familiar with Darwin's ideas. Once students have finished in their small groups, call the students together again for a whole class discussion. Here is an opportunity to highlight some of the ways that Darwin's prior knowledge and beliefs differed from those of Paley and Lamarck. Emphasize that Darwin clearly believed that organisms change over time, but in contrast to Lamarck he thought of this as a change in the frequency of traits of a population rather than at the individual level. Although it is easy and appropriate to make some comparisons at this point, do not spend a lot of time comparing the three models because students will have an opportunity to do that in the next activity (Material 2E). The primary aim of these three days is to give students a preliminary understanding of Darwin's argument-the basic components of the model. They will have a lot of time during future activities to come to a deeper understanding of the mechanism of natural selection.
Teaching Strategies and Student Ideas
First set of reading questions
As mentioned above, students may have difficulty distinguishing between the various aspects of Darwin's argument. Because he builds his argument throughout the excerpt, students sometimes get lost in the details. Help them to see that the basic phenomena he was trying to explain is the observed species diversity and the apparent match between organisms and their environment. Next, if students are having difficulty, you might guide them through each component of the mechanism, having them look at each of these as "mini" arguments. For example, when talking about superfecundity he develops an example about elephants. In this case, the phenomenon is that there are more offspring produced than can survive. The data that he uses to support this observation comes from calculations of the potential for geometric population growth. By calculating how many elephants could descend from a single pair over a 500 year period he illustrates that clearly not all offspring survive or the planet would be overrun with elephants. He then uses this idea and his prior knowledge about animals of the same species eating the same food to infer that there must be a struggle among members for survival. Frequently, the data that supports an aspect of his argument stems from his extensive prior knowledge about the natural world and is developed with specific examples.
Students may have a difficult time understanding how the description of artificial selection figures into his argument. Many students answer question 3 by simply describing artificial selection and do not explain how it figures into his argument. Point out that he used this in a way similar to the way Paley used the analogy between the eye and the telescope. He describes a process that is similar in many ways to the mechanism he is proposing and is making a case that if humans can affect such dramatic change over a short period of time, then it is plausible to believe that natural forces acting over millennia could account for all the diversity of living things we see today.
Differences between Lamarck and Darwin
Interestingly, we have found that students are often confused about the differences between Lamarck and Darwin. They tend to focus in on the apparent agreement between the two that organisms change over time. While this is a common belief of both, it is important for students to see that they proposed very different mechanisms to explain how those changes occur. Emphasize that they differed in the role they attributed to change on the individual level within an organisms' lifetime. Darwin focused on a shift in the frequency of heritable variations in the population, while Lamarck focused on acquired changes at the individual level. One of the key reasons we have students spend time examining the ideas (admittedly greatly simplified) of Lamarck is to directly address some common misconceptions students bring to the study of natural selection. These will be discussed in more detail in the instructional notes for the Three Model Comparison (material 2E).
NEXT PAGES: Student handouts