Figure 1.

Graph showing characteristics of branchiopod populations.

indicates, the results for either dimension show a continuous variation for the two populations. Students observe that regardless of the dimension measured, the mean for the two populations differs.

After the graphs are drawn, Mr. D. asks the students to explain the differences in the populations. The students suggest several general explanations: evolution has not occurred—these are simply different kinds of brachiopods; evolution has occurred—the differences in the means for length and width demonstrate evolutionary change in the populations; evolution has not occurred—the differences are a result of normal variations in the populations.

Mr. D. takes time to provide some background information that the students should consider. He notes that evolution occurs in populations, and changes in a population's environment result in selection for those organisms best fit for the new environment. He continues with a few questions that again challenge the students' thinking: Did the geological evidence indicate the environment changed? How can you be sure that the fossils were not from different environments and deposited within a scale of time that would not explain the degree of evolutionary change? Why would natural selection for differences in length and width of brachiopods occur? What differences in structure and function are represented in the length and width of brachiopods?

The students must use the evidence from their investigations and other reviews of scientific literature to develop scientific explanations for the aforementioned general explanations. They take the next class period to complete this assignment.

After a day's work by the students on background research and preparation, Mr. D. holds a small conference at which the students' papers are presented and discussed. He focuses students on their ability to ask skeptical questions, evaluate the use of evidence, assess the understanding of geological and biological concepts, and review aspects of scientific inquiries. During the discussions, students are directed to address the following questions: What evidence would you look for that might indicate these brachiopods were the same or different species? What constitutes the same or different species? Were the rocks in which the fossils were deposited formed at the same or different times? How similar or different were the environments of deposition of the rocks? What is the effect of sample size on reliability of conclusions?

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