access to some basic concepts in order to see a problem and investigation as meaningful. Over time, they will require additional skills as the investigation advances: they may need a method for collecting relevant data and then a method for analyzing the data. They will almost certainly need structured support in building the logical links that help move them from data to scientific explanation, as well as help them reflect on what they’ve learned in light of previous observations. Like the problems themselves, these and other skills need to be made meaningful to students, and presenting them in the context of a problem to which they can be readily applied helps students understand their utility.

Recently researchers have developed very promising results from building and testing science curriculum units that, from the outset, engage students with problems they will investigate over the course of several weeks or months. These units sequence lessons to gradually build students’ knowledge and skill over time so that they arrive at each phase in an investigation prepared to engage in the necessary work.

“Struggle for Survival” is a six- to seven-week classroom science investigation that supports the learning of core evolutionary concepts. Developed as part of the Biology Guided Inquiry Learning Environments (BGuILE) project at Northwestern University, the unit is designed to support the learning of core concepts in evolutionary biology.1 Using software that depicts a prolonged drought on the Galapagos Island Daphne Major, students investigate how the drought affects the animal and plant populations on the island. They learn background information about the island, read through the field notes of researchers, and examine quantitative data about the characteristics of the island’s species at various times to look for changes in the populations.

The unit unfolds over four phases, which are sequenced to gradually increase the demands of the learning experiences and the sophistication of students’ reasoning about core concepts. The students are presented with a problem at the beginning of the unit—the finch population on the island has declined precipitously. Their job is to examine a range of evidence to determine what has caused this decline. Within this framework, students engage in a study of the problem over a period of approximately six weeks to advance their understanding through reading, posing questions, data analysis, presentation, and debate.

The first phase (10 classes) sets the stage by probing students’ existing knowledge of natural selection, by providing requisite background knowledge about ecosystems and the theory of natural selection, and by building student motivation. In the second phase (five classes), students learn about the Galapagos

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