In the explanation phase of the investigation, students should describe to each other and to the class the conclusions drawn from the data. They then should construct plausible explanations that can account for the observed data. In developing these explanations, students should explore the limitations of the data in providing complete explanations and consider what additional kinds of evidence might be gathered to support their hypotheses.
Students will continue to rely on the Student Reading, the map of the Hawaiian islands, and the data in Table 1 to provide the raw material for their narratives.
Constructing a narrative gives students the opportunity to relate easily understood events—such as the journey of an individual fly from one island to another—to data that may seem to have little relevance to the everyday world. Such accounts can make abstract concepts such as speciation concrete and accessible. Putting the conclusions they draw from the data in the form of a story allows students to demonstrate their understanding of important concepts in their own words, emphasizing the understanding of the idea rather than their knowledge of facts or terminology.
To initiate this part of the investigation, teachers could assign the following task: Write a narrative describing how the four species of picture-winged flies may have evolved from a common ancestor.
The entire class, working together, could write the narrative, with the teacher guiding the class in selecting conclusions consistent with the data. Or the teacher and class could work together to begin the story, with students finishing the story working in groups. Alternately, the students could write the stories individually or in small groups.
One way to begin the writing of narratives would be to provide students with all or part of the following sample narrative:
About four million years ago on the island of Kauai there lived an inter-breeding population of picture-winged flies. Every member of this population had the four chromosomal inversions i, k, o, and b. About three million years ago a storm on Kauai resulted in a large number of flies being swept off the island and carried over the sea. Most of the flies