In the exploration phase of the investigation, students use actual genetic data to trace evolutionary lines of descent for four species of Hawaiian drosophilid flies. By comparing their findings to information about the geology of the Hawaiian islands, they then conclude that new species tend to appear on younger islands.
Students will work from the data in Table 1. Later, they will compare their evolutionary trees to the dates shown on the map of the Hawaiian islands.
The students’ objective is to derive evolutionary pathways from the data in Table 1 for four of the species described in that table: Drosophila heteroneura, D. hanaulae, D. substenoptera, and D. primaeva. Students should work in small groups so they can arrive at a consensus about the most reasonable structure of the tree. They should construct a single tree agreed upon by all members of the group so they gain experience with achieving consensus based on empirical evidence and defending their reasoning.
Table 1 begins by listing 11 chromosomal inversions found in the species D. heteroneura. Following this are the inversions found in 12 other species of picture-winged Drosophila found in the islands. These are incomplete listings of the inversions, since only the 11 inversions found in D. heteroneura are given in this table. Most picture-winged species have some inversions not found in D. heteroneura. For example, D. setosimentum, which also is found on the Big Island of Hawaii, has a total of 21 inversions, as shown on the top line of Table 2. However, it shares only four of its inversions with D. heteroneura, as shown on the bottom line of Table 1. Note, too, that some species have identical inversions so they need to be distinguished using other physical or behavioral characteristics.
Students can perform this exercise without exploring all of the dimensions of the data contained in Tables 1 and 2. At the same time, advanced students can use the data provided in Tables 1 and 2 to extend the exercise in many productive directions.
Once the class has used the data in Table 1 to reconstruct the evolutionary relationships among D. heteroneura, D. hanaulae, D. substenoptera, and D. primaeva, they should be able to conclude, by comparing their trees with the geological ages of the islands, that new species tend to appear on younger islands.