Walruses and whales are both marine mammals. So are dolphins, seals, and manatees. They all have streamlined bodies, legs reduced to flippers, blubber under the skin and other adaptations for survival in the water. Although mammals evolved on land, these species have returned to the sea. Did they evolve from a single ancestor who returned to the ocean, or were there different return events and parallel evolution? We can't go back in time to observe what happened, but DNA sequences contain evidence about the relationships of living creatures. From these relationships, we can learn about the evolutionary history of marine mammals.

In this lab, students use sequence information in GenBank (the public repository of all known DNA and protein sequences from many species) and bioinformatics software to test hypotheses about the relationship between aquatic mammals (seals, whales, dolphins, walruses, manatees, and sea otters) and their potential ancestral relationship to land mammals. In the process, students learn how to build cladograms from molecular data and how to analyze them to make phylogenetic conclusions.

This lab requires a computer lab. Optimally with the teacher's computer projected so that students can initially follow along. In addition, two software products must be installed on each computer ahead of time:

• ClustalX -- you want the graphical version of ClustalX (win.msi for Windows or macosx.dmg for Macs)
• PhyloWin -- for this version of the lab, you need to use the old version of PhyloWin, so I have directed you to the page of legacy versions of the software.

In order to do this in my district, I had to assure my IT staff that (1) "the software is state-of-the-art professional, scientific software, so they didn't have to worry about viruses or malware," (2) "the software is a free download, so it would not cost the district anything," and (3) "the software is not resource-intensive so neither program would tax either the network or the individual computers." I also explained that "the software allows students to make evolutionary family trees for groups of organisms" and that "this lab is necessary because this material is tested on the AP Biology exam. In fact, one of 2009's essays was dependent on understanding this lab exercise."

A sample lab report for this exercise is available to teachers in the Teachers Vault.