Tree thinking is a term biologists use to describe the process of approaching biological problem solving by considering the role of descent with modification based on phylogenetic evidence. This can result in controversies such as that surrounding the evolution of birds. Did the clade that includes birds diverge from a particular group of dinosaurs? Or did it diverge from a line of reptiles that did not include dinosaurs? These types of questions have fueled much debate and extensive research efforts for decades. Because multiple sources of data exist, both the conclusions reached and the trees drawn by independent researchers may not agree. Not only the selection but also the interpretation of characters can support different phylogenies. Biologists try to resolve these questions by carefully weighing evidence from multiple sources.

Look closely at the tree shape in the following figure.  When you investigate the reasonableness of a tree, you should examine all the hypotheses it contains (Donovan and Hornack, 2004). Each branch point in the tree represents a hypothesis about the relationships among members of the ungulates. The order in which groups diverged is also an explicit hypothesis.

After you have reviewed the descriptions of hypothesis A and hypothesis B, describe hypothesis D.

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Hypothesis A: Hippos and whales shared a common ancestor more recently with each other than with other ungulates. The two groups have since diverged. Each possesses derived characters not found in the other groups. A Hypothesis B: The group containing deer and giraffes diverged from the group containing hippos and whales. These two groups also shared a common ancestor, but less recently than the one shared by hippos and whales.