Figure 2.

Answer Key for the PhAT. The key for part A is shown, with symbols mapped onto both tree I and tree II. Note that that the figure shows only one of the multiple correct ways to map the minimum of eight evolutionary changes onto tree I; all correct mappings were given full credit. For example, for both “Large incisors” and “Large canine teeth,” independent losses could have occurred on the branches leading to rat and rabbit, or a loss could have occurred in the common ancestor of rat, rabbit, dog, and cat, followed by a secondary gain in the common ancestor to dog and cat. On the other hand, there is only one way to map characters minimally onto tree II. For the first part of part B (part B′), the correct answer is tree II, which requires seven evolutionary changes, as opposed to tree I, which requires a minimum of eight changes. Refer to Table 2 for correct responses to part B″. For part C, the closest relatives in tree I are the dog and cat, while in tree II, the closest relatives are the rat and rabbit. In both cases, the reason is they share a common ancestor more recently than any other pair of taxa. Refer to Table 2 for correct responses to part C″.