Fig. 1

Alternative hypotheses for the relationships of a set of vertebrate taxa, used to introduce students to the principle of parsimony as applied to phylogenetic analysis. In the two trees shown, we illustrate the concept by showing students that the bony skeleton is hypothesized to have arisen once in tree 1, while tree 2 requires two separate evolutionary origins of this character state. Students are subsequently given an exercise in which they are challenged to count the number of evolutionary changes in each of two trees, and then decide which of the two trees is a better phylogenetic hypothesis. Graphic illustration adapted with permission from the University of California’s Understanding Evolution Web Site (http://evolution.berkeley.edu/evolibrary/article/0_0_0/phylogenetics_08)