Read and construct a simple cladogram, correctly identifying clades, nodes, and the most recent common ancestor of a given pair of organisms.
A phylogenetic tree, or cladogram, is a diagram showing hypothesized evolutionary relationships. A node is a branch point representing a common ancestor and the speciation event that produced two new lineages; a branch represents one lineage over time; a clade is a single ancestor together with all of its descendants, a complete, unbroken branch; and sister taxa are the two groups splitting from the very same node, making them each other's closest relative shown on that tree.
Trees today are built primarily by comparing DNA and protein sequences across species, since greater sequence similarity generally indicates a more recent common ancestor, combined with anatomical and fossil evidence, directly extending Unit 9's molecular evidence. Two common misreadings are worth correcting directly. First, a tree's branches can be rotated at any node without changing its meaning at all -- rotating a branch changes only the left-right order it happens to be drawn in, never who is related to whom. Second, the left-to-right order organisms happen to be listed along the tips of a tree does not mean the rightmost organisms are "more evolved" -- every living species at the tip of a tree has had exactly the same amount of time to evolve since the root, without exception.
Mastery looks like: They can define a clade, identify sister taxa and a most recent common ancestor on a tree, and correctly explain why branch rotation and left-to-right order do not carry biological meaning.
Common stumbles: Reading tip order left-to-right as a ranking from primitive to advanced; thinking rotating a branch at a node changes which species are more closely related.