Final answer:
In cladistic classification, shared derived characters are the most important for determining the evolutionary relationships within a clade and constructing a phylogenetic tree.
Step-by-step explanation:
In cladistic classification, the most important characteristics are those that are shared derived characters. These are traits that appear in some members of a clade and represent changes from the ancestral state. Shared derived characters are essential in determining the evolutionary relationships within a clade, as they help to identify the most recent common ancestor and the branching events that have led to the diversity within the group. On the other hand, shared ancestral characters, while interesting for showing common ancestry, do not provide information about more recent evolutionary branches. Therefore, when constructing a phylogenetic tree or cladogram, evolutionary biologists focus on shared derived characters to map out the lineage diversification and relationships within a group.