Final answer:
Shared derived traits are used to define a clade for more recent common ancestry, while shared ancestral traits are characteristics found in the common ancestor of a group and are not as useful for defining a clade in phylogenetic analysis. The correct answer is option A.
Step-by-step explanation:
Shared derived traits are used to define a clade, while shared ancestral traits are not specifically used for this purpose. A clade is a group of organisms that includes a single common ancestor and all its descendants, and is best defined by traits that have appeared (derived) within the group after they have diverged from other lineages.
Shared ancestral traits are those characteristics that were present in the ancestor of a group, thus shared by the entire clade and other related clades. For instance, the vertebrate backbone is a shared ancestral trait among all vertebrates. However, when constructing phylogenetic trees, these traits do not provide information about more recent evolutionary relationships. On the contrary, a shared derived trait, like the amniotic egg in reptiles, birds, and mammals, is only found in those descendants that have branched off after the trait emerged, and thus it is informative for delineating more recent common ancestry within a clade.
Therefore, the correct answer is A: Shared derived traits are used to define a clade, while shared ancestral traits are not relevant.