Final answer:
A shared ancestral character is a trait that originated in an ancestor outside of a particular clade and is shared by members of that clade. It contrasts with a shared derived character, which is a newer trait that is shared only by the members of a certain clade. These concepts help construct phylogenetic trees and understand evolutionary relationships.
Step-by-step explanation:
The student's question refers to a concept in evolutionary biology known as a shared ancestral character, which is a trait that originated in an ancestor that predates the members of the clade in question. This contrasts with a shared derived character, which is a new trait that arises within a clade and is shared by the members of that clade.
A shared ancestral character is a trait that is found in the ancestor of a group, implying all of the organisms in the taxon or clade share that trait because there has been no change in the trait during the descent of each of the members of the clade. An example would be the vertebrate structure in many animals, which is an ancestral trait shared by a wide range of species.
In contrast, a shared derived character arises at a certain point but does not include all of the ancestors in the tree. An example of a shared derived character is the amniotic egg, which is found in some organisms like reptiles, mammals, and birds but not in their earlier fish or amphibian ancestors.
These concepts are essential for understanding evolutionary relationships and constructing phylogenetic trees, as they help distinguish between different clades and trace the evolutionary history of species.