What is defined as a homologous trait shared by some but not all members of a group?

The correct answer refers to a synapomorphy, which is defined as a derived character or trait that is shared by some, but not all, members of a specific group or clade. This term is particularly important in the context of phylogenetics, where synapomorphies are used to help establish the evolutionary relationships among organisms. By identifying these shared derived traits, researchers can determine which species are more closely related to each other based on their common ancestry.

In contrast, a plesiomorphy refers to an ancestral trait that is retained from a common ancestor and is not necessarily a defining characteristic of a derived group. A taxa, on the other hand, denotes a taxonomic group itself rather than a trait, and an ancestral trait indicates traits present in an ancestor that may not be useful for defining groups derived from that ancestor. Thus, the focus on derived traits that signify a specific lineage makes synapomorphy a key concept for understanding evolutionary relationships.