Final answer:
A synapomorphy is a shared derived characteristic that is present in all descendants of a common ancestor, distinguishing them from other groups and aiding in the construction of phylogenetic trees.
Step-by-step explanation:
A synapomorphy is defined as (c) a shared derived characteristic. It represents a trait that has evolved in a common ancestor of a particular group and is present in all of its descendants, distinguishing that clade from others. In contrast to shared ancestral characters, which are found in all members of a clade and their ancestors, synapomorphies help to clarify evolutionary relationships within a phylogenetic tree by indicating traits that are new to a lineage after it diverges from its common ancestor.
For example, the presence of an amniotic egg is a synapomorphy that unites lizards, rabbits, and humans into a distinct group separate from fish, lampreys, and lancelets. This trait is derived at a specific point in evolutionary history and does not include all of the ancestors in the tree, thus serving as a key indicator in constructing a phylogenetic tree and understanding the divergent paths of evolution within different organisms.