Final answer:
A synapomorphy is a trait that is uniquely shared by all members of a clade, and is used to identify evolutionary relationships within cladistics, distinguishing the clade from the outgroup.
Step-by-step explanation:
A synapomorphy is a homologous trait that is shared among the members of a clade, but not with members of the outgroup. It is a characteristic that arose in the ancestor of the clade and is specific to that clade only. For example, the presence of an amniotic egg in reptiles, birds, and mammals is considered a synapomorphy because it is a derived trait that separates these groups from their more distantly related ancestors, like fish. These traits are critical for constructing phylogenetic trees in cladistics, helping to identify evolutionary relationships by analyzing shared derived characteristics.
When constructing a phylogenetic tree or a cladogram, it is important to use synapomorphies to ascertain the evolutionary connections between species. These traits are contrasted with ancestral characteristics, which are found in both the clade of interest and the outgroup, and thus do not inform us about the more recent evolutionary changes that have occurred within the clade.
Therefore, the correct definition of a synapomorphy is option 3: a homologous trait that is shared among the members of a clade.
A synapomorphy is a shared derived character identifying a clade and its evolutionary separation from outgroup lineages.