Final answer:
Pleisiomorphy refers to shared ancestral characteristics found in a common ancestor of different species. While important for providing a baseline in evolution, they are less useful in cladistics than shared derived traits for determining species relationships.
Step-by-step explanation:
Pleisiomorphy (Sympleisiomorphy) refers to shared ancestral characteristics. These are homologous structures or traits that may also be found in the common ancestor of the species being classified. In cladistics, these ancestral traits are essential as they provide a baseline from which one can determine the characteristics that evolved in the descendant species. However, when determining relationships between species, cladistics emphasizes the importance of shared derived traits because they indicate changes that have occurred since the time of the last common ancestor and can help in understanding the evolutionary relationships between organisms. Unlike shared derived traits, shared ancestral characteristics are not useful in determining the finer details of evolutionary relationships within a clade because they do not change during descent.