Final answer:
Pleisiomorphy, or ancestral trait, is a shared characteristic among a group of organisms that can be traced back to a common ancestor. Sympleisiomorphy, or shared ancestral trait, is a trait present in multiple taxa due to their common ancestry.
Step-by-step explanation:
Pleisiomorphy & Sympleisiomorphy
Pleisiomorphy, also known as ancestral trait, refers to a trait that is shared by a group of organisms and is considered primitive or ancestral. This means that the trait existed in a common ancestor and has been passed down to subsequent generations. On the other hand, sympleisiomorphy, also known as shared ancestral trait, is a trait that is present in multiple taxa due to common ancestry. It is shared by different species that originated from the same common ancestor.
For example, consider the trait of having hair in mammals. This is a pleisiomorphy because it originated in a common ancestor of all mammals and has been inherited by all mammalian species. However, when considering primates, having opposable thumbs is a sympleisiomorphy because it is a shared ancestral trait present in multiple primate taxa, indicating their common evolutionary history.