Final answer:
A paraphyletic group includes a common ancestor and some, but not all, descendants unlike a clade which includes all descendants. It is used to show partial evolutionary relationships on a phylogenetic tree.
Step-by-step explanation:
A paraphyletic group of organisms is defined as a group that includes a common ancestor and some but not all of its descendants. This is in contrast to a monophyletic group or clade, which includes a common ancestor and all its descendants. The definition of a paraphyletic group reflects a partial representation of the evolutionary history and is often used to illustrate organisms on a phylogenetic tree that share a common ancestor but have branched off into different evolutionary paths.
For example, the reptile group is considered to be paraphyletic because it includes dinosaurs (which gave rise to birds) and other reptiles, but does not include birds, which are also descendants of that common ancestor. Therefore, reptiles and birds share a common ancestry, but birds are typically classified separately due to significant evolutionary changes that distinguish them from their reptile relatives.