In taxonomy, a group that includes an ancestor and all of its descendants, living and extinct, is called a clade. The word is derived from the Greek word for "branch," which is an apt metaphor for the relationship between ancestors and their offspring in the tree of life. Non-clades are groups of species that either exclude some descendants of the ancestor or incorporate species that are not descendants.
Mammals are an example of a clade. All mammals descended from a single ancestral species that probably lived during the Permian. No species that are not descended from this ancestor are included in the class Mammalia. Birds are also a clade, likely having emerged during the Jurassic period. Reptiles, however, are not a clade, as some of the species commonly referred to as reptiles are more closely related to birds than they are to other reptiles. Amphibians are also not a clade, as the common ancestor of all modern amphibians was also ancestral to all reptiles, birds and mammals. Humans have a cladistic, or monophyletic, relationship with both species of chimpanzee. Any grouping of apes that includes chimpanzees and excludes humans is therefore not a true clade, as it arbitrarily excludes a descendant species.