Monophyly

Monophyly

In common cladistic usage, a monophyletic group is a taxon (group of organisms) which forms a clade, meaning that it consists of a species and all its descendants. The term is synonymous with the uncommon term holophyly. Monophyletic groups are typically characterized by shared derived characteristics (synapomorphies).

Monophyly is contrasted with the terms paraphyly and polyphyly, which are most easily understood from the second diagram in this article. In current usage, a paraphyletic group consists of all of the descendants of a possibly hypothetical closest common ancestor minus one or more monophyletic groups (most usually one). A paraphyletic group is thus 'nearly' monophyletic (consistent with the meaning of the prefix 'para', namely 'near' or 'alongside'.) A polyphyletic group is any group other than a monophyletic group or a paraphyletic group, which like a paraphyletic group contains only some of the descendants of their closest common ancestor, but unlike a paraphyletic group is not characterized by the missing descendants forming one (or more) monophyletic groups.

These definitions have taken some time to be accepted. When the cladistic school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, taxonomists sometimes used terms without defining them, leading to confusion in the early literature, a confusion which persists.

See also: Crown group

Read more about Monophyly:  Definitions, Controversy