In cladistics for a group of organisms, monophyly is the condition of being a clade —that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants.

Monophyly means common descent from a single ancestor. Biologists have introduced a kind of taxonomy called cladistics. The aim of this is to make biological classification fit what we know about the way organisms evolved . If there is a group made of a common ancestor, and all its descendants, that group is monophyletic.

Monophyly is defined differently in evolutionary biology and cladistics. In evolutionary biology, the term monophyletic refers to a group of organisms descended from its most recent common ancestor. A monophyletic group may include all or only a part of the descendants of the common ancestor.