Monophyly

Monophyly is the condition in which a group includes a common ancestor and all of its descendants. Such a group is called a clade.

Phylogenetic tree: A branching diagram that represents hypotheses about evolutionary relationships among organisms or other entities. Its branches show the ancestor and descendant relationships used to identify monophyletic groups.
Paraphyly: The condition in which a group includes an ancestor but excludes some of its descendants. It differs from monophyly by leaving at least one descendant lineage outside the group.
Cladistics: A method of classifying organisms by shared derived characters and common ancestry. Cladistics seeks classifications composed of monophyletic groups.
Mammalia: The clade of mammals, characterized by traits including hair and mammary glands. Mammals are a familiar example of a group understood as an ancestor and all descendants.
Common ancestor: An ancestral organism or population shared by two or more descendant lineages. A monophyletic group begins with a common ancestor.
Polyphyly: The condition in which a group combines organisms from separate lineages without including their most recent common ancestor. It lacks the inclusive ancestral grouping required for monophyly.
Systematics: The study of biological diversity and the evolutionary relationships among organisms. Systematists use monophyly to organize and explain evolutionary diversity.
Aves: The clade of birds, descended from theropod dinosaurs. Birds illustrate how a monophyletic group can extend beyond an older, narrower classification.
Clade: A group consisting of an ancestor and all its descendants. Clade is the standard name for a monophyletic group.
Traditional classification: A system of biological classification based on inherited conventions and selected similarities. Some traditional groups persist despite excluding descendants of their common ancestor.

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