monophyletic

Monophyletic refers to a group of organisms that includes an ancestor and all of its descendants. It is the defining concept of a clade in modern phylogenetics. A monophyletic group is contrasted with paraphyletic groups, which include the common ancestor and some, but not all, of its descendants, and polyphyletic groups, which do not include the most recent common ancestor of all members.

Monophyly is inferred from phylogenetic analyses that use morphological traits, molecular data, or both. Groups are considered monophyletic when all members descend from a single, common ancestor that is also part of the group, and no lineages outside the group are excluded. Synapomorphies, or shared derived characters, often support monophyly, while incomplete sampling and convergent traits can complicate assessments.

Examples of monophyletic groups include Mammalia (all mammals and their most recent common ancestor and descendants) and Aves (birds). Archosauria, which includes crocodilians and birds, is also treated as monophyletic in many systematic frameworks. By contrast, traditional Reptilia, defined without birds, is often cited as paraphyletic because it excludes the birds descended from the same ancestor. A classic polyphyletic example is the idea of “warm-blooded animals” as a group, which combines mammals and birds but omits their cold-blooded relatives and does not reflect a single common ancestor within the group.

In taxonomy and systematics, prioritizing monophyletic groups helps ensure classifications reflect evolutionary history, with clades representing natural lineages rather than arbitrary assemblages.