monophyletic groups

Monophyletic groups are classifications in biology containing an ancestor and all of its descendants—nothing more, nothing less.

The term appears in taxonomy and evolutionary biology when discussing whether a grouping reflects true evolutionary relationships: Birds are monophyletic because they include all descendants of a common ancestor.

Reptiles, by contrast, exclude birds and are therefore not monophyletic.

The concept is central to cladistics, where only monophyletic groups are considered valid taxa.