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In phylogenetics, a group is monophyletic (Greek: "of one race") if it consists of an inferred common ancestor and all its descendants. A taxonomic group that contains organisms but not their common ancestor is called polyphyletic, and a group that contains some but not all descendants of the most recent common ancestor is called paraphyletic.

The grouping of reptiles and birds is generally believed to be monophyletic.

For example, all organisms in the genus Homo are inferred to have come from the same ancestral form in the family Hominidae, and no other descendants are known. Thus the genus Homo is monophyletic. If, on the other hand, it were discovered that Homo habilis had developed from a different ancestor than Homo sapiens, and this ancestor were not included in the genus, then the genus would be polyphyletic. Since biologists by and large tend to accept groups when they are monophyletic, in this case, they would likely either split the genus or broaden it to include the additional forms. Splitting the genus may split explanations of functional evolution to require convergence.

Some evolutionary taxonomists prefer to use the term holophyletic to describe this sort of group and instead use monophyletic in its older sense, where it refers to both holophyletic and paraphyletic groups.

Sometimes taxonomists are frustrated fitting plant species into a monophyletic group because of polyploidy. There is evidence that some polyploid plant species are of multiple origins (the species has arisen more than once). For example hybrid goat's-beard (Tragopogon miscellus [1]) has formed as many as 20 times in eastern Washington. Unity of explanation through functional evolution is challenged by the atomization required by strict monophyly.

• Taxonomy
• Linnaean taxonomy
• Cladistics
• Evolutionary taxonomy

• Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.