Monophyly

Phylogenetic tree, the blue (left) and red (right) groups represent monophyletic groups, the green group (centre) being paraphyletic.

Cladogram of the primates, showing a monophyletic group (the simians, in yellow), a paraphyletic group (the prosimians, in blue, including the red patch), and a polyphyletic group (the night-active primates, the lorises and the tarsiers, in red)

In common clade, meaning that it consists of an ancestral species and all its descendants. The term is synonymous with the uncommon term holophyly. Monophyletic groups are typically characterized by shared derived characteristics (synapomorphies).

Monophyly is contrasted with paraphyly and polyphyly, which are most easily understood from the second diagram in this article. In current usage, a paraphyletic group consists of all of the descendants of a common ancestor minus one or more monophyletic groups. A paraphyletic group is thus 'nearly' monophyletic (consistent with the meaning of the prefix 'para', namely 'near' or 'alongside'.) A polyphyletic group is characterized by convergent features or habits (e. g., night-active primates, trees, aquatic insects); the features by which the group is differentiated from others are not inherited from a common ancestor.

These definitions have taken some time to be accepted. When the cladistic school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, taxonomists sometimes used terms without defining them, leading to confusion in the early literature,^[1] a confusion which persists.

Definitions

On the broadest scale, definitions fall into two groups.

• Willi Hennig (1966:148) defined monophyly as groups based on synapomorphy (in contrast to paraphyletic groups, based on symplesiomorphy, and polyphyletic groups, based on convergence). Some authors have sought to define monophyly to include paraphyly as any two or more groups sharing a common ancestor.^[2][3][4] However, this broader definition encompasses both monophyletic and paraphyletic groups as defined above. Therefore, most scientists today restrict the term "monophyletic" to refer to groups consisting of all the descendants of one (hypothetical) common ancestor.^[1] However, when considering taxonomic groups such as genera and species, the most appropriate nature of their common ancestor is unclear. Assuming that it would be one individual or mating pair is unrealistic for sexually reproducing species, which are by definition interbreeding populations.^[5]

• Monophyly and associated terms are restricted to discussions of taxa, and are not necessarily accurate when used to describe what Hennig called tokogenetic relationships - now referred to as genealogies. Some argue that using a broader definition, such as a species and all its descendants, does not really work to define a genus.^[5] According to D. M. Stamos, a satisfactory cladistic definition of a species or genus is impossible as many species and even genera may form by "budding" from an existing species, leaving the parent species paraphyletic or may be the result of hybrid speciation.^[6]

See also

• Glossary of scientific naming
• Clade
• Holophyly
• Paraphyly
• Polyphyly

References