Monophyletic, Paraphyletic and Polyphyletic

Having covered the basics of taxonomy and how to read phylogenetic trees it is long past the time I should have dealt with the concept of monophyly and it’s ‘ugly sisters’, paraphyly and polyphyly. While the terms might be unfamiliar, the concepts behind them are simple and practical and I suspect at least a few readers will actually know them even if they couldn’t actually say what they were (so to speak). The three terms all describe different ways of grouping taxa together and are most easily demonstrated on a phylogentic tree (though they don’t need one as such).

Monophyletic groups and lineages are the fundamental basis of taxonomy and evolution as a whole really. Everything within a monophyletic group is the descendent of a single common ancestor and thus are the clades we typically talk about in biology and palaeontology. Here the clade comprising taxa A, B, C and D is monophyletic. Everything that descends from the ancestral lineage or organism that existed at the point marked by the red arrow belongs in that clade and thus it is a monphyletic group. Should we later discover any new relatives that fit anywhere in the tree above that point then they would also by definition be part on that monophyletic clade. Obviously the clade A-D is not the only monphyletic one in this tree, one could name a clade of just A & B, or A-F and this would still be monophyletic.

Paraphyletic groups do not include all of the descendents of a single common ancestor. This means that while the group has a common ancestor, we are artificially ignoring a subset of its descendents. Here the green arrows indicate first the monophyletyic group which we are considering (A-F) and then the second monophyletic group (A & B) that we are not considering, thus leaving us with the paraphyletic clade of C-F. As before, there are other ways of creating a paraphyletic group on this tree and I am merely illustrating one.

This might sound odd, but paraphyletic groups are still used as they have a practical value for describing some groups. The most obvious ones to readers here are the dinosaurs. Birds are dinosaurs, that is, they are the direct descendents of an ancestor that spawned the dinosaurs, yet palaeontologists typically refer to dinosaurs while explicitly not referring to birds. Thus one should formally call them non-avian dinosaurs (basically all dinosaurs except birds) and this does happen quite regularly, thought not always, and certainly not in the press. Two others are worth of mention, one as it relates to this blog regularly and another as a striking example of a paraphyletic group. First off the rhamphorhynchoid pterosaurs (or more properly ‘rhamphorhynchoids’ as the quotes denote it as paraphyletic) which often come up here in my frequent posts on pterosaurs. As with non-avian dinosaurs, the term persists as one of convenience as basically it’s easier to write than “non-pterodactyloid pterosaurs” (or a more extreme example, one can call Plaetosaurus a ‘prosauropod’, or a “non-sauropodan sauropodomorph”). Finally there is the most obvious group of all – fish, or rather ‘fish’. Yes fish are obviously paraphyletic since all vertebrates have a single common ancestor, but tetrapods descended from them but no-one calls mammals or birds fish do they?

Polyphyletic groups are those which have multiple origins and thus do not share a common ancestor or indeed much in common at all aside from whatever trait holds them together. Here the various blue highlighted taxa have been pulled together into a polyphyletic group, and again one could do this in a great many ways on even this small tree. Polyphyletic groups are rarely mentioned (for obvious reasons), but they do come up occasionally when a revision has led to a previously monopyletic or even paraphyletic group being declared polyphyletic as a result of some systematic rearrangement (e.g. ‘we have found that taxa X and Y are not members of clade Q and actually belong to two different clades, thus the clade Q under its current definition is polophyletic and a new definition is required that excludes X and Y’). One obvious example from history is the pachyderms (elephants, rhinos and hippopotamuses) which of course all belong in different mammalian families and were put together based on a few minor characters of their skin.

Hopefully that makes everything clear and explains the fixation with the term monophyletic that often springs up on dinosaur forums (and indeed evolutionary ones in general). Monophyletic groups are the only taxonomically and systematically viable ones – that is they represent evolutionary history. As stated paraphyletic ones are useful for rhetorical simplicity, provided everyone is clear that they are paraphyletic and in what sense they are being applied. Polyphyletic ones are never used, except when referring to changes in taxonomic status, or of course in the most general terms (‘bipeds’ are polyphyletic).