Palaeos: Palaeos Cladistics
and systematics
Cladistic and Linnaean Systems
- Incompatible or Complementary?

Cladistic and Linnaean Systems - Incompatible or Complementary?

The difficulty of reconciling the Evolutionary Linnaean and the Phylogenetic Cladistic systems

The Linnaean taxonomic and the Cladistic phylogenetic arrangements are both very useful systems, although they use very different methodologies. If it assumed that only one is right, then they are incompatible. A better way of looking at things is to say that they are different ways of interpreting the natural world.

To give an example: the Linnaean system distinguishes separate classes for reptiles, birds and mammalsReptiles are cold-blooded and scaly and crawl (or slither in the case of snakes) and lay eggs which they then abandon (the only exception being the Crocodylia which guard their nest), and grow new teeth their whole lives.  Birds are warm-blooded, feathered and fly (or with flightless birds descend from flying ancestors), lay eggs and care for their young, and have erect stance and a toothless beak.   Mammals are warm-blooded, furry, have erect stance, give birth to live young and care for them, and replace their teeth only once.  So there are clear morphological differences.

But when you trace back the evolutionary tree you find that mammals merge into mammal-like reptiles (cynodont therapsids) and birds into bird-like reptiles (theropod dinosaurs).  The cladistic classification has the ancestral amniote (egg-laying) stock giving rise to two lines, sauropsids (reptiles, dinosaurs and birds) and synapsids (mammal-like reptiles and mammals).  Both sauropsids and synapsids start as "reptiles," in a colloquial sense, but one is the branch that leads to birds, while the other is the branch that leads to mammals.  In fact, in cladistics, Amniota is often defined as the last common ancestor of birds and mammals and all of its descendants.   MAK981213

Linnaean system - morphology Cladistic system - sister groups
Class Reptilia
(cold-blooded, scaly, lay eggs)
mammal-like reptile
Division Sauropsida
(common ancestor)
Class Aves (Birds)
(warm-blooded, feathered, lay eggs)
Class Mammalia
(warm-blooded, furry, live young)
Division Synapsida
(common ancestor)
mammal-like reptile

The contrast may be clearer if we look at it from a phylogenetic point of view. See Cladograms. ATW050802

From the cladistic perspective, many conventional Linnaean taxa actually turn out to be paraphyletic (i.e. they include descendants that do not belong within those taxa).  As T. Mike Keesey pointed out in an email, such traditional taxa can be shown as nested lists, e.g.:

Class Reptilia
  Subclass Synapsida --> Class Mammalia
  Subclass Anapsida
  Subclass Diapsida --> Class Aves
Class Aves <-- Order Diapsida
Class Mammalia <-- Order Synapsida
The following diagram (cladogram or more correctly dendrogram) shows the ancestor-descendant links for these taxa
--o Amniota
  |  |--Mesosauridae
  |  `--Reptilia
  |     |--Anapsida
  |     `--Diapsida (including Aves)
  `--Synapsida (including Mammalia)
(cladograms by T. Mike Keesey)

It is not possible to synthesise these two schemas by doing away with paraphyletic taxa. As R. K. Brummitt points out:

"Linnaean classification without paraphyletic taxa is a logical impossibility. Every monophyletic genus in a Linnaean classification must be descended from something (probably a species) in a different genus, which must be paraphyletic. Similarly every monotypic family must be descended from a species in a genus in a different family.  If one denies paraphyletic taxa, where do genera and families come from? Ultimately, one would end up sinking everything into its ancestral taxon, and the whole classification would telescope into its original taxon...

The theory of a Linnaean classification without paraphyletic taxa is nonsensical. Hennig's proposal to eliminate paraphyletic taxa was based on a failure to see the difference between the Linnaean hierarchy in which all taxa are nested in the next higher taxon, and a phylogenetic hierarchy which is not so nested, the lower levels of the hierarchy being not equivalent to the higher levels. Put another way, all the species of a genus together equal the genus but all the offspring of a parent do not equal the parent."

More than this, the Linnaean and Cladistic systems not only do not speak the same language, they are not only different ways of interpreting the natural world, but they use unrelated methodologies and paradigms to describe toally different things. It's as if an astronomy and a sociologist were arguing, and one said only stars and galaxies are real an dnot human societies, and the other said only socio-cultural structures are real and not astronomical bodies. The Linnaean system is a system of taxonomy that classifies living organisms, cladistics a system of phylogeny that presnets hypothes for the evolution of living organisms. Of course, there is also Evolutionary systematics, which is the linnaean system applie dto phylogeny (the evolutionary tre eof life on Earth). So often when phylogenetists say "linnaean", what they are really referring to is the incorporation of Linnaean classification by evolutionary science (phylogeny in the original sense of the word) as developed by Haeckel, Simpson, Romer, Colbert, Carroll, and others. This applies also to the above quote by Brummitt

This is not to say that it isn't possible to come at workable syntheses. There have been a number of attempts on some sites to reconcile the cladistic and Linnaean/evolutionary-systematic positions.  This often results either in a greatly inflated number of classes or in reducing higher grade established taxa to a much lower rank.  In either case it doesn't really work out.   One attempt that wasn't too bad is found in an excellent book called Evolution of the Perissodactyls, ed. by Donald R. Prothero & Robert M. Schoch (New York & Oxford, 1989) where a number of new hierarchical ranks are introduced. Again, Professor Mike Benton of Bristol University has provided a commendable and very useful new classification of the vertebrates, perhaps the first really useful integration of cladistic and Linnaean methodologies, and this approach is not to be sneered at. At Palaeos however we have however decided to leave these two methodologies side by side, each contributing their own insights, and thus acknowledge the multidimensional and multi-perspectival quality of scientific exploration of the natural world. MAK981204 111014

Summing up: both Evolutionary/Linnaean and Phylogenetic/Cladistic schemes are complementary rather than exclusive, and both are necessary and useful, each with strong and weak points.  Reconciling them however is a nightmare.  Monophyletic Linnaean generic and specific taxa can be useful in cladistics, but beyond that the two systems don't work together very well - many higher taxa have very different meanings in each.

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page by M. Alan Kazlev (Creative Commons Attribution 3.0 Unported License) (Creative Commons Attribution 3.0 Unported License) 1998-2002
uploaded on Kheper site 4 & 13 December 1998, on Palaeos site 20 & 26 May 2002, revised MAK120302
checked ATW020705, ATW021107, edited RFVS111203

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