Taxonomy: Cladistic and Linnaean Systems - Incompatible or Complementary?

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

The Linnaean evolutionary systematic taxonomy, and the Cladistic phylogenetics arrangements are both very useful systems, although they use very different methodologies.

By "Evolutionary Systematics" we mean the linnaean system incorporated with the darwinian modern synthesis and applied to both speciation and higher level phylogeny to understand the evolutionary tree of life on Earth. 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 originally by Haeckel, and later in more detail by Simpson, Mayr, Cain and coworkers, and presented in the paleontology textbooks of Romer, Colbert, Carroll, and others.

By cladistics and phylogenetics we mean selecting the most plausible hypotheses of sequence of branching in evolutionary trees (called cladograms and phylograms) based on statistical analysis of morphological and molecular data, as developed by Hennig (phylogenetic systematics using morphology), Fitch (molecular phylogeny), Gauthier (application of paleontology to cladistics) and others

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 mammals. Reptiles 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.

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

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:

■──oAmniota ├─Sauropsida │ ├─Mesosauridae │ └─Reptilia │ ├─Anapsida │ └─Diapsida (including Aves) └─Synapsida (including Mammalia)

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."

Also, speciation often involves a new species, through geographic isolation, budding off from the parent species, which remains unchanged. This isn't a problem with Linnaean Evolutionary taxonomic ranks, as at bottom these are based on evolutionary systematics and principles of speciation. There can be any number of species derived from and co-existing along with an ancestral species. This is revealed in cases where the fossil record is quite complete, such as Neogene invertebrates, late Cenozoic or Quaternary mammals. At this close in detailed scale, standard cladistic formalism is less useful, and where the actual ancestors are known, the concept of a hypothetical common ancestor (suitable when the fossil record is incomplete) becomes irrelevant. One form of cladistics that can be used in this situation is stratocladistics, as this considers the stratigraphic sequence and hence fossil ancestors.

As far as supra-species taxa go - the Linnaean hierarchy of genus, subtribe, tribe, subfamily, family and so on upto phylum and kingdom (and domain if one wants to include that in the Linnaean ranking) - is that evolutionary taxonomic ranks and evolutionary systematics, as formalised by Mayr, Simpson, and others, uses a combination of Linnaean morphological similarity, specxcies diversity, and evolutionary (phylogenic) branching (ancestor-descendent and sister groups) but, as mentioned, does not reject paraphyly (hence there are valid ancestral gradist taxa such as Thecodontia, Condylartha, etc). Whereas in cladistics, the Linnaean hierarchy is determined solely by phylogenetic (branching) sequence. So the first branch would be into domains, the next into kingdoms, and so on. In order to fit every branching event, Archosauria may have to be a superclass, whereas birds (Linnaean class Aves) may only be an infraorder. Obviously, this is totally impractical, even if additional ranks such as parvorder, microorder, epifamily, and so on, which is why Linnaean suffixes such as -idae and -oidea beyond superfamily notation are very rarely applied in cladistic classifications.

There would be no harm in this, if it weren't that the two systems were often conflated. Take our own Evolutionary Linnaean group, Superfamily Hominoidea. Originally - by which we mean in the classic texts of Romer (1966), Carroll (1988) etc, this included the three families Hylobatidae (gibbons), Pongidae (great apes - gorillas, chimps, orangs, and various fossil forms like Dryopithecus), and Hominidae, the "hominids" of popular science literature, by which is meant humans (genus Homo) and the African plio-pleistocene australopithecines, along with the odd extinct taxon like Pliopithecidae. There are clear morphological differences between these groups; for example Hominids walk upright (with all the physiological novelties that entails), have small canines, and, at least in later forms, have much larger brains and use stone tools and fire. No one denies that hominids evolved from pongids; the latter are, cladistically speaking, a paraphyletic taxon, which is not only permissable but essential for Linnaean-Evolutionary systematics. And what distinguishes the hominid line are morphological and ecological factors: they are an evolutionary offspring of african pongids that came down from the trees and took to savannah foraging, developing bipedality as this had significant locomatory advantages over knuckle walking.

The cladistic, or phylogenetic (for the synthesis of cladistic methodology and molecular phylogeny) version is quite different. Even though there is very little physiological difference between chimps and gorillas, in comparison to the highly derived and mostly hairless naked ape line, molecular phylogeny shows that the common ancestor of chimps and humans branched off from the common ancestor of all extant african apes at the same time as the gorilla ancestor did. In other words, there is approximately the same genetic difference between chimps and gorillas as there is between chimps and humans. And even though the african great apes are very similar to Orang Utans, the ancestor of Orangs (representing a south-east Asian lineage) branched off even earlier. The taxonomic confusion arises when this totally empitrical and plausible branching sequence is appointed ranking according to the Linnaean ranking system. Remember that Linnaean-Evolutionary systematics requires paraphyletic taxa, whereas cladistics forbids it. The paraphyletic Pongidae is then sunk in its daugher taxon the Hominidae because the latter has priority in the rules of nomenclature. Hominidae then becomes humans and great apes. The next branching in order of sequence is assigned the rank of subfamily, so Hominae (previously genus Homo, exclusive of Australopithecus) is now all the African apes, whereas Ponginae is retained for the Asian branch; orang utans and their fossil relatives. Because gorillas branched from the human and chimp ancestor the latter two share a tribe, the rank below subfamily, although there is far more difference between them than with, say, members of tribes of plants. Finally the old Hominidae as the term was used upto the 1990s now becomes the Subtribe Hominina, a rank barely above genus, even though as mentioned there are important ecological and morphological differences between the Homo-Australopithecine line and even for that matter between Homo and Australopithecus. Comparing the two gives the following:

Linnaean-Evolutionary taxonomy is consistent in that one can apply the same criteria to any species living or fossil, and also extend the hierarchy up to classes, phyla and kingdoms. Phylogenetics describes the branching order of each clade with great precision, but any attempt at use of linnaean nomenclature becomes totally impractical above a certain level. Moreover, new discoveries would require the addition of further branches (and hence further ranks and subranks), so the only way to keep the above schema is to limit it to extant species only.

The simple fact is, 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 but human societies are not, 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 presents a methodology for testing rival hypotheses regarding the sequence of branching of living organisms in the tree of life.

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.

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. MAK981204 111014 130408