Palæos:		Systematics
SYSTEMATICS		Linnean & Phylogenetic

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Systematics Glossary

References

To give an example: the Linnean 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.  

Linnean system - morphology		Cladistic system - ancestor-descendent
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.

It is getting increasingly difficult to find sources which give a balanced comparison of the Linnean and cladistic methods.  Cladistics has simply swept the field.  Classification has a good, if somewhat wordy, comparison of the two systems.  One of the last, and best, defenses of the Linnean system -- at least for purposes of nomenclature -- is Benton, MJ (2000), Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead? Biol. Rev. 75: 633-648 which can be accessed here.  It would be easy to dismiss these issues as quibbles about nomenclature, but it can make a real difference.  The thoughtful student might look briefly at Lane, A & MJ Benton (2003), Taxonomic level as a determinant of the shape of the Phanerozoic marine biodiversity curve. Amer. Naturalist 162: 265-276.  What this paper means, and whether it means anything, depend entirely on on how seriously we take the concept of taxonomic level and exactly how it is defined.    Taxonomic level is a concept almost without meaning in a cladistic scheme; while it is critical to the Linnean view.  Lane & Benton (2003) conclude that the shape of the biodiversity curve over time depends on on what taxonomic level is being considered.  That issue has important implications in various areas, including public policy.  How can we measure diversity without reference to taxonomic level, particularly for systems in which we cannot account for every species?  ATW050802.