|Page Back: Home||Unit Home||Page Next: Definitions|
|Unit Back: Evolution||Unit Up: Life||Unit Next: Ecology|
Life on Earth
Systematics ‒ History of ideas
The Phylogenetic Tree
Phylogenetic systematics (Cladistics)
Stratigraphy and phylogeny
A new approach
Systematics is the branch of biology that deals with classifying living beings: the diversity and interrelationships of living beings, both current organisms ("neontology") and prehistoric ones ("palaeontology"). It can be divided into three parts.
In pre-modern cultures, the world (including living organisms and inanimate objects) was classified according to the archetypal mandala - the four cardinal points, each associated with an element, a season, a deity, an animal, and so on, which constituted the underlying structure of the world (much as the elementary particles and forces in quantum physics is considered nowadays). This system of symbolic correspondences by association, which is metaphysical rather than scientific, reached great sophistication in the Chinese system of Five States of Change, the traditional Indian (Samkhyan and Tantric) doctrine of tattwas, and more recently (late 19th century) in Hermetic Kabbalah, as well as in "New Age" thought in general.
A different slant on things was given by Aristotle, who introduced the idea of the scala naturae - the natural ladder - according to which the natural world is interpreted in terms of the principle of plenitude, the overflowing abundance of the first principle or Godhead which creates successive beings. The further the beings are from the source the more ontologically impoverished they are. So you have formless matter right at the bottom, then rocks, plants, lower animals, higher animals, man, and finally spiritual and divine beings. This hierarchical view of the world ‒ the Great Chain of Being [see Arthur Lovejoy's classic coverage of this topic], persisted through the middle ages and up until the scientific revolution of the seventeenth and eighteenth centuries, when it was replaced by a sort of monotheistic dualism - there is the material world or creation, and there is God in his heaven.
Though Aristotle's work in zoology was not without errors, it was the greatest biological synthesis of the time, and remained the ultimate authority for many centuries after his death. Animals could be classified by their way of life, their actions, or by their parts. His observations on the anatomy of octopus, cuttlefish, crustaceans, and many other marine invertebrates are remarkably accurate, and could only have been made from first-hand experience with dissection. Aristotle described the embryological development of a chick; he distinguished whales and dolphins from fish; he described the chambered stomachs of ruminants and the social organization of bees; he noticed that some sharks give birth to live young ‒ his books on animals are filled with such observations, some of which were not confirmed until many centuries later.
Aristotle's classification of animals grouped together animals with similar characters into genera (used in a much broader sense than the current Linnaean definition) and then distinguished the species within the genera. He divided the animals into two types: those with blood, and those without blood (or at least without red blood). These distinctions correspond closely to our distinction between vertebrates and invertebrates. The blooded animals, corresponding to the vertebrates, included five genera: viviparous quadrupeds (mammals), birds, oviparous quadrupeds (reptiles and amphibians), fishes, and whales (which Aristotle did not realize were mammals). This basic division (except for the whales) was to be adopted by Linnaeus (for whom the "genera" became classes). The bloodless animals were classified as cephalopods (such as the octopus); crustaceans; insects (which included the spiders, scorpions, and centipedes, in addition to what we now define as insects); shelled animals (such as most molluscs and echinoderms); and "zoophytes," or "plant-animals," (e.g. corals) which supposedly resembled plants in their form.
In the 18th century the Swedish botanist Carl von Linné, better known under the Latinized form of his name, Linnaeus, developed what's known as the binomial system of classification, in order to simplify the chaotic state of affairs around at his time. (Some plants were given names ten words long for example). He used Latin because that was the academic language of the time. The modern system of classification in current widespread use is the binomial hierarchical system introduced by Linnaeus.
The Linnaean system
While Linnaeus founded taxonomy and classification, it was left to Charles Darwin in the 19th century to introduce the theory of evolution and hence make possible phylogenetic reconstruction; that is, the evolutionary relationships and history of the various groups of organisms through geological time (millions of years). That's where things really get interesting, because life as a dynamic process is much more fascinating than life as a static series of unchanging types.
The term is used to describe the Linnaean system of taxonomy presented in evolutionary terms is Evolutionary Systematics.
Cladistics ‒ also called Phylogenetic Systematics or Phylogenetic Taxonomy ‒ is a method of classifying organisms by common ancestry, based on the branching of the evolutionary family tree. Cladistics is currently the most popular paradigm of phylogenetic classification in biological taxonomy. Based strictly on determining branching points in the ancestry of organisms, it establishes groups based on their shared, derived features (synapomorphies), while ignoring primitive features (plesiomorphies) inherited from ancestors. Organisms that share common ancestors (and therefore have similar features) are grouped into taxonomic groups called clades. Like Evolutionary Systematics (which it has currently supplanted) Cladism is a method of classification based on the evolutionary history of organisms, dividing organisms into meaningful groups and subgroups. It was developed by Willi Hennig, an entomologist, in 1950, but was not really accepted until the 1980s.
Clades can be represented in terms of a cladogram. A cladogram is a branching diagram that depicts species divergence from common ancestors. They show the distribution and origins of shared characteristics.
Cladistics is based on three principles:
Cladistic Phylogenetic Systematics acknowledges only Monophyletic groupings as valid. Paraphyletic groups (accepted in Evolutionary Systematics) and Polyphyletic groups are rejected as invalid, as is the whole Linnaean hierarchy above species rank (although sometimes taxa such as family etc are used in a more limited context).
Cladists present cladograms are testable hypotheses of phylogenetic relationships. In this way, cladistic methodology can even be used to predict properties of yet-to-be discovered organisms.
The cladistic revolution of the 1970s and 1980s constituted a major paradigm shift in biology and systematics, with the Evolutionary system falling out of favour and being replaced by the cladistic one. Cladistics is based not on morphological similarity (as in the Linnaean system and more recently phenetics) or on ancestor and descent relationship (as in Evolutionary systematics) but in sister-group relationships between related taxa.
Although the relation between cladistics and evolutionary systematics could be described as the difference between "Vertical" and "Horizontal" Taxonomy, the two systems are quite distinct and, it has no been said (although the present writer (MAK) no longer accepts this) incompatible. Contrary to popular belief, this does not mean that one is right and the other wrong.
Proceedings of a Mini-Symposium on Biological Nomenclature in the 21st Century ‒ suggests replacing the Linnaean system with a cladistic phylogenetic system of nomenclature. However R. K. Brummitt in Quite Happy with the Present Code, Thank You ‒ argues against the tendency to reduce the Linnaean system to the Cladistic one by eliminating paraphyletic taxa.
Dinosaurs and Evolution part 4 - by Jeff Polling, points out weaknesses of the Linnaean scheme and argues for the cladistic, with reference to Mononykus, a prehistoric animal that, like Archaeopteryx, was transitional between dinosaurs and birds (note: this page is part of a longer discussion regarding evolution and creationism).
Phylogenetics Databases and Information
It should be pointed out here that ‒ as with fields of science in general ‒ all of these biotic classification schemes are in a sense arbitrary, dependent on the incomplete state of knowledge at present. Systematic classifications are supposed to convey phylogenetic information according to ancestry and descent, as well as being names for organisms or groups of organisms. But there still is (and perhaps never will be) a complete consensus on the phylogenetic relationships of organisms on Earth. As research progresses, phylogenetic concepts change, and the names that are tied to these concepts change as well. Research also discovers numerous instances of wrongly applied labels, such as when two or more species are found hiding under a single species name, and identifies previously undescribed creatures and lineages for which new names are needed.
[note - part of above paragraph from About Protist Image Data]
More Links: Taxonomy: Classifying Life ‒ John Kimball ‒ excellent overview (part of Kimball's Biology Pages).
Taxonomy, Transitional Forms, and the Fossil Record ‒ Keith B. Miller ‒ online essay, makes some interesting observations.
Biological Systematics : Principles and Applications by Randall T. Schuh (book, Amazon com, many more books could be mentioned here)
|Page Back||Unit Home||Page Top||Page Next: Definitions|