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To decide which classes should go together, and which ones should be splitted, technically we can consider any characteristic. We could, for example, group whales together with olive trees on the basis of their ability to provide us with oils; or whales together with elephants on the basis of their shared grey colour; or whales together with trouts on the basis of their general shape. But, as we are committed to an ontological approach, we have to give priority to those characteristics which are ontologically most meaningful, that is, which are relevant not only for pragmatic purposes, or at a shallow observation, but with reference to the essential nature of the phenomena to be classified. We have to group together the phenomena which are most closely related in their essential nature.
Intuitively, we should thus group the most alike phenomena together, on the basis of their general similarity (morphology). As phenomena are often complex, it is not easy to decide which ones are the most similar. We can easily agree that whales go with other animals rather than plants; but, are they best grouped together with sharks or with seals? We need some sound criterion by which assessing the cumulative value of all similarities and differences, and produce optimal grouping. Numerical taxonomy provides with mathematical techniques to do this [1].
One powerful criterion in this task is the idea of genetic relationships between phenomena [Gno06c]. By considering them, our hierarchical tree can be interpreted as a genealogical tree, where the arches connecting akin concepts express a process of derivation. The process of development of new kinds of phenomena from pre-existing ones is generally known as evolution, and more specifically as phylogenesis, a scientific compound from Greek roots meaning the generation of branches. We are thus working with a dynamic ontology, that is, one where classes are not fixed once and for all from the beginning, but where new configurations can arise and develop [2]. The prototype of this kind of ontology is the evolutionary theory in biology. This idea, supported by Jean-Baptiste de Lamarck and others, introduced a new way of looking at the tree of organisms drawn by Linné (who believed that his groups were fixed as they had been created by God), now seeing it as a tree of evolutionary relationships: if cats and elephants share many characters, it is not just by divine will, but because they are originated from the same ancestors (primitive mammals, in turn derived by reptiles, etc.).
Clearly, when generalizing the notion of phylogenesis from organisms to all kinds of phenomena, one has to avoid an inappropriate translation of the categories of organisms into those of other phenomena. Biological evolution works, among other things, by differential reproduction rates of individuals mixing their genes through sex, recombination and other processes, and trasmitting them to their offspring in new combinations and variations. Other classes of objects are formed by different processes, eg molecules reactions between other molecules and the formation of bonds between atoms. Here, by phylogenesis we mean a more general (and more generic, in the sense that it is not fully studied and understood) process of derivation of new forms from pre-existing and often simpler ones.
Generally speaking, therefore, phylogenetic trees group phenomena according to two macro-characteristics: similarity, and common origin. These correspond to the principles of, respectively, structure and history, which are the main bases for most classifications [3-4].
In some cases, however, these two principles conflict. Dolphins, sharks, and ichthyosaurs all have a similar shape, but this does not depend on their origin. Rather than being a sign of historical relatedness, it has evolved three times independently, due to similar environmental conditions. Such kind of similarity is called analogy. On the other hand, fish fins, bird wings, and human arms all have a common evolutionary origin, despite their different shapes and functions: this is then a case of homology.
The ontological approach suggests that we keep homology in greater consideration than analogy, as common origin has a bigger explanatory power of the nature of phenomena than has shape similarity alone. Once we know that two objects are historically related, we understand their structure in deeper ways, and on this basis we can also predict further characters not manifest at initial inspection: knowing that dolphins are mammals allows us to predict that they breathe by lungs and suckle their offspring, without need of checking this directly for every new dolphin group that is discovered.
A classificatory tree based only on homology would resemble the cladograms produced by biologists of the cladistic school [5], who give common origin greatest priority over similarity. However, this method can also produce some strange results, like classifying birds as a subclass of reptiles, on the basis that birds separated from the main branch of reptiles later than crocodiles (who are also reptiles). Clearly this does not express the fact that birds are much more differentiated from their reptile ancestors than crocodiles are, which is the reason why crocodiles are usually considered still to be reptiles, while birds are not.
For these reasons, other biologists recommend a more balanced approach, taking into account both common origin and similarity [6]. In other words, our classification should express at the same time the relationship of a class with its ancestor classes, and the degree at which it has then differentiated from them, thus forming new kinds of phenomena which are unlike the previous ones. This differentiation can thus form in the course of evolution new grades of beings [7], having such relevant novelties in comparison with their ancestors, that they deserve to be considered as a completely new class. A familiar example is provided by the human species: although evolutionarily derived from pre-existing ape species, it has developed such revolutionary characters (language, culture, spirituality) that representing it just as another ape species, without any further differentiation, would be far too reductionistic.
We have used examples from the classification of organisms, as they are classical and easy to understand. However, the same dialectic between morphological and generical principles can be found in the classification of other phenomena, like climates, languages, or religions. Musical instruments have often been classified by morphological principles, still their history often provides significant contributions to systematics. Zithers include board zithers, where strings are attached directly to a soundboard, like in the harpsicord, and frame zithers, where strings are attached on a frame. Pianos are usually classified with board zithers as their strings were originally attached to the soundboard; however, modern pianos have a cast iron frame (plate), which strictly speaking would make them frame zithers. Similar cases concern the crowth (lyre or not?), and some musical bows (simple or compound chordophone?) [8].
Our classification must give an appropriate account of both the origin of phenomena, and of their various forms and special properties. This requires, beside the classical techniques of grouping, a model of the different levels of reality.
1: Numerical taxonomy / Peter HA Sneath, Robert R Sokal – Freeman : San Francisco : 1973
2: Process theories : crossdisciplinary studies in dynamic categories / Johanna Seibt – Kluwer : Dordrecht : 2003
3: Taxonomy / David L Hull = Routledge encyclopedia of philosophy. v 9. p 272-276 – Routledge : London, New York : 1998
4: The origin of language / Merritt Ruhlen – Wiley : New York : 1994
5: Cladistics : theory and practice of parsimony analysis / Ian J Kitching, Peter L Forey, Christopher J Humphries, David M Williams # ed 2 – Oxford University Press : Oxford : 1998
6: Biological classification: toward a synthesis of opposing methodologies / Ernst Mayr = Science. 214: 1981. p 510-516
7: Evolutionary processes and taxonomy with special reference to grades / Julian S Huxley = Uppsala universitets årsskrift. 1958. 6. p 21-38
8: [E-mail to the author] / Cristina Ghirardini : 2006.04.08
Integrative Levels Classification. Philosophy. Morphology and phylogeny / Claudio Gnoli – ISKO Italy : <http://www.iskoi.org/ilc/book/morphology.php> : 2008.11.07 - 2011.07.29 -
| Integrative Levels Classification | project | scheme | monograph | references |