Classification of Animals: Artificial and Natural

In this article we will discuss about the artificial and natural classification of animals.

Artificial Classification of Animals:

When a classification is constructed on the basis of a few external characters, even a single arbitrarily chosen character and with­out their relationships, called artificial type of classification.

The early taxonomists fol­lowed artificial type of classification up to 18th century, e.g., Aristotle (384-322 B.C.), Pliny (A. D. 23-79), Linnaeus (1707-’78), Georges Cuvier (1769-1822) and others. It is a downward classification and totality of characters is not considered.

Natural Classification of Animals:

In this form classification is made based on the doctrine of organic evolution. It takes into account the available natural morpho­logical characters of the taxa. This system of classification is a phylogenetic one reflecting evolutionary relationships among the differ­ent groups. Simpson (1945) followed natural system of classification in case of classifica­tion of mammals.

Natural system of classifi­cation is a upward one and totality of char­acters is considered. Smith (1965) has con­sidered that natural classification is a phylogenetic one and reflects the evolution­ary relationship of the groups. Blackwelder (1964, 1967) opposes Smith’s view and states that the phylogenetic relationship is totally hypothetical.

Natural classification recognises three types of views, such as:

(i) Phenetic classi­fication

(ii) Cladistic classification and

(iii) Evolutionary classification.

(i) Phenetic classification (or Numeri­cal taxonomy):

Contemporary to Linnaeus, Michel Adanson (1727-1806) a French bota­nist introduced a new system of clas­sification in which he gave importance on overall equally weighted charac­ters.

Recently phenetic classification has developed based on Adanson’s principle. His principle has now been extended to a new type of taxonomy called numerical taxonomy which is based on numerical similarity and dissimilarity in characters.

Phenetic method of classification of Sokal and Sneath (1963) is concerned with the selection of many operational characters and a phyletic lineage (phylogeny) is drawn on the basis of similarities and dissimilarities. The accumulated data is processed through electronic computers to determine the most similar groups and which are different from each other.

In 1963, Sokal and Sneath published their ideas in Principles of Numerical Taxonomy as, “Numeri­cal phenetics is a method of assem­bling individuals into taxa on the basis of un-weighted overall similar­ity”. Gilmore (1961) has claimed that the number of characters used in phenetics is far from the traditional system and help to form a natural classification.

The use of phenetic method in the animal classification is being reduced by the present systematists, but this method is being still used by the plant taxonomists and molecular biologists.

(ii) Cladistic (branching) classification (Phylogenetic classification):

It is a form of classification which is related to the branching sequences of the phylogenesis and the phylogeny is reconstructed on the basis of shared derived (advanced) characters from a common ancestor.

The philosophy of cladistic classification was proposed by a German entomologist Willi Hennig (1950, 1966, 1979) who stated that only similarities grouping spe­cies (synapomorphies) should be used in classification. The plesiomorphic (primitive) characters are not consid­ered and can separate apomorphic (deviates from the original or ances­tral condition) characters from plesiomorphic characters.

Modern zoologists prefer to apply cladistic classification rather than phenetic one for the application of some sophisticated modern tech­niques such as polymerase chain re­action (PCR) to amplify DNA se­quences used in systematics and strong development of computer pro­grams that could handle large data sets.

Christoffersen (1995) has mentioned the differences between cladistic and phylogenetic classification as follows:

(i) In cladistic classification, a cladogram is a predominantly bifurcating, asymmetrical non- truncate dendrogram (also called cladogram) with no defined vertical or horizontal axes in relation to time.

(ii) In phylogenetic classification, the phylogenetic cladogram is a predominantly bifurcating, asymmetrical and truncate dendrogram with time as its vertical axes.

(iii) The use of a cladogram as a graphical model that shows speciation points or branching.

(iv) In phylogenetic classification, the use of a phylogeny (phylo­genetic cladogram) as a graphi­cal model shows the branching of the clades.

(iii) Evolutionary (Phyletic) classification:

The philosophy of this type of classi­fication started from the time of Charles Darwin with the publication of the “Origin of Species by means of Natural Selection”.

Darwin’s prin­ciple is based on the organic evolu­tion which states that each natural group had a common ancestor in the past and is related with the different groups of animals in their descent which are diverged from the parental stock. The separation of taxa must be based on phylogenetic branching and ranking of taxa depends on degree of modifications.