CLADISM (A theory of classification)
Cladism is a taxonomic theory first proposed by Hennig (1950, 1966). According to this theory organisms should be ranked and classified on the basis of “recency of common descent”.
Status of a taxon depends on the position of branching point on the phylogenetic tree and, therefore, recently branched taxa are considered to be more closely related to each other as compared to the ones branched in remote past. For example, snakes and lizards are closely related because they branched apart recently as compared to crocodiles and turtles which branched off in remote past.
Hennig asserts that the process of species cleavage is the characteristic feature of evolution. Cladists believe that evolutionary relationships can be judged by branching alone and not by subsequent divergence of the taxa, which means the rate of evolutionary change in different phyletic lines are of no consequence in Cladism. They presume that all branches evolve at the same rate and all methods that give branching patterns in classification are essentially cladistic in their approach. This was designated as genealogical approach by Gisin (1964), in which rank of a taxon is determined by fixing the branching point from its sister taxon in the geological time scale.
Subordination or hierarchy of a taxon is decided by the age of origin or branching or by the temporal distance between their origin and the present. Unequal divergence is ignored. In other words, the subordination of a category will be decided by the recency of common ancestry in geological time scale.
Another characteristic feature of Cladism is the belief that parental taxon expires when it gives rise to two daughter taxa by splitting, which never happens in phylogeny.
In Cladism a descendant group is always placed in a lower category than the ancestral group from which it has evolved, regardless of the rate of evolution and subsequent divergence. If ancestor is a class then descendant has to be ranked as order. Also the species that has originated from the parent taxon cannot be placed outside the category of that taxon. All descendant species of a genus will have to be placed within that genus and not under any other genus or family.
CRITICISMS OF CLADISM
Cladism has often been confused with phylogenetic classification. Cladists consider only the branching of taxa and not the subsequent divergence. If one line is exposed to so much selection pressure that it becomes genetically very different from its nearest relatives then it would be biological absurdity to call them near relatives. Cladists ignore different rates of evolutionary change leading to speedy or unequal divergence. They consider evolutionary rates to be same in all phyletic lines. Cladism confuses genetic with genealogical relationship.
Ranking of taxa according to the branching point is always misleading. For example, it might necessitate inclusion of chimpanzee in family Hominidae rather than in Pongidae because its branching point from hominids is recent as compared to gibbons.
The principle of recency of common ancestry is used to determine hierarchy of taxa in which temporal (time) distance rather than evolutionary differences are taken into consideration. Then if fish and Amphibia are classes, reptiles should be placed in order and mammals in family, just because fish and amphibians evolved much earlier than the reptiles and mammals have the most recent origin.
Cladists’ view that the parent taxon expires after splitting is not true and it may continue with little change along with the sister group. For example, reptiles gave rise to birds and mammals but the reptilian line still continues.
According to Hennig (1966), no species having arisen from the stem species can be placed outside the parent taxon. That means birds after having arisen from reptiles will have to be placed within class Reptilia.
Cladism is difficult to apply in the absence of fossil record or with poor fossil history. Then branching point will have to be determined by observed similarity.
Hennig (1966) ignores rate of evolution and asserts, “Decisive is the fact that the process of species cleavage is the characteristic feature of evolution.”