The following points highlight the four major branches of Taxonomy. The branches are: 1. Cytotaxonomy 2. Chemotaxonomy 3. Numerical Taxonomy 4. Cladistics Taxonomy.
Branch # 1. Cytotaxonomy:
It is classification based on information provided by comparative cytological studies, number of chromosomes, structure and meiotic behaviour of chromosomes. It is known that fewer and larger chromosomes have been formed in many cases by fusion of smaller chromosomes. Herbaceous plants have larger chromosomes than those of woody plants. Naturally, herbaceous plants are more advanced than the woody plants.
In many genera the same basic chromosome number has been found in different species, e.g., 12 in Solanum species and 9 in Chrysanthemum species.
Human beings have 46 chromosomes while apes have 48. A reduction in number of chromosomes have been achieved through whole arm translocation between two acrocentric chromosomes. Apparently, humans have evolved from ape-like ancestors. Pairing of chromosomes during meiosis helps to bring out relationships between species.
Branch # 2. Chemotaxonomy (Biochemical Systematics):
The system of classification is based on characteristics of various chemical constituents of organisms like amino acids, proteins, DNA sequences, alkaloids, crystals, betacyanins, etc. Chemical constituents of plants are generally specific and stable.
They do not change easily. Ancient medical men based their identification of plants on fragrance, taste and other chemical characteristics. Crystals of calcium oxalate like raphides are restricted to 35 families. Similarly, certain alkaloids are restricted to a few related families, e.g., benzylisoquinoline alkaloid in Papaveraceae, Berberidaceae and Ranunculaceae.
Branch # 3. Numerical Taxonomy:
It evaluates resemblances and differences or primitiveness and advancement through statistical methods based on a large number of characters obtained from all disciplines of biology.
This is followed by assigning them number and codes of computer like plus (+), minus (-), θ (data not available), followed by computer analysis. It establishes the numerical degree of relationship among individuals. The relationship or affinity values are then used to erect taxonomic categories.
However, its effectiveness depends upon the judgement of the biosystematics in selecting characters and current knowledge about them.
Branch # 4. Cladistics Taxonomy (Gk. clados- sprout):
It searches similarity due to common phylogeny or origin from a common ancestor. These are two types of characters, ancestral and derived. Ancestral characters are traits of basic body design which would be present in an entire group.
Derived characters are those traits whose structures and functions differ from those of ancestral characters. They appear during evolution and cause the formation of new subgroups. One or more derived characters would be shared by an entire subgroup.
In cladistics taxonomy (cladistics) each evolutionary step produces a branching. All the members of a branch would possess the derived character. It will be absent below the branch point.
Arranging organisms on the basis of their shared similar or derived characters that differ from ancestral characters, will produce a phylogenetic tree called cladogram. Depending upon the type of system of classification, organisms are classified into two kingdoms or three kingdoms, four kingdoms, five kingdoms and now into six kingdoms.