Taxonomic Categories

It is also called Linnaean hierarchy or taxononmic hierarchy or Taxonomic classification. It was first proposed by Linaeus. Hierarchy of categories is the classification of organism in a definite sequence of categories (taxonomic categories) in a descending order starting from kingdom. The number of similar characters of categories decreases from lowest rank to highest rank. The hierarchy includes seven obligate categories – kingdom, division or phylum, class, order, family, genus and species. The categories are arranged in descending sequence keeping the kingdom at the top. In order to make taxonomic position of species more precise, certain categories have been added to this list. They are called intermediate categories.

Types of Taxonomic Categories

There are 7 main taxonomic categories. They are obligate categories i.e, they are strictly used at the time of any plant classification.

1. Species: Species (used both as singular and plural) is a natural population of individuals or group of population which resemble one another in all essential morphological and also reproductive characters so that they are able to interbreed freely and produce fertile offspring. Mango is species indica of genus mangifera (mangifera indica). Potato is species tuberosum of genus solanum (solanum tuberosum).

2. Genus: it is a group or assemblage of relate species which resemble one another in certain correlated characters. Correlated Characters are those similar or common features which are used in delimitation of a taxon above the rank of species. All the species of genus are presumed to have evolved from a common ancestor.

3. Family: it is taxonomic category which contains one or more related genera. All the genera of a family have some common features or correlated characters. They are separable from genera of a related family by important and characteristic differences in both vegetative and reproductive features. Thus the genera of cats (felis) and leopard (panther) are included in the family felidae.

4. Order: the category includes one or more related families. Thus the family solanaceae is placed in the order polemoniales along with four other related families (convolvulaceae, boraginaceae, hydrophyllaceae and polemoniaceae). Similarly, the families fekidae and canidae are included under the order carnivore alongwith hyaenidae (hyaenas) and ursidae (bears).

5. Class: a class is made of one or more relates order. For example, the class dicotyledoneae (dicotyledonae, dictoyledons) of flowering plants contains all dicots which are grouped into several orders (e,g. Rosales, passiflorales, polemoniales, sapindales, ranales, etc.) likewise, class mammalian of animals includes all mammals which range from bats (order chiroptera),kangaroos (order marsupialia). Rodents (order rodentia), whales (order cetacean), carnivores (order cornivora) to great apes and man (order primate).

6. Division or phylum: it is a category higher than that of class. The term phylum is used for animals while division is commonly employed for plants. A division or phylum is formed of one or more classes. The phylum chordate of animals contains not class mammalian but also aves (birds), reptilian (reptiles), amphibian (amphibians), cyclostomata, chondrichthyes, osteichthyes (fishes) etc.

7. Kingdom: it is the highest taxonomic category. All plants are included in kingdom plantae while all animals belong to kingdom animalia.

There are some extra categories like sub division, sub order, sub family, tribe, sub tribe,. etc. They are not regularly used. They are used only when they are needed.

Species Concept

John Ray: Proposed the term and concept of species.

To explain the species, different concepts were proposed, which are as follows:

(A) Biological concept of species:

(1) Mayer proposed the biological concept of species.

(2) Mayer defined the "species" 'in the form of biological concept.

(3) According to Mayer "All the members that can interbreed. among themselves and can produce fertile off springs are the members of same species"

But this definition of Mayer was incomplete because this definition is applicable to sexually reproducing living beings because there are many organisms that have only asexual mode of reproduction

Example: Bacteria, Mycoplasma, BGA

(4) The main character in determination of any species is interbreeding. But this, character is not used in taxonomy. In taxonomy, the determination of species is based on other characters.

Example: Mainly morphological characters

(5) In higher plants, the determination of species is mainly based on the morphology of flower (floral morphology). Because floral (reproductive) characters are more conservative as compared to vegetative (Root/Stem, Leaf) characters i.e. they do not shows any major changes.

(6) When the species is determined on the basis of interbreeding then it is called as biological species.

Example: All the 'humans in this world can interbreed among themselves.

So all the humans are the members of one biological species.

(7) When the determination of species is based on other characters then it is' called as taxonomic species.

Example: These 3 have same morphological characters. Therefore they belong to same taxonomic species i.e. one taxonomic species. But these three cannot interbreed among themselves. Therefore on the basis of interbreeding these are three biological species.

(B) Static concept of species:

(1) The static concept of species was proposed by Linnaeus. According to Linnaeus "species is unchangeable" i.e. there is no change in the character of species. The species of present day are same as they were in past and they will remains same in future.

(2) Linnaeus believed in the "Theory of Special creation"

Father Suarez - gave the principle of special creation. According to this theory "All the living organisms are created by God (Every life is created 'by God) and God gave the basic size and shape of all living organisms, they are still present in their actual former form. But Lamarck rejected this hypothesis.

(C) Dynamic concept of species:

(1) It was proposed by "Lamarck".

(2) According to this concept "Species is always changeable". Changes always occur in the characters of species from one generation to next generation. And these changes are known as "evolution".

(D) Typological concept:

(1) It was proposed by "Aristotle" and "Plato".

(2) According to this concept 'There is a definite type or pattern of characters in the each species of every living organisms and all the members of species show maximum resemblance with this pattern. (Typological concept is based on single individual of species)The species in which a fixed pattern of characters is present are called as monotypic species.

Example: Bacteria, blue green algee.

(3) In many species, more than one type' or pattern of characters are present. These are called

"Polytypic species" or "Macrospecies",

Example: Brassica oleracea → Cauliflower, Cabbage, Knolkhol

Polytypic Species are of three types:

• Biotype: Members of same species inhabiting similar environment and having some genetic variations are known as biotypes. Variations found in these members are permanent. These members cannot interbreed among themselves.

    Example: Cauliflower, Cabbage, Knol-Khol are three biotypes of one species

• Eeotypes: Members of same species inhabiting different environment and having some genetic variations are known as ecotypes. Variations are permanent. These members can interbreed among themselves but due to geographical barrier they can not interbreed.

    Example: Crow (Corvus splendense) found in different regions are ecotype of one species

    Corvus splendense splendense - Indian crow

    Corvus splendense insolence   - Myanmar crow

    Corvus splendense protegatus - Srilankan crow

• Eeads or Ecophenes: Members of same species having some non genetic variations due to environment is called Ecads. These variations are temporary.

     Example: Every living being

     This hypothesis is believed to be most acceptable.

     Some definition related to species:

• Linneon species: Those taxonomic species whose determination is based on morphology. They are called as linneon species. They are also called morpho-species or taxonomic species.

     Example: Most of species in taxonomy are linneon species

• Microspecies or Joardan's species: Those species in which variations are very less are called Joardan's species. They reproduce asexually so they have very less variations.

• Sibling species or Cryptic species: Members of species which are morphologically similar but reproductively isolated are known as Sibling species i.e. they cannot interbreed among themselves.

     Sibling species is one taxonomic species (because these members have similar morphology) but they are different biological species. [Because they cannot interbreed]

     Example: Brassica oleracea

• Allopatric species: Those species that are found in different geographical regions and have geographical barriess between them are known as allopatric species.

     Geographical barriess are hills, oceans, himalyan mountains.

• Sympatric species: The species found in similar geographical regions are sympatric species.

• Allochronk species: The species found in different time periods.

     Example: Man and Dinosaurs.

• Synchronic species: Those species that are found in same era.

     Example: Dinosaurs and Archaeopteryx.

• Palaeo species: Those species that are extinct now and are found in the form of fossils.

     Example: Dinosaurs
 

Types of Taxonomy

1. Alpha taxonomy or classical taxonomy: It is based on external morphology, origin and evolution of plants.

2. Beta taxonomy or Explorative taxonomy: Besides external morphology, it also includes internal characters' like embryological, cytological, anatomical characters etc.

3. Omega taxonomy or Encyclopaedic taxonomy: Omega taxonomy has widest scope. It is based on all the informations or data available about plants.

4. Cytotaxonomy: The use of cytological characters of plants in classification or in solving taxonomic problems is called cytotaxonomy. Cytological characters constitute an important aid to plant taxonomy, especially in determining affinities at the generic and infrageneric levels.

5. Chemotaxonomy: The uses of chemical characters of plants in classification or in solving taxonomic problems is called chemotaxonomy or chemical taxonomy. It is based on the chemical constitution of plants. The fragrance and taste vary from species to species.

The basic chemical compounds used in chemotaxonomy are alkaloids, carotenoids, tannins, polysaccharide, nucleic acids, fatty acids, amino acids, aromatic compounds etc.

6. Karyotaxonomy: Based on characters of nucleus and chromosomes. Pattern of chromosomal bands (dark bands and light bands) is most specific characters. 

Question 1: Artificial system of classification classifies plants on the basis of- 

a.    One or two characters

b.    Phylogenetic trends

c.    Many naturally existing characters

d.    None of the above

Question 2: The term new systematics was introduced by –

a.    Linnaeus

b.    Bentham

c.    Hutchinson

d.    Huxley 

Question 3: Group of organisms that closely resemble each other and freely interbreed in nature, constitute a-

a.    Species     

b.    Genus

c.    Family      

d.    Taxon

Question 4: The scientific naming of plants began with publication of Linnaeus book –

a.    Genera plantarum

b.    Systema naturae

c.    Species plantarum

d.    Charaka sanhita

Question 5: The basic unit of classification is –

a.    Genus

b.    Species

c.    Order       

d.    All of the above

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