Obligate Categories

The Living World of Class 11

Obligate Categories

The obligate categories used in classification are explained below:

Species (John Ray)

  • Species occupies a key position in classification. It is the lowest taxonomic category.
  • It is a basic unit for understanding taxonomy as well as evolution. It is a natural population of individuals or group of populations which resemble one another in all essential morphological and reproductive characters, carry same type and amount of genetic material so that they are able to breed freely among themselves under natural conditions in order to produce fertile offspring. The species is also called genetically distinct and reproductively isolated natural population e.g., mango (Mangifera indica), potato (Solanum tuberosum), etc. In this case, indica, tuberosum; are species of genera Mangifera, Solanum respectively.
  • As per rules of binomial nomenclature, a species can be named only if it is assigned to a genus.
  • A species may have subgroups, called subspecies or varieties, showing certain distinct features of their own.


  • A genus may have a single species known as monotypic genus eg. Homo sapiens.
  • A genus may have several species called as polytypic.


  • Family is represented by a group of related genera. All the genera of a family resemble one another in certain correlated characters indicating a common ancestry.
  • In plants the family ends in the suffix - aceae subfamily in - oidae.
  • while in animals the suffixes are -ideal for family, inae for sub family.-ini for tribe (between subfamily and genus) Order
  • It is an assemblage of families resembling one another. These characters are less similar as compared to many genera put in a family.
  • An order ends in suffix - ales in plants. Different suffixes are used in case of animals.


It is a taxonomic category made up of one or more related orders.

  • It ends in suffixes - phyceae, - opsida and - ae in plants. The suffix is not fixed in case of animals.


Phylum or division consists of one to several related classes having similar correlated characters.

The division is given the suffix - phyta and the sub division - phytina.


In general it includes all organisms that share a set of distinguishing common characters. This is the highest category of biological classification. Plants are put in plant kingdom and animals are put in animal kingdom.

R.H. Whittaker (1969) has recognized five kingdoms of organisms - Monera, Protista, Fungi, Plantae, and Animalia.

System of classification

Three major systems of classification for different animals and plants in use are

  • Artificial System
  • Natural System
  • Modern or Phylogenetic System

Artificial System

A system of classification based on one or a few superficial characters chosen arbitrarily is called an artificial system of classification. No weightage was given to natural as well as phylogenetic relationships. This was prevalent before and also during the period of Linnaeus. Even before him, Aristotle had also classified organisms on the basis of their habitat. The habitat was recognized as a criterion to group animals into land, air and water. Some of the artificial systems are given below:

  • Theophrastus (370-285 B.C.), a Greek botanist, classified plants into four groups herbs, under shrubs, shrubs and trees on the basis of their habit.
  • Pliny the Elder (23-79 A.D.) distinguished animals into flight and non flight ones. Flight animals included bats, birds and insects. He divided plants into herbs, shrubs, under-shrubs, tree, vines, succulents, aquatic and terrestrial.
  • Carolus Linnaeus (1707-1778), a Swedish naturalist classified plants into 24 groups on the basis of sexual characters. He took only the number, length and union of stamens and carpels into consideration. For example, he proposed classes Monandria (1 stamen), Diandria (2 stamens) and Tri-and Polyandria (3 and more stamens). Hence, Linnaeus’s system is also known sexual system of classification.

Artificial system is easier to practise in the field, however it has several drawbacks in this system of classification that are follows:

  • It lacks the natural relationship amongst the organism.
  • Organisms do not show a clear cut evolutionary line.
  • It leads to heterogeneous assemblage of unrelated organisms. In this system many closely related species are classified in separate groups and many unrelated species are placed together in one group. For example whales, fishes are placed in one group and insects, birds and bats are put together in flying animals.

Natural system

This system is based on natural affinities of plants. In this system all available information about the constant and natural characters of plants are used as a basis of classification. Natural classification is mainly based on relationships realizing all information available at the time of collection of plants. The natural systems remained dominant before the idea of evolution was accepted. However, evolutionary characters are not considered.

  • A.L. de Jussieu (1686-1758) attempted a natural classification in his Genera Plantarum (1789). He laid emphasis on number of cotyledons, presence or absence of petals and stamens.
  • A.P. de Candolle in 1819, published a system of classification in his book Theorie élémentaire de la botanique. He was first to use the characteristic of vascular tissues in the classification of plants and recognized two group of plants Vasculares: (Vascular plants with cotyledons), Celulares: (Plants without vascular bundles or cotyledons).
  • George Bentham (1800 - 1884) and Joseph Dalton Hooker (1817 - 1911) worked together at Royal Botanic Gardens, Kew, England and proposed the most important and the last of the natural system for classification of seed plants. The system presented by them was published in Latin language in their treaties Genera Plantarum which appeared in three volumes. In these volumes, they have described some 97,205 seed plants according to their classification. An important feature of their classification is that they have given Gymnosperms a rank equal to Dicotyledons and Monocotyledons. Bentham and Hooker’s system of classification is used by most of the well known herbaria of the world.
  • Natural system of classification not only brings out natural relationships but also studies the evolutionary tendencies and phylogeny with the help of all the available data including fossils. This system of classification is certainly better than any artificial system of classification because

It establishes natural relationships amongst organisms.

It places only related organisms in a group.

It prevents grouping of unrelated organisms.

It brings out phylogenetic relationships and indicates the possible origin of different taxa.

Phylogenetic system

The evolutionary history of a group of organisms is called phylogeny. The system of classification reflecting the evolutionary sequence as well as the genetic interrelationships of organisms is called phylogenetic system. Adolf Engler (1884-1930) and his associate Karl Prantl (1849-1893) published a phylogenetic system in the monograph Die Naturlichen Pflanzenfamilien. They placed families and orders of the flowering plants in ascending series based on the complexity of floral morphology. The characters like one whorl of perianth or no perianth, unisexual flowers and pollination by wind were considered primitive as compared to perianth with two whorls, bisexual flowers and pollination by insects. The plant Kingdom according to their classification, is further divided into divisions, subdivisions, classes, orders and families. Asteraceae (Compositae) among dicots and Orchidaceae (Orchid family) among monocots are considered highly advanced. According to them monocots are more primitive than dicots.

This system considered evolution of angiosperm from a single source and the sequence of order and families show parallel evolution.

Ideally, a classification must reflect possible evolutionary relationships. Organisms belonging to same taxa are believed to have a common ancestor and may be represented in the form of a family tree (Cladogram). The systems of classification proposed after Darwin’s theory of natural selection, are mostly claimed to be phylogenetic.

Modern attempts at developing a phylogenetic classification of flowering plants are those of Cronquist, Hutchinson (1959), Takhtajan (1967). However, a phylogenetic system is not static but highly dynamic. Its major source is fossil record. As newer and better fossils are discovered, our knowledge about past organisms and their linkage with the present forms is liable to undergo change. Therefore, phylogenetic classification is under refinement ever since the concept was born.

  • First book on botany is Historia Plantarum by Theophrastus (Father of Botany).
  • First book on zoology is Historia Animalium by Aristotle (Father of Zoology).
  • Father of Ayurveda is regarded to Charaka.
  • The term phylogeny was introduced by Lamarck but the concept was established by Ernst Haeckel (1866).
  • Linnaeus wrote three major books: Systema Naturae, Genera Plantarum and Species Plantarum.
  • The term taxon was introduced by Adolf Meyer (1926) for animal groups.
  • A cladogram is an assembled data with regard to their shared characters forming a tree like diagram.
  • Plant names are effective from 1st May 1753.
  • Animal names are effective from 1st August 1758.
  • Among all the prevailing organisms in the world, the degree of diversity is more in the animals than in plants.
  • Takhtajan described taxonomy without phylogeny as bones without flesh. (A system of phylogeny of Flowering plant, 1967).
  • Phylogenetic classification is the ideal classification.
  • Of various categories of classification, only the species have real existence in nature.
  • The higher the category lesser will be the number of common characteristics of organisms.

Two kingdom system of classification

Linnaeus (1707-1778) divided the living organisms into two kingdoms: Plantae and Animalia. Each kingdom was split up into phyla or divisions. Each phylum or division was divided into classes. A class is subdivided into orders. An order is broken up into still smaller groups, the families. Each family comprised of many genera and in each genus were included one or numerous species.

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