Biological Classification – General Science

PMF IAS Environment

Biological Classification: Monera, Protista, Fungi, Plantae, Animalia, Viruses, Viroids, Lichens. Biodiversity, Classification, Taxonomic Categories.

Source | Credits | Picture Credits: NCERT General Science

Biodiversity

  • Classification of life forms will be closely related to their evolution. Charles Darwin first described this idea of evolution in 1859 in his book, The Origin of Species.
  • The number of species that are known and described range between 1.7-1.8 million. Rough estimates state that there are about ten million species on the planet. This refers to biodiversity or the number and types of organisms present on earth.
  • The warm and humid tropical regions of the earth, between the tropic of cancer and the tropic of capricorn, are rich in diversity of plant and animal life. This is called the region of megadiversity.
  • Of the biodiversity of the planet, more than half is concentrated in a few countries within tropics.

In alphabetical order, the 17 megadiverse countries are:

  1. Australia
  2. Brazil
  3. China
  4. Colombia
  5. Democratic Republic of the Congo
  6. Ecuador
  7. India
  8. Indonesia
  9. Madagascar
  10. Malaysia
  11. Mexico
  12. Papua New Guinea
  13. Peru
  14. Philippines
  15. South Africa
  16. United States
  17. Venezuela

megadiverse countries

Picture credits: Environment.gov.au

Classification of Biodiversity

  • There is a need to standardize the naming of living organisms such that a particular organism is known by the same name all over the world. This process is called nomenclature.
  • Obviously, nomenclature or naming is only possible when the organism is described correctly and we know to what organism the name is attached to. This is identification.
  • For plants, scientific names are based on agreed principles and criteria, which are provided in International Code for Botanical Nomenclature (ICBN).
  • Animal taxonomists have evolved International Code of Zoological Nomenclature (ICZN). The scientific names ensure that each organism has only one name.
  • Biologists follow universally accepted principles to provide scientific names to known organisms. Each name has two components – the Generic name and the specific epithet.
  • This system of providing a name with two components is called Binomial nomenclature. This naming system given by Carolus Linnaeus is being practised by biologists all over the world.
  • The scientific name of mango is written as Mangifera indica. In this name Mangifera represents the genus while indica, is a particular species, or a specific epithet. Other universal rules of nomenclature are as follows:
  • Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.
  • The first word in a biological name represents the genus while the second component denotes the specific epithet.
  • Both the words in a biological name, when handwritten, are separately underlined, or printed in italics to indicate their Latin origin.
  • The first word denoting the genus starts with a capital letter while the specific epithet starts with a small letter. it can be illustrated with the example of Mangifera indica.
  • Name of the author appears after the specific epithet, i.e., at the end of the biological name and is written in an abbreviated form, e.g., Mangifera indica Linn. It indicates that this species was first described by Linnaeus.
  • Since it is nearly impossible to study all the living organisms, it is necessary to devise some means to make this possible. This process is classification.
  • Classification is the process by which anything is grouped into convenient categories based on some easily observable characters.
  • The scientific term for these categories is taxa. Here you must recognise that taxa can indicate categories at very different levels. ‘Plants’ – also form a taxa. ‘Wheat’ is also a taxa. Similarly, ‘animals’, ‘mammals’, ‘dogs’ are all taxa – but you know that a dog is a mammal and mammals are animals. Therefore, ‘animals’, ‘mammals’ and ‘dogs’ represent taxa at different levels.
  • Hence, based on characteristics, all living organisms can be classified into different taxa. This process of classification is taxonomy.
  • External and internal structure, along with the structure of cell, process and ecological information of organisms are essential and form the basis of modern taxonomic studies.
  • Hence, characterisation, identification, classification and nomenclature are the processes that are basic to taxonomy.
  • Human beings were, since long, not only interested in knowing more about different kinds of organisms and their diversities, but also the relationships among them. This branch of study was referred to as systematics.
  • The word systematics is derived from the Latin word ‘systema’ which means systematic arrangement of organisms. Linnaeus used Systema Naturae as the title of his publication.
  • The scope of systematics was later enlarged to include identification, nomenclature and classification.
  • Systematics takes into account evolutionary relationships between organisms.

Taxonomic Categories

  • Classification is not a single step process but involves hierarchy of steps in which each step represents a rank or category.
  • Since the category is a part of overall taxonomic arrangement, it is called the taxonomic category and all categories together constitute the taxonomic hierarchy.

Biological classification - Taxonomic Categories

Species

  • Taxonomic studies consider a group of individual organisms with fundamental similarities as a species.
  • Let us consider Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion). All the three names, indica, tuberosum and leo, represent the specific epithets, while the first words Mangifera, Solanum and Panthera are genera and represents another higher level of taxon or category.
  • Each genus may have one or more than one specific epithets representing different organisms, but having morphological similarities. For example, Panthera has another specific epithet called tigris (Panthera tigris) and Solanum includes species like nigrum and melongena.
  • Human beings belong to the species sapiens which is grouped in the genus Homo. The scientific name thus, for human being, is written as Homo sapiens.

Genus

  • Genus comprises a group of related species which has more characters in common in comparison to species of other genera.
  • We can say that genera are aggregates of closely related species. For example, potato and brinjal are two different species but both belong to the genus Solanum.
  • Lion (Panthera leo), leopard ( pardus) and tiger (P. tigris) with several common features, are all species of the genus Panthera. This genus differs from another genus Felis which includes cats.

Family

  • The next category, Family, has a group of related genera with still less number of similarities as compared to genus and species.
  • Families are characterised on the basis of both vegetative and reproductive features of plant species.
  • Among animals for example, genus Panthera, comprising lion, tiger, leopard is put along with genus, Felis (cats) in the family
  • Similarly, if you observe the features of a cat and a dog, you will find some similarities and some differences as well. They are separated into two different families – Felidae and Canidae, respectively.

Order

  • You have seen earlier that categories like species, genus and families are based on a number of similar characters. Generally, order and other higher taxonomic categories are identified based on the aggregates of characters.

Class

  • This category includes related orders.

Phylum

  • Classes comprising animals like fishes, amphibians, reptiles, birds along with mammals constitute the next higher category called Phylum.

Kingdom

  • All animals belonging to various phyla are assigned to the highest category called Kingdom Animalia in the classification system of animals.
  • The Kingdom Plantae, on the other hand, is distinct, and comprises all plants from various divisions. Henceforth, we will refer to these two groups as animal and plant kingdoms.

Taxonomical Aids

Herbarium

  • Herbarium is a store house of collected plant specimens that are dried, pressed and preserved on sheets. Further, these sheets are arranged according to a universally accepted system of classification.
  • The herbarium sheets also carry a label providing information about date and place of collection, English, local and botanical names, family, collector’s name, etc.
  • Herbaria also serve as quick referral systems in taxonomical studies.

Botanical Gardens

  • These specialized gardens have collections of living plants for reference.
  • The famous botanical gardens are at Kew (England), Indian Botanical Garden, Howrah (India) and at National Botanical Research Institute, Lucknow (India).

Museum

  • Museums have collections of preserved plant and animal specimens for study and reference. Specimens are preserved in the containers or jars in preservative solutions.

Zoological Parks

  • These are the places where wild animals are kept in protected environments under human care and which enable us to learn about their food habits and behavior.

Key

  • Key is used for identification of plants and animals based on the similarities and dissimilarities.
  • The keys are based on the contrasting characters generally in a pair called couplet.
  • Flora, manuals, monographs and catalogues are some other means of recording descriptions.

Biological Classification

  • In Linnaeus’ time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed.
  • This system did not distinguish between the eukaryotes and prokaryotes, unicellular and multicellular organisms and photosynthetic (green algae) and non-photosynthetic (fungi)
  • Classification of organisms into plants and animals was easily done and was easy to understand, but, a large number of organisms did not fall into either category. Hence the two kingdom classification used for a long time was found inadequate.
  • Biologists, such as Ernst Haeckel (1894), Robert Whittaker (1959) and Carl Woese (1977) have tried to classify all living organisms into broad categories, called kingdoms.
  • The classification Whittaker proposed has five kingdoms and is widely used:
  1. Monera,
  2. Protista,
  3. Fungi,
  4. Plantae and
  5. Animalia

Biological Classification - kingdoms

  • The main criteria for classification used by him include cell structure, thallus organisation, mode of nutrition, reproduction etc.
  • It brought together the prokaryotic bacteria and the blue green algae with other groups which were eukaryotic.
  • It also grouped together the unicellular organisms and the multicellular ones.
  • The classification did not differentiate between the heterotrophic group – fungi, and the autotrophic green plants, though they also showed a characteristic difference in their walls composition – the fungi had chitin in their walls while the green plants had a cellulosic cell wall.
  • When such characteristics were considered, the fungi were placed in a separate kingdom – Kingdom Fungi.
  • All prokaryotic organisms were grouped together under Kingdom Monera and the unicellular eukaryotic organisms were placed in Kingdom Protista.
  • Kingdom Protista has brought together Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both having cell walls) with Paramoecium and Amoeba (which were earlier placed in the animal kingdom which lack cell wall).

At present the biological classification includes:

  1. Kingdom Monera
  2. Kingdom Protista
  3. Kingdom Fungi
  4. Kingdom Plantae
  5. Kingdom Animalia
  6. Viruses, Viroids and Lichens
  • Further classification is done by naming the sub-groups at various levels as given in the following scheme: KPC OF GS
  1. Kingdom,
  2. Phylum (For Animals) / Division (For Plants),
  3. Class,
  4. Order,
  5. Family,
  6. Genus,
  7. Species.
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