NEET Biology Notes – General Classification of Living Things
General Classification of Living Things
All living things have been categorised into different groups on the basis of various common features shared by them. These are as follows:
On the basis of body temperature, living things can be divided into two groups
An animal, whose body temperature is maintained by heat received from the environment is called ectothermic,
e.g. desert lizards
whereas animals, whose body heat is generated through its own metabolic activities are called endothermic
e.g. birds, mammals and a few fishes like tuna fish and sword fish.
Some organisms, which are capable of maintaining a stable body temperature independent of the environmental temperature are called homeothermic or warm-blooded or endothermic, e.g. birds, mammals, whereas some organisms, whose body temperature fluctuates considerably with environment is called poikilothermic or cold-blooded or ectothermic
e.g. amphibians, reptiles, insects, etc.
- Hibernation Winter sleep or dormancy during winter is called hibernation.
- Aestivation Dormancy during summer or dry weather is called aestivation.
On the basis of osmoregulation, animals are divided into two groups
(ii) Osmoregulators Animals
whose body is iso-osmotic with outer environment are called osmoconformers
e.g. hag fishes (jawless members of the class-Agnatha), elasmobranchs, coelocanths (Latimeria) and most of the marine invertebrates. Animals, whose body fluids are not isotonic with the outside environment are called osmoregulators. They must either discharge excess water when they live in a hypotonic environment or continuously in take of water is required when they live in a hypertonic environment
e.g. all freshwater and many marine animals along with humans and other terrestrial animals.
On the basis of aquatic habitat, animals can be divided into two groups
Anadromous fishes live and mature in marine water (sea water) and migrate to freshwater for breeding
while catadromous fishes live and mature in freshwater and migrate to the ocean (marine water) for breeding
e.g. European and American eels and Anguilla.
On the basis of salinity, animals can be divided into two groups
Most fishes are stenohaline, which can tolerate only a narrow range of salinities and a few are euryhaline, which can tolerate a wide range of salinity. These fishes can migrate between fresh and salt water.
Diversity existing among living organism is called biological diversity.
It refers to the number and types of organisms present on earth. There are millions of plants and animals present in the world. Every distinct geographical location has its own set of flora and fauna.
In order to facilitate the study, a number of scientists have established procedures to assign a scientific name to a particular species. It is nearly impossible to study all the living organisms, so it is necessary to devise some means to make this possible. This process is called classification.
It is the process by which anything is grouped into convenient categories based on some easily observable characters. It is the systematic arrangement of organisms into groups on the basis of their affinities or relationships. The modern system of classification began in 1758, when Carolus Linnaeus (1707-1778), a Swedish botanist published the tenth edition of his book ‘Systema Naturae.
Types of Classification
There are three main systems of classification:
It is based on arbitrary, easily observable, single or a few morphological characters such as habit, colour, number, form or similar features
e.g. Linnaeus system of classification.
It is based on overall similarities or affinities derived from morphology, anatomy, embryology, phytochemistry, ultrastructure and all other fields of study
e.g. Bentham and Hooker’s system.
It is based on the evolutionary descent of a group of organisms, the relationship depicted through a phylogram, phylogenetic tree or a cladogram
e.g. Engler and Prantl’s system, Takhtajan classification
The local names of organisms would vary from place to place, even within a country. Hence, there is a need to standardise the names. The naming of living organisms is dove in such a way that a particular organism is known by the same name in all over the world. Nomenclature is a science of providing distinct and proper names to organism so that they can easily be recognised and differentiated from others. It is done by using rules and recommendations of the code, prescribed by different scientific bodies as mentioned below :
ICBN International Code of Botanical Nomenclature
ICZN International Code of Zoological Nomenclature
ICNB International Code for Nomenclature of Bacteria
ICNCP International Code of Nomenclature for Cultivated Plant
ICTV International Committee for the Taxonomy of Viruses
The ICBN recognises several kinds or types depending on the way in which a type of specimen is selected.
Types of Specimen
We know the plants and animals in our own area by their local names. These are the names given to the organisms in a particular language in different region of world. These are not valid names.
Scientific names are given to the organisms by biologists based on agreed principles and criteria for their acceptability all over the world. These are:
Before 1750, this system was used, in which, organism’s name consisted of series of Latin descriptive words. Such names were lengthy and difficult to learn.
e.g., Caryophyllum saxatilis, Folis gramineus, umbellatis and corymbis.
This system is used to describe sub-species
e.g. Homo sapiens sapiens.
Swedish naturalist Carolus Linnaeus established binomial nomenclature, though, it was first proposed by Casper Bauhin. Linnaeus gave some principles of binomial nomenclature in Philosophica Botanica (1751), but the nomenclature was used first in Species Plantarum (1753), in which, names and description of 5900 plant species were given. Later he published Systema Naturae (1758), in which 4326 animal species were described.As per binomial system, name of any organism consists of two parts or epithets, i.e. generic epithet and specific epithet, e.g. the botanical name of mango is Mangifera indica L. In which, Mangifera is generic epithet, which represents its genus and indica is specific epithet, which represents its species. represents the name of the scientist (Linnaeus), who gave the name of mango.
Some rules for binomial nomenclature are
- Names should be in Greek or Latin language.
- Generic name begins with capital letter (Mangifera) and is placed before species name, while species name begins with a small letter (indica).
- The scientific name should be either underlined or italicised.
- Name of the authority should be written after specific epithet in an abbreviated form.
Rule of Priority
It is the most important rule of ICBN. In case, where two or more names are given, the oldest, i.e. the name given first is recognised as valid name and all other names are called synonyms. Names with same generic and specific names are called tautonyms,
e.g. Rattus rattus. These are not valid in botany.
It is the branch of Biology concerned with reconstructing phylogenies and with naming and classifying species. The term ‘Systematics’ was coined by Linnaeus and G Simpson (1961) distinguished systematics, taxonomy and classification. Julian Huxley (1940) proposed the term ‘New systematics’. It deals with the study of diversity of organisms and all their comparative and evolutionary relationship based on comparative anatomy, ecology, physiology and biochemistry.
It is the branch of Biology dealing with the identification, nomenclature and classification of living organisms. The term ‘Taxonomy’ was coined by AP de Candolle. Carolus Linnaeus known as Father of Taxonomy and Father of Systematic Botany, whereas Santapau is known as Father of Indian Taxonomy.
Modern Trends in Taxonomy
- Phenetic classification is based on the overall similarity of organisms evaluated without any regard to phylogeny.
A modern method of classification called cladistics, it is based on evolutionary history.
- Arranging organisms on the basis of their shared similar or derived characters that differ from ancestral characters will produce a phylogenetic tree called cladogram.
- The phylogenetic tree is also known as genealogical tree or dendrogram.
- Karyotaxonomy is based on nucleus and chromosomes. Numerical taxonomy is based on statistical methods, whereas experimental taxonomy is based on experimental determination of genetic inter-relationships.
- Numerical taxonomy is now easily carried out using computers. Each character is given equal importance and at the same time, hundreds of characters can be considered.
- Cytotaxonomy is based on cytological information like chromosome number, structure and behaviour.
- The chemotaxonomy put emphasis on the chemical constituents of the organisms to resolve confusions and are also used by taxonomists these days in order to determine evolutionary closeness.
Classification is not a single step process but involves hierarchy of steps in which each step represents a rank or category. .
A taxon is a taxonomic group of any rank. It is a taxonomic group of real organisms assigned to a category, whereas taxonomic category represents rank or level in a hierarchy and does not represents the living organisms.
For example, Reptilia is a taxon but class is a category similarly Poaceae is a taxon but family is a category.
Correlated characters are those similar or common features which are used in delimitation of a taxon above the rank of species. According to ICBN, the names of different categories must end in the standard endings (suffixes).
Suffixes used in the animal kingdom are
- Super-family — oidea
- Family — idae
- Sub-family — inae
- Tribe — ini Tribe is an intermediate category between sub-family and genus. Hierarchy of categories is also called as Linnaean hierarchy because it was first proposed by Carolus Linnaeus.
Linnaeus introduced five . categories in the taxonomic hierarchy viz,
Later on, three more categories,
i. e. Kingdom, Division (in plants) or ‘Phylum (animals) and Family were added and Variety was discarded to make a hierarchy of seven obligate categories, viz.
The term sub-species is used more commonly by zoologists, while variety is used commonly by botanists. Race is a group of individuals within a species forming a nearly permanent breed. Deme, cline, variety and form are termed as infraspecific categories.
Organisms with their Taxonomic Categories