Taxonomy is a branch of biological science dealing with the principles, procedures and rules of classification and nomenclature.
Principles of Classification
Diversity is a characteristic feature of organisms. Every organism has its own individuality. It resembles its own species but differs from the members of other species.
If you go to a nearby farm you would find crops such as plants, shrubs, grasses and trees of different kinds. On the same farm, you might see animals such as cows, ox, buffalo, dogs, rats and squirrels.
Now, if you happen to go on a pond or lake or seashore, you would find many planktons floating over the water surface or suspended in water. A number of them are unicellular and some are multicellular.
You may find algae because of their green colour. Saprophytes like fungi are colourless or might be of different colours.
The nektons, i.e., organisms moving by their own efforts will be numerous. They include fishes of different kinds and sizes, lamprey, turtles, aquatic birds, seals and whales.
Among invertebrates, there will be crayfishes, crab, squids, octopus, starfishes, jellyfishes, sponges and protozoa of innumerable varieties.
Organisms have diversity of form, structure, physiology, mode of life and behavior.
The immense biological diversity is a consequence of organic evolution.
Variations are widespread. Genetic processes and reproductive processes give rise to new variations continuously.
The environment and climatic conditions differ from one place to another. The topography of land surface, habitat and availability of food impose on organisms certain pressure to adapt themselves to varied circumstances. Thus, organisms are continually adapting to the changed conditions and are becoming more and more diversified.
The diversification of organisms creates problems for biologists. There are about 1.7 Millions species of plants and animals put together at present. Many are being discovered.
This has thrown challenges before biologists as how to study them and store information regarding them.
To give an idea of the diversity of animals alone, the Order–> Insecta has over 8,50,000 species. Arthropoda including insects has about 9,00,000 species.
Other phyla have quite an enormous number of species. The invertebrates put together excluding arthropods have about 1,40,000 species.
Some Terms Related to Taxonomy
- Taxonomy: It is a branch of biological science dealing with the principles, procedures and rules of classification and nomenclature.
- Classification: It is a method of grouping or arranging organisms into related groups according to their resemblances (based on phylogeny ) and differences. According to Simpson(1961), the zoological classifications is the ordering of animals into groups or sets on the basis of their relationships, that is of association by contiguity, similarity or both.
- Nomenclature: It is a system of giving distinctive scientific names to each of the groups or taxa, recognised in any system of classification accepted internationally.
What is the need of biological classification?
We have seen that organisms are much diversified. We can never ignore them since they touch the circle of our life at many points.
Some of them are useful to us, while some others spread diseases, cause diseases to human or destroy crops. So it is necessary to classify animals.
Purpose of Biological Classification
- All the organisms that are so varied, can not be studied by any person in his lifetime. Therefore, organisms are grouped based on their resemblances. A typical organism from each group can be studied to give us an idea of the nature of organisms of that group. Thus, classification is very convenient to biologists.
- Classification is a means of storing information. Until we divide organisms into groups, how can we store information about the organisms? If we know that a certain animal belongs to the group Mammalia, we can conveniently collect and store the information regarding that animal under the group Mammalia.
- Classification makes it easy to exchange information regarding organisms.
- Above all, the classification helps us in establishing the natural relationships among organisms. We know that organisms have common ancestry and they are related to each other because of the evolutionary process they have undergone.
A Brief History of Taxonomy
Aristotle (384-322 B.C.), the father of Zoology was the first to classify living beings. He divided the animal kingdom into two main division and they are:
- Anaima: (Invertebrates)- Animal without red blood.
- Enaima: Vertebrates) – Animal with red blood.
He further sub divided Enaima into two categories:
- Ovipara: The animal which lay eggs such as fishes, amphibians, reptiles, birds etc.
- Vivipara: The animal which gives birth to young ones such as mammals.
Charaka – (First century A.D) who is popularly known as the “Father of Ayurveda”, identified some kind of animals and 340 kinds of plants. One can find the description of these in the book Charaka Samhita. The Vedas also give descriptions of Munis describing various animals and plants.
After Aristotle, John Ray (1627-1705) was the first biologist to introduce the concept of species and to make some efforts to classify a few groups.
Carolus Linnaeus (1707-1778) gave scientific classification which is in use today with a few modifications. He divided the animal kingdom into Phylum, Class, Order, Family, Genera and Species.
In his book Systema Naturae (tenth edition, 1758) he recognised six “classes” – Mammalia, Aves, Amphibia (reptiles and amphibians), Pisces, Insecta and Vermes (all other invertebrates). He also introduced binomial nomenclature.
Other biologists such as Cuvier (1769-1832) divided animals into four branches: Vertebrate (mammal to fish), Mollusca (molluscs to barnacles), Articulata (annelids, crustaceans, insects, spider) and Radiata (Echinoderms, nematodes, cnidarians, rotifers).
Lamarck (1801-1812), Haeckel (1864), Lankester (1877) etc. gave many views on simplification of the classification of the animal kingdom.
The other scientists who contributed to the knowledge regarding classification are Leukart (1848), Owen (1801), Milne Edwards (1855) and Agassez (1859).
Ernest Heinrich Haeckel (1864) and E. Ray Lankaster (1877) outlined the principal features of zoological classification which are in use today.
Various zoologist has improved upon the classification in place of the existing two-kingdom classification.
Basis of classification
To arrange the animals into groups and sub-groups, some criteria have been selected. These include different approaches to discover the characters to decide the resemblances and differences among the animals.
The diversities (i.e., variations) among animals are remarkable and one would naturally feel the necessity of arranging these innumerable living forms in some orderly groups according to the habitat or structure or function or according to the utility of man.
The accepted principle of classification is based on organic evolution which states that all organisms had a common ancestor and have inter-relationship.
A different group of animals in their descent diverged from a common stem. In other words, the classification is based on the evolution of animals that are naturally graded in order of increasing complexity.
Before beginning the study of organisms, it is essential to identify them correctly. This leads to systematics as a discipline in animal science.
Therefore, identification, nomenclature and classification are the different sub-divisions of the taxonomy or systematics.
Natural System of Classification
The natural system of classification is based on natural relationships, i.e., based on phylogeny. To discover the phylogeny, different approaches have been adopted and those are as follows:
- Phylogenic Approach
- Numerical Approach
- Artificial Classification
Let us now discuss about all these approach briefly.
Means the evolutionary history concerning a particular group. The real basis of taxonomy is phylogeny. A successful classification must be able to reflect the phylogenic relationship.
Classification thus must be able to represented by a genealogical tree. Where ancestral forms should form the trunk, various classes and orders should form the branches and the top should be occupied by modern species.
This, however, is subjected to modification in the light of new facts and researches accumulating every day.
In modern times a taxonomist takes into account several characters from different fields or branches, e.g., he collects characters regarding the organisms to be classified from their morphology, history, embryology, physiology, biochemistry and palaeontology.
All the characters so obtained are carefully analysed to show natural relationships, i.e., phylogeny. When phylogeny has been established, only then the place of the organism is decided and it is classified accordingly.
Recently, there is an emphasis on studying the chemical constituents present in the body of an organism to understand the natural relationship of that organism. This is known as Chemotaxonomy.
It has a significant role in establishing remote relationships. Chemotaxonomy is quite popular among modern taxonomist. It takes into account the structure of DNA, proteins, carbohydrates, fats, immunoglobins and enzymes.
Further development in this field of taxonomy is numerical taxonomy, which seeks to evaluate the similarity and differences between taxonomic unit objectively through similarities if they are in majority indicating the relationship.
A classification based on the comparison of one or two characters in deciding the position of a group is known as artificial classification.
For example, if a biologist takes into account a character such as possession of wings as the basis of classifying the animals. He would put a bird, a bat and an insect in one common group.
On the other hand, if we take into account other characters too, such as body organisation, development, exoskeleton etc., then we might discover important differences that will not justify putting them together in a group.
A classification plan consists of several group or Taxa under which various organisms are placed. These are arranged in a Taxonomic hierarchy.
Species is the lowermost Taxon in any classification. However, it is divisible into two subspecies in some cases.
Various groups which are used for the purpose of classification are as under:
Let us now briefly discuss about all these one by one.
Species: (Pl. Species)
- A species is a population of individuals which are morphologically similar.
- They share a common gene pool that is distinct from the gene pool of other species.
- There is free gene flow in a gene pool of a species.
- The members of a species can interbreed freely (flow of gene) and produce viable reproductive individuals.
- A species is reproductively isolated from other species.
- Each species occupies a particular ecological niche that is not utilized by other species.
- The species have a capacity to change and give rise to new species. (Evolution).
For example, a population of lions represents the species, Panthera leo, and a population of tigers represents the species Panthera tigris.
Lion can breed to give rise to the next population of lions, i.e., they can interbreed freely and can produce lions which in turn can also reproduce.
But lions can not mate with tigers or produce viable offsprings, i.e., they are reproductively isolated from tigers. All lions are morphologically similar and differ from tigers.
Genus (Pl. Genera):
A genus is a group of related species. It comprises one or more species, which although different, has many features in common.
Obviously, all the species in a genus have more features in common than in other closely rated groups of species. Further, it is also applicable that all species under one genus have evolved from a common ancestor.
For example, Panthera leo and Panthera tigris are two distinct species and are grouped in one genus Panthera, for they share some common characteristics.
The genus Panthera is differentiated from the related genus Felis which includes Jungle cat (Felis chaus), Leopard cat (Felis bengalensis), Golden cat (Felis temmincki) and fishing cat (Felis viverrina).
This category comprises one or more genera, which have some common characteristics.
For example, the family Felidae comprises lions, tigers, leopards and cats.
It is a higher taxon consisting of many families. The order Carnivora contains all the carnivorous mammals such as lion, panther, cats, dog, seal and walrus, belonging to different families. They all are carnivorous and have a carnassial set of teeth.
It comprises one or more orders. Class Mammalia consists of sixteen living orders. The order primates which includes man, monkey, gorilla, gibbon and the order Carnivora which comprise of a lion, tiger, cat, dog etc are included in the class Mammalia.
It includes one or more classes having board characters in common. The phylum Chordata includes many classes such as Elasmobranchii, Teleostomi, Amphibian, Reptilians, Aves and Mammalia.
It contains many phyla. It is the highest taxonomic category. All animals are included in the animal kingdom or kingdom Animalia.
Take this example of Lion for easy understanding of the systematic position.
On the whole, the scheme of classification may be compared to a tree bearing many leaves that represent different species of animals, fine stem bearing leaves are equivalent to genera.
Several stem from a twig which represents a family. The twigs in their turn unite to form small branches or orders and several small branches join to form larger branches that stand for classes.
The larger branches form the main framework representing phylum and the entire tree stands for the animal kingdom.
Taxonomic Hierarchy: To indicate, the taxonomic hierarchy see the example the as follows:
- Kingdom- Animalia
- Sub Kingdom- Metazoa
- Species- sapiens
Nomenclature: Another concern of taxonomy is nomenclature. That is the naming of species and other higher categories of any recognised system of classification.
Two Kingdom classification
The System of classification given by Linnaeus classifies all living organisms into two main kingdoms: Plantae (including all plants) and Animalia (including all animals).
Animals are normally distinguished by their movements, nutritions, patterns of growth, body organisation etc. But there are some exceptions to all these characteristics.
For example, there are many non-green plants. They feed like animals, e.g., mushrooms and other fungi grow on dead organic matter.
Lichens can grow on the bark of the tree and even do not look to be alive. Sponges (animals) are fixed just like plants and have a branching pattern similar to plants.
It also becomes difficult to classify unicellular organisms seen in a pond or lake to be called plants and animals. They are coloured green, yellow, brown or red. They are also photosynthetic like green plants.
Shortcomings of Two Kingdom Classification:
Two kingdom of classification has following shortcomings:
- Classifies prokaryotic (true nucleus absent) cell and eukaryotic (true nucleus present) cells together in one group.
- It has little regard for diversified forms.
- Photosynthetic green plant and non-photosynthetic green plants are also grouped together.
- Places unicellular organisms which are so diversified in one group, i.e., Protozoa.
Five kingdom classification:
To overcome the shortcomings occurred by two system classification, Whittacker (1969) proposed a new five-kingdom classification.
The Five Kingdom classification is based on following factors:
- The complexity of cell structure, i.e., prokaryotic or eukaryotic unicellular organisms has been separated.
- The complexity of body organisation, i.e., unicellular or multicellular or tissue grade of organisation.
- Phylogenic relationships.
- Mode of nutrition, i.e., autotropic, heterotropic
The five kingdoms that have been recognized are Monera, Protista, Fungi, Plantae and Animalia.
- Monera: Unicellular, prokaryotic organisms, e.g., blue-green algae, other bacteria
- Protista: Unicellular, eukaryotic, saprophytic or holozoic or autotrophic organisms e.g., Protozoa
- Fungi: Multicellular, syncytial, saprozoic, e.g., mushroom, mould, slime
- Plantae: Multicellular, tissue well-formed, autotrophs, e.g., algae, flowering and non-flowering plants
- Animalia: Eukaryotic, multicellular tissue well-formed, motile, holozoic, e.g., all metazoans animals.
Note: The classification of animals is concerned with only Protista and Animalia kingdom.