“The arrangement of organisms into groups and sub-groups on the basis of similarities is called biological classification.” Over 1.5 million species of animals and over half a million species of plants are known. There should be some system or the classification of these organisms. Many types of classifications are possible.
For example: classification of the flowering plants on the basis of their colour, height, or any other character. It does not give information about the similarities and differences among different individuals. So this type of classification is not accepted.
Basis of Classification
Evolutionary Histories All the organisms have some related evolutionary histories. They are related at some common point. Some organisms are more closely related than the others. For example, sparrows are more related to pigeons than insects. So both sparrows and pigeons are placed in the same groups.
The living organisms are classified on the bases of following characteristics:
- Comparative Biochemistry
Structures that are similar because of their common organ but perform different function due to present in different environment are known as Homologous and their study is known as Homology. The living organisms of a characteristic group have some similar characters of body organs is called homology.
The organs of the body which have same fundamental structure but different in their function are called homologous organs for example legs of horse, flippers of turtles, wings of birds are homologous organs. They have the same fundamental plan, show different structure but perform different ways of locomotion of the body.
The homologous organs but different animals show relationship and prove that they have evolved from a common ancestor. In this way, homology is a very important character for classification of living organisms.
2. Biochemistry (Chemical Constituents):
Biochemistry is also a basic character for the classification of living organisms. The study of chemical nature is useful to identify and classify them, because they have similar cellular structure.
In biochemistry certain techniques are used, such as chromatography and electrophoresis so their chemical constituents can be studied properly. Amino acid sequence in the proteins or arrangement of Nitrogen bases in DNA help to classify the organism and to determine their evolutionary relationship.
Cytology also helps in the classification of living organisms. By the help of electron microscope it is proved that in bacteria and Cyanobacteria (blue green algae), incomplete nucleus is present so they are placed in the same Kingdom-Monera.
Prokaryotic and eukaryotic organisms have been identified on the basis of microscopic study. Animal and plant cells have different characters from each other.
Cell study has also helped to know about the number of Chromosomes in different organisms. All plants and animals contain particular fixed number of chromosomes.
4. Genetics (Gene = unit of Heredity):
It is the branch of Biology which deals with the study of inheritance of character from one generation to the next generation. It is the final technique, which is applied in taxonomy, because all characters are inherited from parents to new generation. All morphological, biochemical and cytological characters are based upon genetic combination. DNA study is very useful in the classification of organisms.
The organisms are grouped into large distinct categories called taxa. Each taxa or category is more general than the taxa or category below it. The members of lower category resemble more with one another than the higher texon. Following units of classification are used in biology:
Species is the basic unit of classification. “A species is a group of natural population which can interbreed freely among them and produce fertile offspring, but are reproductively isolated from all other such groups in nature”. However, species formation by “Inbreeding cannot be applied to asexually reproducing organisms.
Each species has its own distinct structural, behavioral characteristics. Different species do not exchange genes between them. So specie is an independent unit from the evolutionary unit.
It is composed of one or more similar species.
Similar genera form a family.
Similar families form an order.
Similar orders form a class. .
Similar class form phylum. Phylum is also called division in plants like algae and fungi.
Similar phyla form a kingdom. There are two kingdoms: Animal kingdom and plant kingdom.