BIOREADERS: Biological classification

Biological classification

Histology of classification:

Scientists have been made many attempts in classifying living organisms into different groups. In ancient period Greek scholars Hippocrates (460-377 BC) and Aristotle (382-322 BC) arranged animals into four major groups like insects, birds, fishes and whales. Aristotle is regarded as the first taxonomist and he sub-divided plants and animals based on their habitat (locality of habitat). He classified plants into trees, shrubs and herbs; animals into two groups, those which had red blooded and those that did not. Many scientists took part in classifying living organisms; John Ray first used Latin language for writing names of organisms. Theophrastus (370-285), Father of botany classified plants on the basis of their habitats, forms, and shape into four categories- trees, shrubs, undershrub’s and herbs. He wrote book called Historia Generalis Plantarum in which he gives description of 480 plants. Carolus Linnaeus (1707-1778) a Swedish naturalist, known as father of taxonomy developed Binomial nomenclature (scientific names of living organisms). He described 5900 species of plants in his book Systema Naturae (1735). These scientists considered only limited features in the classification based on the studies of ribosomal RNA which is the common ancestor in all living organisms. This taxonome is referred as artificial system of classification. Later the natural system was classified based on the natural affinities their related characters. A modified form of natural system of classification is Phenatics (Numerical taxonomy) emerged. This system of classification is considered better because it uses a large number of comparable characters to place the species in particular or specific position in the taxonomy. Studies of homologous structures, comparative embryology, sequence analysis in DNA, RNA and amino acids sequences forms the basis of today's modern taxonomy. The improved tools and techniques for classification resulted in development of new branches of systematics, such as cytotaxonomy, chemotaxonomy, or biochemical taxonomy. The Cytotaxonomy is based onthe cytological information on cell, chromosomal number, and behaviour of chromosomes during meiosis. Chemotaxonomy or biochemical taxonomy is based on serology (scientific study of blood serum), molecular biology, comparative study of DNA sequence, chemical nature of protein, chemical constituent of plant species. Phenetic classification is based on overall similarities of organisms without regard to Phylogeny.

Classification:

There are 3 types of classification:

1. Artificial system of classification

2. Natural system of classification

3. Phylogenetic system of classification

1. Artificial system of classification:-

in this type of classification, only limited characters are selected randomly and the organisms are classified into groups based on their characters. This type of classification was first given by Pliny the Elder (28-29 AD) in his book Historia Naturalis. He divided the organisms based on habit and habitat. Linnaeus system of classification is also an artificial system which is commonly known as sexual system as it is based on the numerical strength of sex organs. The following are the drawbacks of this system:

1. The organisms with close similarities get separated into different categories.

e.g. bat, whale (common feature: echolocation).

2. The organisms with different features get placed together in a single group. For example, Pliny distinguished animals into flight and non-flight. Birds, bats, insects categorised into same group though they different features. Insects have two pairs of wings, while bats and birds each have one pair. Insect wings lack bones, but bird and bat wings have them. Butterfly wings are covered in scales, bird wings in feathers, and bat wings with bare skin.

2. Natural system of classification:-

This system considered all important characteristics and the organisms are classified according to their related characters. This well-known natural system is known as Bentham and Hooker’s system of classification. This system has following advantages compared to artificial system of classification.

1. It grouped natural relationships amongst organisms into same categories.

2. Only closely related organisms are placed in the same group.

3. This system indicates phylogenetic relationship i.e., the sequence of their evolution from simpler to more complex forms of life.

4. This system gives an idea about unknown organisms based on their similarities and dissimilarities between them.

3. Phylogenetic system of classification:

This type of classification is based on their evolutionary and genetic similarities between them. However, it is difficult to classify the organisms on the basis of their evolutionary history due to incomplete fossil records. This system was proposed by Engler (1886), Hutchinson (1926), Tippo (1942). This system of classification led to the emergence of new systematics or Biosystematics. Father of new systematics is Sir Julian Huxley (1940).

Phenetics:

Phenetics also known as taximetrics, is the method used to classify organisms based on overall similarities usually morphology or other observable traits rather than their phylogeny or evolutionary relation. Or it is the study of relationship among a group of organisms based on their degree of similarity between them, be that molecular similarity, phenotypic or anatomical.

Phylogeny:

The evolutionary relationship of organisms is referred to as phylogeny.

Phylogenetic tree:

A phylogenetic tree or evolutionary tree is a branching diagram or tree showing the evolutionary relationships among various biological species or other entities—their phylogeny based upon similarities and differences in their physical or genetic characteristics. The phylogenetic tree is also called as Genealogical tree or dendrogram.

Clade: clade is a group of organisms that evolved from a common ancestor.

Cladistics: It is referred to as the biological classification in which the organisms are categorised into same groups based on the shared traits(characteristics) or common ancestor. Or it is the study of pathways of evolution.

Cladogram:

it is a branching diagram (dendrogram) showing cladistic principle and method is called a Cladogram.

THE TWO KINGDOMS CLASSIFICATION:

Carolus Linnaeus, AL de Jussieu and GL Cuvier established the two-kingdom classification. They divided organisms into two kingdoms namely Plantae (includes all plants and bacteria) and Animalia (includes all animals).

However, this system has the following drawbacks

1. This system did not distinguish between eukaryotes and prokaryotes.

2. This system did not distinguish between unicellular and multicellular.

3. This system did not distinguish between photosynthetic and non-photosynthetic organisms.

4. It brought together the prokaryotic bacteria and the blue-green algae with other groups which are eukaryotic.

5. This system did not distinguish between heterotrophic group, fungi and autotrophic green plants.

6. At the lower level of organisation difference between plants and animals is not very clear.

Example: euglena, sponges, unicellular algae like Chlamydomonas.

7. In this system all the prokaryotes are classified under Kingdom-Plantae. Even though after the invention of microscope they considered that viruses and prokaryotes have no relation with plants, but they are studied under botany.

8. Lichens have neither any plant character, nor any animal character. They represent as example between algae and fungi. But also, they are not given appropriate place in Linnaeus system.

Hence the two-kingdom classification used for a long-time was found inadequate. Besides gross morphology classification also includes characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships etc. Classification systems fir living organisms have hence, undergone several changes overtime.

In the view of above-mentioned taxonomic difficulties, during past 50 years, biologists have recognized that most basic differences among groups of organisms is not between animals and plants but that between prokaryotes and eukaryotes. Bacteria are classified into separate kingdom under the name Monera. Ernst Haeckel was the innovator of kingdom Monera for bacteria.

Herbert F Copeland (1956) reclassified all the microorganisms, recognising Protoctista (Protista) for the nucleated microrganisms.