Categories of classification

This system of classification / method of classifying organisms was based on one or two (few) superficial similarities or based on mere easy features of identification.

Or

The system which was based on simple arbitrary chosen criterion instead of an evolution of totality of character.

For example Aristotle (384-322 BC) a Greek biologist produced the first classified animals by looking at the way they move e.g. bats = insects and birds were grouped together based on their ability to fly.

He classified plants based on the appearance and size e.g. he grouped plants into herbs, shrubs and trees.

Other criteria could be used for example utility where we have edible and non-edible plants, medicinal and non-medicinal plants etc.

An artificial system of classification is based on mere easy features of identification.

Advantages / Merits

1. It is useful for quick reference and identification of organisms.
2. Identification requires few characteristics which are not difficult to study and apply.
3. Organisms which are poorly known can be filled in the system very easily.
4. It is a time saving since it is quick and easy to prepare.
5. It is very stable, does not subjected to changes.

Disadvantages / Demerits

1. The system did not reflect any natural relationship existing among the organisms.
2. The system limits amount of information about members i.e. it is rigid.
3. It did not consider many important characteristics and could not explain the evolutionary significance.
4. The system put unrelated organisms under one heading.
5. It cannot add any information about an organism.

This is the system of classification based on characters of similarities which indicate natural relationship. The basis of a natural classification is similarities.

The system of classification based on the real nature of organism(s).

In this system more numbers of characters rather than single character are used for determining similarities. For example phylogenetic and phenetic.

Phylogenetic system of classification

This system of classification is based on evolution and genetic relationship among organisms. This system enables us to find the ancestors or derivatives of any taxon. It reflects the true relationship among organisms. The system was proposed by two German botanists Adolf Engler (1844-1930) and Karl A. E. Prantl (1887-1893).

Relationship based on the evolutionary aspect of organisms. Darwin's concept tells on regional relationship among organisms.

It relies on fossils records and is dynamic.

They vary between zoologists and botanists as zoologists rely on structural aspect while botanists don't.

Phenetic classification

Type of phylogenetic classification which relies on similar and dissimilar features present in today's organism without including evolutionary and other reflected aspects.

Cladistics.

The system is based on similarities of form known as phenetic. Phenetic system of classification emphasizes overall similarity. Phenetic reflect evolutionary relationship.

The character taken for identifying similarities are homologous not analogous structures.

Homologous structure - these are structural or biochemical features which are shared between organisms by virtue of common ancestry e.g. backbone, pentadactyl limb plan of mammals, birds and reptiles.

Analogous structure - structural features which have the same function but basically different in structure e.g. wings of bats, birds and insects.

Features studied in natural classification

Various features of the organisms like external form (morphology), internal structure (anatomy), cell structure (cytology), life process (physiology), and biochemistry etc.

Homologous organs.

Molecular similarities (DNA, RNA and comparative proteins).

Advantages of natural system of classification

1. Close related species are placed in the same group and hence reflect the evolutionary relationship.
2. Many characteristics are considered in classifying organisms by so doing it is more accurate than artificial classification.
3. Like artificial system of classification, it is useful for reference identification of organisms.
4. It is flexible hence can easily accommodate newly discovered organisms.
5. It is based on biological important similarities of organisms, it offers information (knowledge) about organisms.
6. The system brings out natural relationships among organisms.

Disadvantages of natural system

1. It is time consuming since it takes more time and much effort also many attributes are taken into consideration.
2. It is difficult as too much information is needed to group organisms.
3. The system is not stable since it is liable to change with time increase of knowledge of living organisms. Poorly known organisms are not easy to classify.
4. It always needs a skilled personnel hence it is expensive since identification uses scientific instruments.

This system of classification is based on evolution and genetic relationship among organisms.

This system enables us to find out the ancestors or derivatives of any taxon. It reflects the true relationship among the organisms.

The system was first proposed by two German botanists Adolf Engler (1844-1930) and Karl A. E. Prantl (1887-1893).

1. The process is time consuming hence tiresome work.
2. Some organisms may show living and non-living properties hence difficult to place them in any group e.g. viruses.
3. The process always needs skilled personnel and instrument to place living organisms to their correct group therefore it is very expensive.
4. Sometimes evolutionary information about an organism can be missing hence difficult to classify. Classification is subject to change.