Classification - (GCSE Biology)

Classification

Classification

Living organisms must be classified, in order to compare between them and see the differences between different types of organisms. Classification systems aim to reflect evolutionary relationships and organisms can be classified into groups by the features that they share.

Artificial classification was the earlier classification system. It only used observable features in organisms to place them in groups e.g. can fly.

Natural classification uses information about the organisms’ common ancestors and structural features to place them in groups e.g. similar bone structure. This has occurred due to the increased understanding of evolutionary relationships between organisms. The 5 kingdom system is an example of natural classification.

Carl Linnaeus devised this system in the 1700s. Organisms are split into the following system:

Kingdom
Phylum
Class
Order
Family
Genus
Species

Kingdom

• Organisms are split into five kingdoms. Plants, animals, fungi, protists and prokaryotes make up the five kingdoms. Examples of protists include Plasmodium and Amoeba. Examples of prokaryotes include bacteria, such as Staphylococcus aureus.

Organisms can be placed into kingdoms, their main groups and sub groups based on their features. This is shown in the table below:

 

Phylum

• Phylum lies in between Kingdom and Class. The phylum of an organism is a generalisation based on the body plan of an organism. It is a classification that lies in between Kingdom and Class. The class of humans is Chordata.

Class

• Class is another division in organisms. You may be familiar with some of the divisions of class. This includes mammals, fish, reptiles and birds. We are mammals.

Order

• Order comes after class. Orders of organisms can be defined. They are formed after class. Mammals are divided into bats, carnivores, rodents and many other sections.

Family

• Families are made from classes. Families are a subdivision of class. Birds for example can be divided into families of pigeons and doves and ducks along with many more.

Genus

• The genus is a subdivision of families. Families can be further divided into the genus. An example of a genus is Felix, that of domestic cats.

Species

• The species is the final division. We generally know what an organism is through its species. It is the final division of an organism. You can define species as a group of organisms that can reproduce to produce fertile offspring.

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An Overview of Classification

The binomial system is used to name organisms. It is the internationally agreed system where you use the genus and species to name the individual species of organisms.

Here are some key points about the system:

• There is no overlap between groups. An organism can only be in one group at each level).
• Higher groups have many organisms. There are more organisms in the early groups, and less and less as you move down the classification system.
• Organisms lower down are more closely related. Organisms in the same lower groups are more closely related. For example, organisms in the same species are more closely related than organisms in the same domain.
• The system is always updating. The classification system is continually evolving and changing as new types of analysis are discovered (e.g. gene sequencing).

Importance of Technological Developments

Classification is traditionally based on studies of morphology and anatomy. However, as microscopes have developed over time, they have allowed us to improve our understanding. Using microscopes has improved our understanding of different organisms and the distinctions between them.

We can use this understanding to learn more and more about different sorts of organisms, pertaining to their biochemical processes and other such processes. Linnaeus for example, struggled to see the differences between lichens and mosses.

As time went by, chemical analysis has led to the advent of Carl Woese’s ‘three-domain system.’ This model was only introduced in the 1970s after RNA analysis. Now the sequences of bases in DNA and of amino acids in proteins are used as a more accurate means of classification. Organisms that are more closely related and share a more recent ancestor have base sequences in DNA that are more similar than those that share only a distant ancestor. This is all part of molecular phylogenetics – looking at the evolutionary history of groups of organisms by exploring their molecules e.g. DNA.

The ‘three-domain system’ involves dividing organisms into three categories:

Archea

Archea are more primitive bacteria that have become accustomed to living in extreme environments. They have no nucleus. Examples include the halobacteriales.

Bacteria

Bacteria cells are the true bacterial cells. They are again prokaryotic and examples include the Spirochates.

Eukaryota

Eukaryota encompass the rest of the organisms. It includes fungi, plants, animals and protists. Anything with a membrane bound nucleus fits into this section.

Evolutionary Trees

• Evolutionary trees show how organisms are related. Evolutionary trees show the relationships between species. Living organisms are mapped using current data, however fossils are used to map extinct organisms. These trees therefore span generations upon generation and show how extinct organisms are linked to living organisms.