Classification, The Five Kingdoms, Phylogeny, Evolution of Classification Systems, Five Kingdoms/Three Domains

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Classification is the act of arranging organisms into groups based on their similarities and differences so that it is easier for scientists to identify and study them. 

Taxonomy is the study of classification. All classification systems involve placing organisms into a taxonomic hierarchy. 

Domain -> Kingdom -> Phylum -> Class -> Order -> Family -> Genus -> Species

Further down the hierarchy there are more groups but fewer organisms in each group. The species group contains only one type of organism.

The nomenclature (naming system) used for classification is the binomial system - all organisms are given one internationally accepted scientific name in two parts:

Genus species e.g. Homo sapiens

This helps to avoid confusuon of using common names which are different in different countries and cultures.

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The Five Kingdoms


Prokaryotic, unicellular, no nucleus, less than 5 micrometers, e.g. bacteria


Eukaryotic, usually live in water, unicellular or simple multicellular, e.g. algae, protozoa


Eukaryotic, chitin cell wall, saprotophic, spore reproduction, e.g. moulds, yeasts, mushrooms


Eukaryotic, multicellular, cellulose cell wall, can photosynthesise, contains chlorophyll, autotrophic e.g. mosses, ferns, flowering plants


Eukaryotic, multicellular, no cell walls, heterotrophic, e.g. fish, reptiles, insects, birds, mammals

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Phylogeny is the study of the evolutionary history of groups of organisms. It shows who's related to whom and how closely related they are.

All organisms evolved from shared common ancestors. Closely related species diverged more recently.


Members of the Hominidae family evolved from a common ancestor. Orangutans first diverged, followed by gorillas, then humans, then bonobos and chimpanzees.

Humans are closely related as they diverged recently. Humans and orangutans are more distantly related as they diverged longer ago. 

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Evolution of Classification Systems

Early classification systems only used observable features e.g. anatomical, behavioural. However scientists do not always agree on the relative importance of different features and physical features not always accurate in showing relations, e.g. sharks and whales look very similar and both live in the sea, but sharks are cartilaginous fish, whereas whales are vertebrate mammals. New technologies e.g. new DNA techniques/better microscopes, lead to new discoveries which can be shared in scientific journals and meetings. Classification is constantly revised to take into account new findings.

Molecular evidence

Analysing similarities in protein and DNA, comparing DNA storage, sequence of DNA bases, sequence of amino acids in proteins. More closely related organisms will have more similar molecules. 

Embryological evidence

Looking at similarites in the early stages of an organism's development

Fossil record

Studying fossils can provide evidence of how organisms evolved from one another and how closely related they are.

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Five Kingdoms/Three Domains

The Three Domains system was proposed in 1990 as three large superkingdoms above the kingdoms in the taxonomic hierarchy. 

Cells containing nucleus olaced in domain Eukarya. Organisms that were in the kingdom Prokaryotae were separated into two domains - Archaea and Bacteria. 

Molecular evidence

  • Enzyme RNA polymerase is different in Bacteria and Archaea
  • Archaea but not Bacteria have similar histones to Eukarya

Cellular evidence

  • Bonds of lipids in celle membranes of Archaea and Bacteria are different
  • Development and composition of flagellae in Bacteria and Archaea are different

Dichotomous keys are used to identify organisms based on observable features.

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