The Three Domains of Life (Edexcel International A Level (IAL) Biology): Revision Note

Why Classify Organisms?

• Taxonomy is the practice of biological classification

• The phylogenetic classification system enables us to arrange species into groups based on their evolutionary origins and evolutionary relationships

• By grouping organisms into taxa, it can make them easier to understand and remember

• There are several different ranks or levels within the hierarchical classification system used in biology

  • There is no overlap between groups

  • Each group is called a taxon (plural taxa)

  • Multiple smaller taxa can be put in the same larger taxa

• Species is the lowest taxonomic rank in the system

  • Similar species can be grouped in a genus

  • Similar genera can be grouped in a family

  • Similar families can be grouped into an order

  • Similar orders can be grouped into a class

  • Similar classes can be grouped into a phylum

  • Similar phyla can be grouped into a kingdom

  • Similar kingdoms can be grouped into a domain

• Domains are the highest taxonomic rank in the system

  • They include the Archaea (prokaryotes), Eubacteria (prokaryotes) and Eukarya (eukaryotes)

• There are a few different rhymes that exist to help you remember the different ranks in the taxonomic classification system. You can always make up your own but the one below is super helpful!

  • The first letters of all the different ranks below the domains can be remembered as:

    • Kings Play Chess On Fancy Gold Squares

    • Kingdom Phylum Class Order Family Genus Species

The different levels within the hierarchical classification system - the higher ranks contain more organisms with less similarity between them, while the lower ranks contain fewer organisms with more similarity between them

• Many different species with similar genotypes and phenotypes can be grouped together in the same genus

• They are, however, still separate species since they cannot produce fertile offspring 

  • This is known as the species concept

• Early classification systems only relied on the outward appearance (phenotype) of organisms to classify them

  • Looks can be deceiving however as many non-related organisms share similar phenotypes - e.g. whales and sharks

  • Advances in technology, such as DNA sequencing, have made it possible to compare the genotypes of organisms and provide clarity on the evolutionary relationships between them

Binomial system

• Binomials are the scientific names given to individual species

• It consists of the organism’s genus and species name in modern Latin

  • The genus name is written first and is capitalised, while the species name comes second and is not capitalised

  • Both parts of a binomial are italicised

• For example, the binomial for humans is Homo sapiens and the binomial for dogs is Canis familiaris

• Binomials are extremely useful for scientists as they allow for species to be universally identified - the binomial for a species is the same across the entire globe

Five kingdoms

• Before the three-domain system, organisms were placed into one of five kingdoms based on observable features such as cell structure and how they obtain nutrients:

  • Prokaryotae: prokaryotic cells with no nucleus (e.g. bacteria)

  • Protoctista: eukaryotic organisms that do not fit into the other kingdoms; mostly single-celled (e.g. algae, protozoa)

  • Fungi: eukaryotic; cell walls made of chitin; feed by extracellular digestion (e.g. moulds, yeasts, mushrooms)

  • Plantae: multicellular eukaryotes with cellulose cell walls; autotrophs that photosynthesise (e.g. mosses, ferns, flowering plants)

  • Animalia: multicellular eukaryotes with no cell walls; heterotrophs (e.g. insects, fish, mammals)

• Molecular evidence (e.g. comparing base sequences in DNA and RNA) showed that prokaryotes form two very different groups, Bacteria and Archaea, which led to the three-domain system

The four kingdoms that fall under the domain Eukarya, representing the eukaryotes

Evidence for Taxonomy

• In the past, scientists encountered many difficulties when trying to determine the evolutionary relationships between species

• Using the physical features of species (such as colour/shape/size) has many limitations and can often lead to the wrong classification of species

• Advances in genome sequencing and immunology has allowed scientists to further investigate the evolutionary relationships between species

• This has led to many organisms being reclassified or changes being made to the classification system structure

• Scientific knowledge is constantly changing, as more technological advances are made

• One such example is the three domain classification system, which was based on new information obtained from molecular analysis of RNA genes in particular

  • Using this new information,scientists have realised that using cell type to classify organisms is insufficient, and that prokaryotes could be divided into two separate groups (domains)

  • The three domains are:

    • Archaea (prokaryotes)

    • Bacteria (prokaryotes)

    • Eukarya (eukaryotes)

The taxonomic classification system within the three domains

• This new classification system was based on molecular phylogeny

• Phylogeny investigates the evolutionary history of different groups of organisms and can specify how closely related they are

• Molecular phylogeny relies on similarities or differences of molecules (such as DNA, RNA or proteins) between groups of organisms to determine how closely related they are

• The more similar the molecules, the more recent the common ancestor is that they share

• Based on the evidence that molecular phylogeny provided, it was determined that the organisms in the kingdom Prokaryotae would be reclassified into two separate domains - the Archaea and Bacteria

• The evidence suggested that archaea and bacteria are less closely related to each other than scientists originally thought

• Organisms from the other four kingdoms where placed in another domain - Eukaryota

Characteristics & Features of the Three Domains Table