5.2.2 Classifying Organisms

• Biological classification involves putting organisms into groups, or taxa (singular taxon)
• The taxa form a hierarchy
  • A hierarchical system is one in which larger groups contain smaller groups with no overlap between groups

• The taxonomic hierarchy contains the following taxonomic groups in descending order of size:
  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

Examples of classification

• The wolf, Canis lupus, is an example of an organism in the animal kingdom
  • A wolf belongs to the following taxa
    • Domain: Eukaryote
    • Kingdom: Animalia
    • Phylum: Chordata
    • Class: Mammalia
    • Order: Carnivora
    • Family: Canidae
    • Genus: Canis
    • Species: lupus

• The flowering plant Hibiscus rosa-sinensis is an example of an organism in the plant kingdom
  • It belongs to the following taxa
    • Domain: Eukaryote
    • Kingdom: Plantae
    • Phylum: Angiospermae
    • Class: Dicotyledonae
    • Order: Malvales
    • Family: Malvaceae
    • Genus: Hibiscus
    • Species: rosa-sinensis

The Classification of the Wolf and the Hibiscus Plant Table

• The taxonomic classification system organises species into groups based on their evolutionary origins and relationships
• This means that all members of a taxonomic group share a common ancestor
  • A common ancestor is one individual, or pair of individuals, from whom all individuals in a group have descended, e.g.
    • For a group of full siblings, the common ancestors will be their parents
    • For a group of cousins, the common ancestors will be their grandparents

• Grouping organisms into taxa that share a common ancestor is known as natural classification and a group of organisms classified in this way is known as a natural group
  • E.g. eukaryotic cells appear to have only evolved once, meaning that one eukaryotic cell was the ancestor of all other eukaryotes; the eukaryote domain is therefore a natural group

• It is possible to carry out artificial classification, grouping together organisms from different ancestors, e.g.
  • Dolphins and sharks could in theory be grouped together as they are both groups of aquatic animals that share a similar body shape, but they in fact belong to different classes
    • Dolphins are mammals and sharks are fish
    • Their streamlined body shapes evolved separately rather than originating in one common ancestor

  • Cacti and euphorbia are two groups of desert plant recognisable by their spiny leaves and branching, succulent stems, and it would be reasonable to assume from their appearance that they are closely related to each other, but they belong to different orders of plant
    • Cacti are found in the deserts of the Americas, while euphorbia are found in Africa
    • They evolved separately, but adapted to similar environments, hence they have similar characteristics; this kind of evolution is known as convergent evolution

• It can be difficult to carry out accurate natural classification from observation alone, but the development of DNA sequencing technology means that natural classification on the basis of DNA sequence data has become possible

It would be possible to group both sharks and dolphins, and butterflies and bats, together on the basis of their shared characteristics, but this would not be a natural classification as they do not share recent common ancestors

The advantages of natural classification

• Natural classification enables identification of unknown species
  • E.g. when a sample of insects is collected and the researcher doesn't recognise all of the species collected
  • Using the natural classification system an unknown species can be identified by logically working through the taxa
    • This means first identifying the domain, then the kingdom, then phylum etc., narrowing down the options until a species is identified
    • An identification method known as a dichotomous key works using this principle

• Natural classification enables predictions to be made about the characteristics of a species
  • If all of the members of a taxon descend from a common ancestor means that it is likely that the members will share common characteristics
  • This can be useful when e.g. searching for new drugs; if researchers know that one member of a plant genus produces a chemical that is useful in treating a disease, then it is likely that other members of that genus may produce the same chemical
    • This allows researchers looking for new drugs to focus their search rather than testing every single plant species

• There can be difficulties when trying to determine the ancestry of species based on observation of shared characteristics
  • This can lead to artificial rather than natural classification
  • • Convergent evolution can lead to groups of organisms sharing similar characteristics when they did not evolve from a shared common ancestor

• Advances in DNA, RNA and protein sequencing has allowed scientists to further investigate the relationships between species
  • This has revealed the true ancestry of taxa

• This sometimes leads to reclassification of taxa
  • Taxa may be split if taxonomists decide that they do not descend from a common ancestor
  • Taxa previously classified as separate may be grouped together if it is discovered that they do descend from a common ancestor

• An example of a taxon in which there has been a great deal of reclassification since the introduction of sequencing data is the Hominidea group to which humans belong
  • Humans were originally thought to have their own taxon separate from the rest of the great apes, but are now known to be part of the same family

The taxon containing humans has been frequently reclassified