Classification

The Basis of Classification

  • Organisms are classified based on shared characteristics which reflect their evolutionary relationships.
  • The Linnaean system includes groups which are organised hierarchically being: Kingdom, Phylum, Class, Order, Family, Genus, and Species, with Species being the most specific level.
  • Biologists often use binomial nomenclature, where every organism is given a two-part name – the first part indicates its genus and the second part denotes the species within that genus.
  • Taxonomy is the science of naming, describing and classifying organisms and includes all plants, animals and microorganisms of the world.

The Five Kingdoms

  • The Ray’s five-kingdom system includes Prokaryotae, Protoctista, Fungi, Plantae, and Animalia.
  • Prokaryotae consists of unicellular organisms without nuclei like bacteria.
  • Protoctista includes unicellular eukaryotes and algae.
  • The Fungi kingdom includes eukaryotic, heterotrophic organisms which are not classified as plants or animals.
  • Plantae encompasses multicellular, autotrophic organisms.
  • Animalia represents multicellular, heterotrophic organisms.

Virus Classification

  • Virus classification primarily depends on their genomic structure and replication methods.
  • Each virus belongs to a family which is named using the suffix -viridae.
  • Genomic structures of viruses comprise DNA or RNA which may be either single or double-stranded.
  • Viral replication methods include lytic, lysogenic, or a combination of both cycles.

Molecular Phylogenetics

  • Molecular phylogenetics uses sequences of gene or protein to identify and compute relationships between species or individuals.
  • Molecular data, especially DNA, are used to establish taxonomic and phylogenetic relationships. Genetic similarities indicate organisms with a relatively recent common ancestor.
  • Differences in base sequence of DNA are resulted from mutations which accumulate over time. Therefore, the greater the difference, the more remote the last common ancestor.

Three-Domain System

  • The three-domain system includes Bacteria, Archaea, and Eukarya.
  • Bacteria and Archaea consist entirely of prokaryotes, but they are distinct from each other on the molecular level.
  • Eukarya includes all organisms that have cells with nuclei. This includes simple, single-celled microbes like yeast as well as complex, multicellular organisms like plants and animals.
  • Each domain reflects a distinct evolutionary path.