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.