A species is the basic unit of biological classification, defined as a group of organisms that share common characteristics and, in sexually reproducing organisms, can interbreed to produce fertile offspring.
Explanation
In classical taxonomy the biological species concept describes species as populations whose members can mate with one another to produce viable, fertile offspring but cannot successfully breed with members of other species. This concept works well for animals and plants but is less applicable to organisms that reproduce asexually. Microbiologists therefore use genetic and phenotypic criteria to delineate bacterial and archaeal species: isolates with ≥97 % similarity in 16S rRNA gene sequences and ≥70 % DNA–DNA hybridization are generally considered the same species. Species names are written using binomial nomenclature: the genus name capitalized followed by the species epithet in lowercase and both italicized (e.g., Homo sapiens, Escherichia coli). Species are nested within higher taxonomic ranks such as genus, family, order and phylum. Speciation—the evolution of new species—occurs through reproductive isolation and genetic divergence. In microbial taxonomy, horizontal gene transfer complicates species boundaries, and new sequencing techniques continually refine species definitions.
Examples and Notes
- The human species is Homo sapiens, distinguished from other primates by its anatomy and ability to interbreed within the species.
- Escherichia coli is a bacterial species with numerous strains; most are harmless intestinal commensals, but some, such as E. coli O157:H7, are pathogenic.
- In fungal taxonomy a species may be defined by morphological traits and reproductive structures (e.g., Aspergillus fumigatus).
- Viruses lack a universal species concept; the International Committee on Taxonomy of Viruses defines species based on genome sequence similarity, host range and ecology.
- Species names follow rules set by international codes of nomenclature to ensure consistency and stability.
Species classification provides a framework for organizing biological diversity, communicating about organisms and understanding evolutionary relationships. Despite challenges in defining species across all life forms, the concept remains central to biology.
Related Terms: Genus, Taxonomy, Binomial Nomenclature, Speciation, Phylogeny