A genus is a rank in the biological classification system used to group species that are closely related; the plural form is genera.
Explanation
In biological taxonomy, the genus is situated between family and species in the hierarchical classification of life. Species assigned to the same genus share a combination of morphological, physiological or genetic traits that indicate common ancestry. The genus name forms the first part of an organism’s binomial scientific name and is always capitalized and italicized. For example, in Escherichia coli the genus Escherichia honours the paediatrician Theodor Escherich. Traditionally, genera were defined by observable characteristics, but modern systematics increasingly relies on molecular phylogenetics to delimit genera; this has led to the splitting or merging of genera when DNA sequence data reveal unexpected relationships. Because the plural of genus is genera, taxonomists may refer to multiple genera within a family. The genus concept dates back to Linnaeus and remains central to nomenclature in zoology, botany, microbiology and virology. The breadth of variation allowed within a genus varies among groups: some genera encompass dozens of species, while others contain only one extant species. In microbiology, genera provide a convenient means of organizing the immense diversity of bacteria and archaea based on phenotypic and genetic coherence.
Examples and important facts
The genus Homo includes modern humans (Homo sapiens) and several extinct relatives such as Homo neanderthalensis and Homo habilis. The genus Mycobacterium encompasses acid‑fast bacteria like M. tuberculosis and M. leprae, though recent genomic analyses have transferred some species to new genera such as Mycobacteroides. Yeasts used in baking and brewing belong to the genus Saccharomyces, including S. cerevisiae and S. pastorianus. The genus Bacillus comprises spore‑forming, rod‑shaped bacteria like B. subtilis and B. anthracis. In zoology, the genus Panthera contains large felids such as lions (Panthera leo) and tigers (Panthera tigris). After the first mention, genus names may be abbreviated to their initial letter (e.g., E. coli).
Genera provide an intermediate category that groups related species and stabilizes nomenclature. Understanding this rank helps scientists communicate about evolutionary relationships and organize the diversity of life.
Related Terms: Taxonomy, Species, Family, Binomial nomenclature, Phylogeny