A species is a fundamental unit of biological classification and a central concept in evolution. In taxonomy, species are named groups of organisms; in evolutionary research, they are commonly understood as lineages that evolve separately from other lineages. The familiar definition based on interbreeding applies chiefly to sexually reproducing organisms, rather than to all life. Different species concepts emphasize reproductive compatibility, observable characteristics, ancestry, or ecological relationships, and these criteria do not always identify identical boundaries. (pubmed.ncbi.nlm.nih.gov)
Classification and scientific names
Species names follow binomial nomenclature, a system associated with Carl Linnaeus. Each name combines a genus name with a specific name or epithet. In Homo sapiens, for example, Homo identifies the genus and sapiens supplies the second component. The genus begins with a capital letter, the second component with a lowercase letter, and both are conventionally italicized. Neither component alone constitutes the complete species name. (iczn.org)
A scientific name and a biological definition perform different functions. In zoological nomenclature, a name-bearing type provides the reference that fixes a name’s application, even when researchers revise the boundaries of the group. A type specimen is therefore a nomenclatural anchor, not necessarily an average or especially representative member of its species. Naming rules provide continuity without requiring taxonomists to agree permanently about classification. (code.iczn.org)
Major species concepts
The biological species concept identifies species through actual or potential interbreeding in nature and reproductive isolation from other groups. Producing viable, fertile offspring is an important criterion. Isolation may involve differences in mating behavior, breeding seasons, or reproductive structures, as well as reduced survival or fertility of offspring. This concept is useful for many sexual organisms, but cannot be applied directly to asexual organisms or tested experimentally for extinct forms. Geographically separated populations also present difficulties because their reproductive compatibility may be unknown. (openstax.org)
Morphological species concepts distinguish groups by anatomical or other observable traits. Phylogenetic species concepts emphasize diagnosability and evolutionary ancestry; different formulations use different criteria for recognizing distinct groups. These approaches can disagree because recognizable traits, reproductive barriers, and genealogical separation need not develop simultaneously during divergence. Consequently, no single operational test resolves every case. (pubmed.ncbi.nlm.nih.gov)
The general lineage concept treats species as separately evolving population lineages. Under this framework, reproductive isolation, distinctive morphology, and genealogical evidence help establish lineage separation rather than each serving as a universally required definition. It distinguishes the conceptual question—what a species is—from the empirical question of how researchers recognize its boundaries. (pubmed.ncbi.nlm.nih.gov)
Formation and reproductive boundaries
Speciation is the emergence of new species through evolutionary divergence. Geographic separation can reduce gene flow, allowing populations to follow different trajectories. Mutation introduces genetic variation, while natural selection and genetic drift can change the frequencies of variants. If divergence produces reproductive barriers, populations may remain distinct even after contact resumes. Geographic separation alone, however, does not demonstrate that speciation has occurred. (openstax.org)
Speciation can also occur without complete geographic separation. Differences in resource use, habitat preference, or mating behavior may restrict reproduction within a shared area. In plants, polyploidy—the possession of additional complete sets of chromosomes—can create reproductive barriers relatively rapidly. Changes in chromosome number may prevent successful reproduction with the parental population while allowing reproduction among individuals with the new chromosome complement. (openstax.org)
Species boundaries are not always impermeable. Hybridization between closely related species may occur where their distributions overlap. The survival and fertility of hybrids influence whether boundaries persist, weaken, or become reinforced. Thus, the existence of occasional hybrids does not by itself settle whether two populations should be classified as one species or two. (openstax.org)
Identification and delimitation
Species identification assigns an organism to a recognized group; species delimitation investigates where the boundaries between groups lie. Genetic data provide powerful evidence for delimitation, but genetic differentiation can also arise among populations within one species. A 2017 study demonstrated that widely used multispecies-coalescent methods can detect population structure without necessarily distinguishing it from species-level separation. Genetic clusters therefore require biological interpretation rather than automatic conversion into species names. (pmc.ncbi.nlm.nih.gov)
Researchers can compare DNA from multiple sampled individuals and analyze variation across genomes. Combining genetic evidence with reproductive, geographic, and morphological information helps distinguish competing explanations. Approaches that explicitly model the speciation process attempt to separate population divergence from the formation of independent species lineages. Their conclusions remain dependent on the data sampled and the assumptions of the models employed. (pmc.ncbi.nlm.nih.gov)
Microbial species
For bacteria and archaea, interbreeding is not a generally applicable species criterion. Genomic comparisons instead provide important operational measures. Average nucleotide identity measures similarity across shared genomic sequences; approximately 95 percent identity is a widely used threshold for distinguishing prokaryotic species. This is a practical microbial criterion, not a universal percentage applicable to every organism. Horizontal gene transfer further complicates relationships by moving genetic material between lineages. (pmc.ncbi.nlm.nih.gov)
Biodiversity and conservation
Species are one level of biodiversity, alongside variation within species and among ecosystems. Counting species alone therefore does not describe all biological diversity. The Convention on Biological Diversity explicitly includes these different levels in its definition, recognizing that variation among populations and ecological communities also matters. (cbd.int)
Species are also important assessment units in conservation biology. The IUCN Red List evaluates extinction risk using documented information about recognized taxa. Taxonomic revisions or improved knowledge can change an assessment without indicating a genuine change in biological risk, so comparisons across assessments must distinguish changes in classification from changes in a species’ condition. (nrl.iucnredlist.org)