A species is a fundamental unit of biological classification representing a group of organisms that are considered sufficiently closely related to constitute a distinct evolutionary lineage. There is no single definition that works perfectly for every organism and every biological situation. The biological species concept commonly defines a species as a group of natural populations whose members can potentially interbreed and produce fertile offspring while being reproductively isolated from other such groups.
This definition works particularly well for many sexually reproducing animals but becomes difficult for asexual organisms, fossils and organisms that hybridise extensively. Other concepts include the morphological species concept, ecological species concept, phylogenetic species concept and evolutionary species concept. Consequently, the word species has both a practical taxonomic meaning and several more specialised theoretical meanings depending on the biological context.
The biological species concept is strongly associated with Ernst Mayr and twentieth century evolutionary biology. Under this concept, reproductive isolation is central. Members of the same species can potentially exchange genes through reproduction, while barriers prevent substantial gene flow with other species. Prezygotic barriers act before fertilisation and include differences in mating behaviour, breeding season, habitat, courtship signals and reproductive structures. Postzygotic barriers act after fertilisation and can involve hybrid inviability, hybrid sterility or reduced fitness of hybrid offspring. A familiar example is the horse and donkey, which can produce a mule, but mules are generally sterile. This illustrates why successful mating or even viable offspring alone does not necessarily establish that two populations belong to the same biological species.
Species boundaries are often more complicated than a simple yes or no reproductive test. Closely related species may hybridise occasionally while remaining distinct because most gene flow is prevented or because ecological and behavioural differences maintain separate populations. Plants frequently exhibit hybridisation and polyploidy, sometimes producing fertile lineages with chromosome numbers different from their ancestors. Ring species provide another instructive pattern in which neighbouring populations can interbreed while terminal populations are reproductively isolated. Asexual organisms cannot be evaluated using ordinary interbreeding criteria at all. These examples show that species are evolutionary populations rather than merely collections of individuals that look alike.
The morphological species concept identifies species primarily through observable structural characteristics. It has historically been essential for describing fossils because reproductive behaviour cannot usually be observed in extinct organisms. It is also useful when rapid identification is required or when genetic information is unavailable. However, morphology can mislead because individuals of one species can vary considerably due to age, sex, environment or developmental stage, while unrelated species can evolve similar appearances through convergent evolution. Cryptic species are especially important because they are genetically and reproductively distinct despite being difficult to distinguish visually. Molecular techniques have revealed many such cases, demonstrating that species diversity can be underestimated when classification depends exclusively on morphology.
The phylogenetic species concept generally emphasises diagnosable evolutionary lineages identified through shared derived characteristics. Genetic sequences can be used to determine whether populations form distinct branches in a phylogenetic tree. This approach can be powerful for organisms that reproduce asexually and for identifying cryptic diversity. However, the precise threshold for recognising a separate species can vary, and different genes can produce different evolutionary histories.
The evolutionary species concept treats a species as a distinct lineage maintaining its identity and evolutionary tendencies through time. The ecological species concept emphasises adaptation to a particular ecological niche. These concepts often overlap, but they can produce different classifications in difficult cases because they prioritise different biological properties.
Species are also central to evolutionary biology because speciation explains how new species arise. Allopatric speciation occurs when populations become geographically separated and diverge through mutation, natural selection, genetic drift and other processes. Sympatric speciation occurs without complete geographic separation and can result from ecological specialisation, sexual selection, polyploidy or other mechanisms. Parapatric and peripatric patterns describe additional geographic relationships. Reproductive isolation can accumulate gradually as populations diverge. Once gene flow becomes sufficiently limited, differences can become increasingly independent and the populations may follow separate evolutionary trajectories. Species formation is therefore a population process rather than an instantaneous event in which one individual suddenly becomes a new species.
Species names are used in binomial nomenclature and normally consist of a genus name followed by a specific epithet, together forming the scientific name of the species. For example, humans are Homo sapiens, with Homo representing the genus and sapiens representing the specific epithet. In formal scientific writing, the full binomial is conventionally italicised, with the genus capitalised and the specific epithet written in lower case. The term species is abbreviated as sp. when referring to one unspecified species within a genus and spp. when referring to multiple species. Scientific names reduce ambiguity caused by common names, which can vary between regions and languages.
The species concept has practical importance in conservation, ecology, medicine, agriculture and biodiversity research. Legal protection may apply specifically to species, so accurate species identification can affect conservation policy and management. Pathogens may contain closely related species or strains with different host ranges and clinical characteristics. Agricultural breeding programmes may depend on understanding reproductive compatibility among crop relatives. Biodiversity estimates depend heavily on how species boundaries are defined.
Because species concepts have limitations, taxonomists sometimes use multiple evidence sources including morphology, genetics, ecology, behaviour, geography and reproductive compatibility. The most scientifically useful species assessment therefore depends on the organism and the evidence available rather than assuming that one universal test can resolve every species boundary.
