Species concept

A species concept is a framework for deciding how biological populations are defined as distinct species. Different concepts emphasize reproductive isolation, shared ancestry, diagnosable traits, or ecological role.

Biological species concept: A species definition based on reproductive compatibility within populations and reproductive isolation from other populations. It makes interbreeding and barriers to gene flow the decisive criteria.
Reproductive isolation: The restriction of gene flow between populations through barriers to mating or successful reproduction. Under biological definitions, these barriers distinguish one species from another.
Population: A group of organisms of the same species living in a defined area and capable of interbreeding. Species concepts often begin by deciding when populations count as separate units.
Ring species: A chain of neighboring populations whose endpoints coexist but are reproductively isolated. They expose how gradual variation can resist a single clear species boundary.
Species delimitation: The process of identifying and drawing boundaries between species using biological evidence. It applies species concepts to real populations and competing lines of evidence.
Phylogenetic species concept: A species definition that identifies the smallest diagnosable group sharing a common ancestor. It draws boundaries from ancestry and distinguishing traits rather than reproductive compatibility.
Gene flow: The transfer of genetic variants between populations through migration and reproduction. Its presence can blur boundaries, while its restriction helps maintain species distinctions.
Taxon: A named group of organisms at any rank in biological classification. A species is a taxonomic rank, but its circumscription depends on a species concept.
Cryptic species: Distinct species that are difficult to distinguish by visible morphology alone. They reveal the limits of morphological identification and the value of genetic evidence.
Biodiversity: The variety of life across genes, species, and ecosystems. Counts of species depend partly on how species boundaries are defined.

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