A species concept is a rule for deciding where one evolutionary lineage ends and another begins. No single rule works equally well for every organism, so biologists use several complementary concepts.

For sexually reproducing organisms, the biological species concept emphasizes reproductive isolation: populations belong to the same species when they can exchange genes through successful reproduction, while different species are sufficiently isolated that gene flow between them is strongly reduced.

This definition is useful because it connects species boundaries to evolutionary independence. If two populations stop exchanging genes, mutation, drift and selection can make them diverge separately.

However, the biological species concept has limits. It is difficult to apply to:

• fossils, for which mating cannot be observed;
• organisms that reproduce asexually;
• geographically separated populations that never meet;
• lineages that occasionally hybridize while remaining distinct.

Other concepts therefore emphasize different evidence. A morphological species concept distinguishes groups by consistent form, while a phylogenetic species concept identifies the smallest diagnosable evolutionary lineages on a phylogenetic tree.

The important idea is not that species are arbitrary. Rather, species boundaries are hypotheses about separately evolving lineages, supported by evidence such as reproductive isolation, morphology, ecology and ancestry.