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The biological species concept groups naturally interbreeding biological populations with viable, fertile offspring, distinguished from other such groups by reproductive isolation. An operational test examines mating, fertilization, offspring survival and offspring reproduction, rather than only whether two adults can be paired. Sexual reproduction is its scope: fossils and organisms studied only through asexual reproduction cannot supply the same mating evidence. ^[ref]

For an original hypothetical example, imagine two separated biological populations that reproduce successfully after contact. The separation alone did not demonstrate reproductive isolation. Conversely, failure to mate in one unfamiliar chamber could reflect the assay conditions. Field observations of gene flow and tests under suitable conditions address the biological question more directly. A successful laboratory pairing does not establish frequent interbreeding in nature.