Subspecies, Semispecies, Superspecies

DISTINCT POPULATIONS THAT REPLACE EACH OTHER GEOGRAPHICALLY were recognized either as full species or as lower-level ‘‘varieties’’ or ‘‘forms’’ under the original Linnaean taxonomy. In zoology, a practical resolution of this ambiguity took place largely between about 1880 and 1920, with the recognition of a single additional taxonomic rank, the geographic subspecies (while in botany, many infraspecific ranks remain valid). Since the 1980s, the fashion has changed once more. Some systematists are again recognizing geographical replacement forms as full species, even when they blend together at their boundaries.