ring species A group of subspecies that are contiguous along a cline. Members of each population are able to mate successfully with members of adjacent populations, but the group as a whole forms a ring, with sufficient morphological differentiation in some places to prevent interbreeding between overlapping populations. Gulls of the genus Larus comprise a circumpolar ring species in the northern hemisphere. Moving westwards from Britain, the herring gull (L. argentatus) occurs in North America (where one variant has developed into a distinct species, L. glaucoides), but is somewhat different from the British race. Between central Asia and north Europe, the races increasingly resemble the black-backed gull (L. fuscus) and in northern Europe the two species overlap and do not naturally interbreed.

ring species Two species with a looped or ringlike distribution pattern, for example circumpolar, which comprises a series of interbreeding forms that are intermediate between the two species. The latter occur where the two ends of the `ring' meet. Ring species thus demonstrate how the evolution of differences among the different populations or subspecies leads eventually to the formation of new species.