In
biology, a
ring species is a connected series of neighbouring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential
gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may
co-exist in the same region thus closing a "ring". The German term
Rassenkreis, meaning a ring of populations, is also used.