The kind of ring species you refer to would also be possible, although far less likely. The ancestor species could form a geographical ring, and if the members

The kind of ring species you refer to would also be possible, although far less likely. The ancestor species could form a geographical ring, and if the members are not too populous, nor too migratory, then mutations that cause local incompatibility would be selected out (because it makes it much harder to find a mate) whereas mutations that cause non-local incompatibility may persist (if the species is largely non-migratory, so non-local compatibility is not as valuable). Over time, the non-local incompatibilities can build up and create a ring species like the one you describe. But something like this is far less likely than a ring species with incompatible ends. Because such "complete" rings can decay into incomplete rings through the extinction of a connecting subspecies, and similarly, incomplete rings can segment further, and then grow again as two separate ring species. But a complete ring, once broken, is very unlikely to re-form.