ring species

In a ring species, gene flow occurs between neighbouring populations of a species, but at the ends of the ring the populations don't interbreed. The coloured bars show natural populations (colours), varying along a cline. Such variation may occur in a line (e.g. up a mountain slope) as in A, or may wrap around as in B.

Where the cline bends around, populations next to each other on the cline can interbreed, but at the point at which the beginning meets the end again, as at C, the differences along the cline prevent interbreeding (gap between pink and green). The interbreeding populations are then called a ring species. In biology, a ring species is a connected series of neighbouring populations, each of which interbreeds 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 linked neighbouring populations. Such non-breeding, though genetically connected, end populations may co-exist in the same region (sympatry) thus closing a ring. The German term Rassenkreis, meaning "circle of races", is also used.