Ring species also present an interesting problem for those who seek to divide the living world into discrete species, as well as for those who believe that evolution does not create new species. After all, all that distinguishes a ring species from two separate species is the existence of the connecting populations - if enough of the connecting populations within the ring perish to sever the breeding connection, the ring species becomes two distinct species.
The coloured bar to the right shows a number of natural populations, each population represented by a different colour, varying along a cline (a gradual change in conditions which gives rise to slightly different characteristics predominating in the organisms that live along it). Such variation may occur in a straight line (for example, up a mountain slope) as is shown in A, or may bend right around (for example, around the shores of a lake), as is shown in B.
In the case where the cline bends around, populations next to each other on the cline can interbreed, but at the point that the beginning meets the end again, as is shown in C, the genetic differences that have accumulated along the cline are great enough to prevent interbreeding (represented by the gap between pink and green on the diagram). The interbreeding populations in this circular breeding group are then collectively referred to as a ring species.
The problem, then, is whether to quantify the whole ring as a single species (despite the fact that not all individuals can interbreed) or to classify each population as a distinct species (despite the fact that it can interbreed with its near neighbours). Ring species illustrate that the species concept is not as clear-cut as it is often understood to be.
A classic example of ring species is the Larus gulls circumpolar species "ring". The range of these gulls forms a ring around the North Pole. The Herring Gull, which lives primarily in Great Britain and Ireland, can hybridize with the American Herring Gull (living in North America), which can also hybridize with the Vega or East Siberian Herring Gull, the western subspecies of which, Birula's Gull, can hybridize with Heuglin's gull, which in turn can hybridize with the Siberian Lesser Black-backed Gull (all four of these live across the north of Siberia). The last is the eastern representative of the Lesser Black-backed Gulls back in north-western Europe, including Great Britain. However, the Lesser Black-backed Gulls and Herring Gull are sufficiently different that they do not normally hybridize; thus the group of gulls forms a continuum except in Europe where the two lineages meet. A recent genetic study has shown that this example is far more complicated than presented here (Liebers et al, 2004). This example only speaks of classical Herring Gull - Lesser Black-Backed Gull complex and does not include several other taxonomically unclear examples which belong in the same superspecies complex, such as Yellow-Legged Gull, Glaucous Gull and Caspian Gull.
Other examples include: