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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). Interbreeding between two populations is shown by a grey zone. 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, 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 classify 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 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, can breed with the American Herring gull (living in North America), which can also breed with the Vega Herring gull, which can breed with Birula's gull, which can breed with Heuglin's gull, which can breed with the Siberian lesser black-backed gull (all four of these live across the top of Siberia), which can breed with the Lesser black-backed gull back in Northern Europe, including Great Britain. However, the Lesser Black-backed gull and Herring gull are sufficiently different that they cannot interbreed; thus the group of gulls forms a ring species. A recent genetic study has shown that this example is far more complicated than presented here. For more information about this see: The herring gull complex is not a ring species, D Liebers, P de Knijff, AJ Helbig, Biological Sciences, 2004 Volume 271.
Other examples include:
- The Ensatina salamanders, which form a ring round the Central Valley in California
- The greenish warbler (Phylloscopus trochiloides), around the Himalayas
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