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.
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