Ring speciesA species which has gradually expanded its breeding distribution from its point of origin right round the world at the latitudes concerned until the two ends of its ring-like range meet. Each stage of the expansion has involved slight changes in the birds' characteristics, so that by the time the two ends of the ring join and overlap the two stocks are sufficiently different for them not to interbreed or, in other words, speciation has taken place. The Lesser Black-backed Gull Larus fuscus and the Herring Gull L. argentatus are thought to represent the two ends of such a ring, the British Isles being in the area where they overlap.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver