Figure 1 The ring species of Larus gulls. The numbers around the Arctic Circle are placed at the approximate geographical centre of the range of each subspecies. 1, Larus fuscus (lesser black-backed gull); 2, Larus heuglini (Heuglin’s gull); 3, Larus argentatus birulai (Birula’s gull); 4, Larus vegae (east Siberian herring gull); 5, Larus smithsonianus (American herring gull); 6, Larus argentatus (herring gull). The double-headed arrows show gene flow. Each subspecies can breed with its neighbour subspecies except over northern Europe where the herring gull and the lesser black-backed gull do not interbreed, and where speciation is complete
Speciation, the generation of new species from an ancestral species, requires reproductive isolation. If the geographical area that a species occupies is larger than the average life-time travel of any individual, then although the population is continuous, gene flow will be restricted to geographic neighbours. Could collections of genetic changes at the extremes of a geographic range of a population result in speciation?
This has been observed in ring species, where an ancestral species encountered a geographical barrier, migrated around it in two directions and the populations then met again.
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