Return of the Ring

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Science  16 Apr 2004:
Vol. 304, Issue 5669, pp. 361
DOI: 10.1126/science.304.5669.361a

A classic example of an evolutionary pattern may need revision after a new analysis based on mitochondrial DNA. In the ring species model, an organism expands its geographical range around an uninhabitable area, so that when the extremities of the population finally meet, they have diverged sufficiently to become reproductively isolated from each other (“isolation by distance”). An oft-cited example is the herring gull complex, which contains more than 20 species and subspecies of large gull distributed around the Northern Hemisphere. According to the original ring species model, the modern complex originated in the Caspian/Aral Sea region and spread northwest to Scandinavia, Britain, and France, giving rise to lesser black-backed gulls, and eastward to Siberia, Alaska, and North America, giving rise to herring gulls. Finally, the North American herring gulls were thought to have spread eastward across the Atlantic, where they coexist with but are reproductively isolated from the European black-backed gulls.

A phylogeographic analysis by Liebers et al. suggests that the picture is more complex. Rather than evolving via a series of interconnected populations, the gull taxa differentiated through events involving long-distance dispersal, isolation, and fragmentation of populations. The DNA evidence indicates that the ring was never closed by colonization of Europe from North America; in fact, lesser black-backed gulls are currently expanding westward and may close the ring in the opposite direction. — AMS

Proc. R. Soc. London Ser. B 10.1098/rspb.2004.2679 (2004).

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