94 What Are Ring Species?

Key Idea: A ring species is a connected series of closely related populations, distributed around a geographical barrier, in which adjacent populations in the ring can interbreed, but those at the extremes of the ring are reproductively isolated. The ring species concept was proposed by Ernst Mayr in 1942 to account for the circumpolar distribution of species of herring gulls (Larus species). The idea of a ring species is attractive to biologists because it appears to show speciation in action. However, such examples are rare, and rigorous analysis of supposed ring species, including the herring gull complex, have shown that they do not meet all the necessary criteria to be ring species as defined. Although ring species are rare, the concept is still helpful because it can allow us to reconstruct the divergence of populations from an ancestral species. Ring species also provide evidence that speciation may occur without complete geographical isolation.

What is a ring species?

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A: Natural populations along a cline (environmental gradient). Each population varies slightly from the next.

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2 B

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C

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The variation in populations may occur in a geographical ring, e.g. around a continental shoreline (B). Adjacent populations in the cline can interbreed. If the ring closes (C), the populations at the extremes of the ring may meet but are too different to interbreed.

Criteria for a ring species

` Ring species develop from a single ancestral population with isolation by distance. ` They show expansion of their range around a geographic barrier, such a mountain range or desert. ` Adjacent populations in the ring can interbreed to produce fertile offspring (gene flow). ` The terminal populations are reproductively isolated.

The circumpolar distribution of Larus subspecies (still often cited in many texts) inspired Mayr to propose the ring species hypothesis. However, mtDNA studies (Liebers et al. 2004) have indicated that there were two ancestral gull populations (not one) and most of Mayr's subspecies in fact deserve species status. Moreover, the Larus complex includes several species, excluded by Mayr, whose taxonomy is unclear. Ring species appear to be a very rare phenomenon if they exist at all. In contrast, cryptic (hidden) species, which are morphologically identical but behave as (reproductively isolated) true species, appear to be common.

Omasz G. Sienicki CC 3.0 Photo left: The herring gull (front) and black-backed gull (rear) do not interbreed at the ends of the circumpolar ring where they coexist. However, genetic analyses do not support a ring species.

Photo right: Greenish warbler populations occupy a ring around the Tibetan Plateau. Eastern and western populations meet in Siberia but do not interbreed. Analyses support their status as a ring species.

1. What is a ring species?

2. Why is the phenomenon of ring species interesting to evolutionary biologists?

3. Explain why the populations the two extremes of a geographical ring, as depicted in the diagram, cannot interbreed:

JM Garg CC 3.0