In biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" species. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing a "ring".
Ring species provide important evidence of evolution in that they illustrate what happens over time as populations genetically diverge, and are special because they represent in living populations what normally happens over time between long deceased ancestor populations and living populations, in which the intermediates have become extinct. Richard Dawkins observes that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension."
Formally, the issue is that interfertile "able to interbreed" is not a transitive relation – if A can breed with B, and B can breed with C, it does not follow that A can breed with C – and thus does not define an equivalence relation. A ring species is a species that exhibits a counterexample to transitivity.
Problem of definition
Ring species also present an interesting case of the species problem, for those who seek to divide the living world into discrete species. After all, all that distinguishes a ring species from two separate species is the existence of the connecting populations - if enough of the connecting populations within the ring perish to sever the breeding connection, the ring species' distal populations will be recognized as two distinct species.
The problem, then, is whether to quantify the whole ring as a single species (despite the fact that not all individuals can interbreed) or to classify each population as a distinct species (despite the fact that it can interbreed with its near neighbours). Ring species illustrate that the species concept is not as clear-cut as it is often thought to be.
Currently, there are four known forms of life that appear to be matching the definition of a ring species. The classic example is that of the Larus gull, which in 1925 was found to form a chain of varieties around the Arctic Circle by Jonathan Dwight, but lately doubts have arisen as to whether this is an actual ring species. In 1938, Claud Buchanan Ticehurst hypothesized the Greenish Warbler spread from Nepal around the Tibetan Plateau, while adapting to each new environment, meeting again in Siberia where the ends could no longer interbreed. These and other discoveries led Ernst Mayr to first formulate a theory on ring species in his 1942 study Systematics and the Origin of Species. Also in the 1940s, Robert C. Stebbins recognised the Ensatina around the Californian Central Valley as a ring species. Finally in 2012, the first example of a ring species in plants was found in a Spurge form around the Caribbean Sea.
The European Herring Gull (L. argentatus argenteus), which lives primarily in Great Britain and Ireland, can hybridize with the American Herring Gull (L. smithsonianus), (living in North America), which can also hybridize with the Vega or East Siberian Herring Gull (L. vegae), the western subspecies of which, Birula's Gull (L. vegae birulai), can hybridize with Heuglin's gull (L. heuglini), which in turn can hybridize with the Siberian Lesser Black-backed Gull (L. fuscus). All four of these live across the north of Siberia. The last is the eastern representative of the Lesser Black-backed Gulls back in north-western Europe, including Great Britain.
The Lesser Black-backed Gulls and Herring Gulls are sufficiently different that they do not normally hybridize; thus the group of gulls forms a continuum except where the two lineages meet in Europe.
However, a 2004 genetic study has shown that this example is far more complicated than presented here (Liebers et al., 2004): this example only speaks to the complex of species from the classical Herring Gull through Lesser Black-backed Gull. There are several other taxonomically unclear examples that belong in the same superspecies complex, such as Yellow-legged Gull (L. michahellis), Glaucous Gull (L. hyperboreus), and Caspian Gull (L. cachinnans).
The Greenish Warbler (Phylloscopus trochiloides) forms a species ring, around the Himalayas. It is supposed to have spread from Nepal around the inhospitable Tibetan Plateau, to rejoin in Siberia, where the plumbeitarsus and the viridanus appeared to no longer be able to mutually procreate.
In 2012, Cacho and Baum described the first example of a ring species in plants. They showed the Euphorbia tithymaloides is a group within the Spurge family, that has reproduced and evolved in a ring through Central America and the Caribbean, meeting in the Virgin Islands where they appear to be morphologically and ecologically distinct.
- Dawkins, R. The Ancestor's Tale, 2004:303
- Brown, Rob. ""Same Species" vs. "Interfertile": concise wording can avoid confusion when discussing evolution""
- Liebers, D.; De Knijff, P.; Helbig, A. J. (2004). "The herring gull complex is not a ring species". Proceedings of the Royal Society B: Biological Sciences 271 (1542): 893. doi:10.1098/rspb.2004.2679.
- The greenish warbler ring species, by Darren Irwin
- A closer look at a classic ring species: The work of Tom Devitt
- Liebers, Dorit; de Knijff, Peter; Helbig, Andreas J. (2004). "The herring gull complex is not a ring species" (PDF). Proc. Roy. Soc. B 271 (1542): 893–901. doi:10.1098/rspb.2004.2679. Electronic Appendix
- Alström, Per (2006). "Species concepts and their application: insights from the genera Seicercus and Phylloscopus" (PDF). Acta Zoologica Sinica 52 (Suppl): 429–434.
- Patten and Pruett, Per (2009). "The Song Sparrow, Melospiza melodia, as a ring species: patterns of geographic variation, a revision of subspecies, and implications for speciation" (PDF). Systematics and Biodiversity 7 (1): 33–62.
- This species ring forms the subject of "The Salamander's tale" in Richard Dawkins' The Ancestor's Tale, 2004.
- Cacho & Baum (2012) "The Caribbean slipper spurge Euphorbia tithymaloides: the first example of a ring species in plants", Procedings of the Royal Society B
- Irwin DE, Irwin JH, Price TD (2001). "Ring species as bridges between microevolution and speciation" (PDF). Genetica. 112-113: 223–43. doi:10.1023/A:1013319217703. PMID 11838767.
- Futuyma, D. (1998). Evolutionary Biology (3rd ed.). Sunderland, MA: Sinauer Associates. ISBN 0-87893-188-0.
- Moritz, C., Schneider, C.J., et al. (1992). "Evolutionary relationships within the Ensatina eschscholtzii complex confirm the ring species interpretation". Systematic Biology 41: 273–291.
- Nova Scotia Museum of Natural History: Birds of Nova Scotia
- Adriaens, P. Hybrid Gulls Breeding in Belgium
- Greenish Warbler
- Greenish Warbler maps and songs
- Ensatina Salamander
- Ring species -- the abridged version, by Peter Hadfield alias potholer54