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In this diagram, interbreeding populations are represented by coloured blocks. Variation along a cline may bend right around, forming a ring. 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."[1] 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. 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.[2] Contents 1 Explanation of the diagram 2 Problem of definition 3 Larus gulls 4 Other examples 5 See also 6 References 7 External links Explanation of the diagram The coloured bar to the right shows a number of natural populations, each population represented by a different colour, varying along a cline (a gradual change in conditions which gives rise to slightly different characteristics predominating in the organisms that live along it). Such variation may occur in a straight line (for example, up a mountain slope) as is shown in A, or may bend right around (for example, around the shores of an ocean), as is shown in B. In the case where the cline bends around, populations next to each other on the cline can interbreed, but at the point that the beginning meets the end again, as is shown in C, the differences that have accumulated along the cline are great enough to prevent interbreeding (represented by the gap between pink and green on the diagram). The interbreeding populations in this circular breeding group are then collectively referred to as a ring species. Problem of definition 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. Larus gulls The Larus gulls interbreed in a ring around the arctic (1 : Larus argentatus argentatus, 2: Larus fuscus sensu stricto, 3 : Larus fuscus heuglini, 4 : Larus argentatus birulai, 5 : Larus argentatus vegae, 6 : Larus argentatus smithsonianus, 7 : Larus argentatus argenteus) A Herring Gull, Larus argentatus (front) and a Lesser Black-backed Gull. Larus fuscus (behind) in Norway: two phenotypes with clear differences. A classic example of ring species is the Larus gulls' circumpolar species "ring". The range of these gulls forms a ring around the North Pole, which is not normally transited by individual gulls. The Herring Gull L. argentatus, 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 recent genetic study entitled The herring gull complex is not a ring species has shown that this example is far more complicated than presented here (Liebers et al., 2004)[3]: this example only speaks to the complex of species from the classical Herring Gull through Lesser Black-backed Gull.[improper synthesis?] There are several other taxonomically unclear examples which belong in the same superspecies complex, such as Yellow-legged Gull L. michahellis, Glaucous Gull L. hyperboreus and Caspian Gull Larus cachinnans. Other examples There are other examples. The Ensatina salamanders form a ring round the Central Valley in California.[4] The Greenish Warbler (Phylloscopus trochiloides) forms a species ring, around the Himalayas (Alström 2006). See also Cline (biology) Speciation Parapatric speciation Dialect continuum Cryptic species Species concept Species group Species problem References ^ Dawkins, The Ancestor's Tale, 2004:303 ^ Brown, Rob. ""Same Species" vs. "Interfertile": concise wording can avoid confusion when discussing evolution"".  ^ 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 ^ This species ring forms the subject of "The Salamander's tale" in Richard Dawkins' The Ancestor's Tale, 2004. Alström, Per (2006). "Species concepts and their application: insights from the genera Seicercus and Phylloscopus" (PDF). Acta Zoologica Sinica 52 (Suppl): 429–434.  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 0878931880.  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" External links Greenish Warbler Greenish Warbler maps and songs Ensatina Salamander [1] Ring species -- the abridged version, by Peter Hadfield alias potholer54 v · d · eSpeciation Basic concepts Species · Cline · Chronospecies · Speciation Modes of speciation Allopatric · Heteropatric · Peripatric · Parapatric · Sympatric · Polyploidy · Paleopolyploidy Auxiliary mechanisms Sexual selection · Assortative mating · Punctuated equilibrium Intermediate stages Hybrid · Species complex · Ring species · Haldane's rule