thumb|300px|In a ring species, gene flow occurs between neighbouring populations of a species, but at the ends of the ring the populations don't interbreed.

thumb|300px|The coloured bars show natural populations (colours), varying along a [[cline (biology)|cline. Such variation may occur in a line (e.g. up a mountain slope) as in A, or may wrap around as in B.

Where the cline bends around, populations next to each other on the cline can interbreed, but at the point at which the beginning meets the end again, as at C, the differences along the cline prevent interbreeding (gap between pink and green). The interbreeding populations are then called a ring species.]]

In biology, a ring species is a connected series of neighbouring populations, each of which interbreeds 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 linked neighbouring populations. Such non-breeding, though genetically connected, end populations may co-exist in the same region (sympatry) thus closing a ring. The German term , meaning "circle of races", is also used.

Ring species represent speciation and have been cited as evidence of evolution. They illustrate what happens over time as populations genetically diverge, specifically 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. The evolutionary biologist Richard Dawkins remarks 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 interfertility (ability to interbreed) is not a transitive relation; if A breeds with B, and B breeds with C, it does not mean that A breeds with C, and therefore does not define an equivalence relation. A ring species is a species with a counterexample to the transitivity of interbreeding. However, it is unclear whether any of the examples of ring species cited by scientists actually permit gene flow from end to end, with many being debated and contested.

History

<gallery class="center" mode="nolines" widths="300px">

File:Ring species seagull.svg|The Larus gulls interbreed in a ring around the arctic. 1:&nbsp;L. fuscus, 2: Siberian population of L. fuscus, 3:&nbsp;L.&nbsp;heuglini, 4: L. vegae birulai, 5: L. vegae, 6: L. smithsonianus, 7: L. argentatus

File:PT05 ubt.jpeg|Herring gull (Larus argentatus) (front) and lesser black-backed gull (Larus fuscus) (behind) in Norway: two phenotypes with clear differences

</gallery>

The classic ring species is the Larus gull. In 1925 Jonathan Dwight found the genus to form a chain of varieties around the Arctic Circle. However, doubts have arisen as to whether this represents an actual ring species. In 1938, Claud Buchanan Ticehurst argued that the greenish warbler had spread from Nepal around the Tibetan Plateau, while adapting to each new environment, meeting again in Siberia where the ends no longer interbreed. These and other discoveries led 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 described the Ensatina salamanders around the Californian Central Valley as a ring species; but again, some authors such as Jerry Coyne consider this classification incorrect.

Speciation

The evolutionary biologist Ernst Mayr championed the concept of ring species, stating that it unequivocally demonstrated the process of speciation. A ring species is an alternative model to allopatric speciation, "illustrating how new species can arise through 'circular overlap', without interruption of gene flow through intervening populations…" However, Jerry Coyne and H. Allen Orr point out that rings species more closely model parapatric speciation. or, "taxonomic zeal" The following examples provide evidence that—despite the limited number of concrete, idealized examples in nature—continuums of species do exist and can be found in biological systems.

  • Acanthiza pusilla and A. ewingii
  • Acacia karroo
  • Alauda skylarks (Alauda arvensis, A. japonica and A. gulgula)
  • Aulostomus (trumpetfish)
  • Camarhynchus psittacula and C. pauper
  • Ensatina salamanders
  • 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.)
  • The greenish warbler (Phylloscopus trochiloides) forms a species ring, around the Himalayas. 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 species complex, such as yellow-legged gull (L. michahellis), glaucous gull (L. hyperboreus), and Caspian gull (L. cachinnans).
  • Pelophylax nigromaculatus and P. porosus/P. porosus brevipodus
  • Phellinus
  • Platycercus elegans (Crimson rosella) complex
  • Drosophila paulistorum
  • Phylloscopus collybita and P. sindianus
  • Powelliphanta
  • Rhymogona silvatica and R. cervina with the subspecies heermanni and fallax meeting in the vicinity of the San Gorgonio Pass.
  • Todiramphus chloris and T. cinnamominus