Ring species

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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.
The coloured bars show natural populations (colours), varying along a 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 that 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 each "linked" population and the next.[1] Such non-breeding, though genetically connected, "end populations" may co-exist in the same region (sympatry) thus closing a "ring". The German term Rassenkreis, meaning a circle of races, is also used.

Ring species represent

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".[2]

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.[3] 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.[4]

History

  • The Larus gulls interbreed in a ring around the arctic. 1: L. fuscus, 2: Siberian population of L. fuscus, 3: L. heuglini, 4: L. vegae birulai, 5: L. vegae, 6: L. smithsonianus, 7: L. argentatus
    The Larus gulls interbreed in a ring around the arctic. 1: 
  • Herring gull (Larus argentatus) (front) and lesser black-backed gull (Larus fuscus) (behind) in Norway: two phenotypes with clear differences
    Herring gull (Larus argentatus) (front) and lesser black-backed gull (Larus fuscus) (behind) in Norway: two
    phenotypes
    with clear differences

The classic ring species is the

spurge, forming a ring around the Caribbean Sea.[9]

Speciation

The biologist Ernst Mayr championed the concept of ring species, stating that it unequivocally demonstrated the process of speciation.[10] 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…"[11] However, Jerry Coyne and H. Allen Orr point out that rings species more closely model parapatric speciation.[4]

Ring species often attract the interests of evolutionary biologists, systematists, and researchers of speciation leading to both thought provoking ideas and confusion concerning their definition.[1] Contemporary scholars recognize that examples in nature have proved rare due to various factors such as limitations in taxonomic delineation[12] or, "taxonomic zeal"[10]—explained by the fact that taxonomists classify organisms into "species", while ring species often cannot fit this definition.[1] Other reasons such as gene flow interruption from "vicariate divergence" and fragmented populations due to climate instability have also been cited.[10]

Ring species also present an interesting case of the

species problem for those seeking to divide the living world into discrete species. 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 then the ring species' distal populations will be recognized as two distinct species. The problem is whether to quantify the whole ring as a single species (despite the fact that not all individuals interbreed) or to classify each population as a distinct species (despite the fact that it interbreeds with its near neighbours). Ring species illustrate that species boundaries arise gradually and often exist on a continuum.[10]

Examples

Ensatina salamanders example of ring species
Speculated evolution and spread of the greenish warbler, Phylloscopus trochiloides.
  P. t. trochiloides
  P. t. obscuratus
  P. t. plumbeitarsus
  P. t. ludlowi
  P. t. viridanus
Note: The P. t. nitidus in Caucasus Mountains not shown

Many examples have been documented in nature. Debate exists concerning much of the research, with some authors citing evidence against their existence entirely.[4][13][self-published source?] 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.[10] This is often characterized by sub-species level classifications such as clines, ecotypes, complexes, and varieties. Many examples have been disputed by researchers, and equally "many of the [proposed] cases have received very little attention from researchers, making it difficult to assess whether they display the characteristics of ideal ring species."[1]

The following list gives examples of ring species found in nature. Some of the examples such as the Larus gull complex, the greenish warbler of Asia, and the Ensatina salamanders of America, have been disputed.[13][14][15][16]

See also

References

External links