Do ring species exist?

Question

In trying to understand evolution better, I have been looking at examples of speciation, and have thus come across the topic of ring species.  I have tried to find concrete examples of how these work, but have been unable to.  This paper deals with one of the frequently quoted examples of ring species, but concludes:

In conclusion, although ring speciation is theoretically
  possible, the few well-studied examples suggest that it
  occurs infrequently, because the dynamics of species’
  ranges are more likely to result in fragmentation, i.e. periods of allopatry, before the slow process of isolation by
  distance leads to sufficient divergence to allow for circular overlap.

The paper does, however, cite an article on a bird that appears to be a strong example of a ring species, but buying access to it is more expensive than a year´s supply of toilet paper.

Is there hard evidence that ring species exist?  And if so, what is the evidence and what does it teach us about the nature of speciation?

Tyler Mc · Accepted Answer

While no perfect example exists, there are various different 'ring species' in nature where you have which species are able to interbreed with closely related populations, but there are least two "end" populations in the series. Examples include:

Alauda arvensis, A. japonica and A. gulgula: Species of Skylark birds that coexist around Central Asia

In central Asia, a northern form,, arvensis, a southern form,gulgula, and Oriental,japonica , coexist without interbreeding.These forms are separated
by a gap in distribution in northern China, but a morphologically intermediate
form, japonica, occurs in Japan. Northern japonica are similar to arvensis and
southern japonica resemble gulgula. -Ring species as bridges between microevolution and speciation

Lalage leucopygialis, L. nigra, and L. sueurii: Species of triller birds that coexist on Sulawesi Island.

The existence of ring species like this can, as biologist Ernst Mayr puts it, illustrate "how new species can arise through 'circular overlap', without interruption of gene flow through intervening populations…" and offers proof of speciation through a method other than allopatric speciation: speciation that happens when two populations of the same species become isolated from each other due to geographic changes.

Do ring species exist?

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