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Ring Species: Unusual Demonstrations of Speciation

Darren E. Irwin

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Ring species provide a unique glimpse into how some species came to be.

  • A ring of populations encircles an area of unsuitable habitat.
  • At one location in the ring, two distinct forms coexist without interbreeding.
  • Around the rest of the ring, the traits of one species change gradually through intermediate populations into the second species’ traits.

August 2002

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Large-blotched Salamander, an Ensatina subspecies, of California. Source: Chris Brown, USGS.

To understand how evolution has produced the diversity of life, we need to study two fundamental processes:

Speciation usually takes too long to observe in one lifetime.
  • How a single species changes through time.
  • How a single species becomes two or more species.

The first process has been observed and studied extensively in many species, for example in evolving beak sizes of Darwin’s finches1 and in evolving body sizes and developmental rates of guppies.2 The second process, called speciation, is more difficult to observe directly, primarily because it usually takes much longer than a biologist’s lifetime to occur.

Geography and speciation

Speciation can happen when two related species are isolated geographically.

One way to study speciation indirectly is to examine geographical variation, or how the characteristics of organisms differ between different locations. We can then infer from the variation how speciation occurs. During the early 20th century, biologists such as David Starr Jordan3 and Ernst Mayr4 used this approach and noticed that, in most cases, two closely related species do not occur at the same location nor are they distantly separated. Rather, they usually occur in geographically adjacent regions that are separated by a geographical barrier such as a mountain range or a body of water. The biologists concluded from this pattern that:

  • Speciation often begins when a single species becomes geographically separated into two populations. Individuals cannot travel between the populations, preventing the two populations from interbreeding.
  • Because the two populations cannot exchange genes, and because they may be subject to different environmental conditions, they slowly evolve differences.
  • Eventually the two populations become different enough that they do not interbreed even if they come into contact (in other words, they are ‘reproductively isolated’), and are therefore separate species.

These conclusions were based on broad patterns in the distribution and relationships of many species. But determining how speciation occurs in any particular case can be difficult, because we are usually only presented with the outcome of the process (two species) and we often have no record of their common ancestor or the intermediate forms that occurred during speciation.

Ring species

Ring species acquire new traits as they move away from the ancestral home.

Ring species provide unusual and valuable situations in which we can observe two species and the intermediate forms connecting them. In a ring species:

  • A ring of populations encircles an area of unsuitable habitat.
  • At one location in the ring of populations, two distinct forms coexist without interbreeding, and hence are different species.
  • Around the rest of the ring, the traits of one of these species change gradually, through intermediate populations, into the traits of the second species.
Ring species: a ring of populations in which there is only one place where 2 distinct species meet.

A ring species, therefore, is a ring of populations in which there is only one place where two distinct species meet. Ernst Mayr4 called ring species “the perfect demonstration of speciation” because they show a range of intermediate forms between two species. They allow us to use variation in space to infer how changes occurred over time. This approach is especially powerful when we can reconstruct the biogeographical history of a ring species, as has been done in two cases.

Ensatina salamanders

One well-studied ring species consists of salamanders in the Ensatina eschscholtzii group, distributed in mountains along the west coast of North America. In 1949, Robert Stebbins5 described a fascinating pattern of geographical variation in these salamanders:

California salamanders exhibit ring species traits.
  • Two distinct forms of Ensatina salamanders, differing dramatically in color, coexist in southern California and interbreed there only rarely.
  • These two forms are connected by a chain of populations to the north that encircles the Central Valley of California, and through this ring of populations the color patterns of the salamanders change gradually.
DNA analysis supports a common ancestor for these salamanders.

Stebbins thought that this situation arose when an ancestral population of salamanders, in northern California, expanded southward along two fronts, one down the Sierra Nevada mountains, and the other down the coastal mountains. The two groups gradually became different as they moved south. When they met again in southern California, the two expanding fronts were so different that they rarely interbred, and were therefore different species. More recently, a team of researchers led by David Wake6-8 has examined genetic relationships among salamander populations using DNA sequences and other molecular traits, and the genetic evidence has supported Stebbins’ hypothesis. The geographical variation, when combined with the inferred history revealed by the molecular traits, allows us to envision the small steps by which a single ancestral species in the north gave rise through evolutionary divergence to two species in southern California.

Greenish warblers

Greenish warblers, a ring species, are found in parts of Asia and eastern Europe.

Another ring species that has provided valuable insights into speciation consists of the greenish warblers (Phylloscopus trochiloides). These small, insect-eating songbirds breed in the forests of central and northern Asia and eastern Europe. In the center of Asia is a large region of desert, including the Tibetan Plateau and the Taklamakan and Gobi Deserts, where the warblers cannot live. Instead, they inhabit a ring of mountains surrounding this region, as well as the forests of Siberia to the north. The warblers have remarkable geographic variation:9-11

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Greenish warblers are found in parts of Asia and eastern Europe. Speciation can happen when related species are isolated geographically. Source: Wikimedia Commons.

  • In Siberia, two distinct forms of greenish warblers coexist, one in the west and one in the east, their distributions narrowly overlapping in central Siberia, where they do not interbreed. These forms differ in color patterns, the songs that males sing to attract mates, and genetic characteristics. Also, males of each form usually do not recognize the song of the other form, but respond strongly to their own.
  • The traits that differ between the two Siberian forms change gradually through the chain of populations encircling the Tibetan Plateau to the south.
  • Thus two distinct species are connected by gradual variation in morphological, behavioral, and genetic traits.
DNA evidence points to an ancestor somewhere in the Himalayas.

Claude Ticehurst,9 who during the 1930s studied variation in museum specimens of greenish warblers, hypothesized that the present pattern of variation arose when an ancestral species in the south, perhaps in the Himalayas, expanded northward along two pathways, one on the west side of Tibet and the other on the east. The two expanding fronts gradually became different, resulting in two distinct Siberian forms. More recently, studies of genetic variation and song variation have strongly supported this view.10-11

The pattern of song variation is particularly interesting:

  • Songs are short and simple in the south, but to the north songs become gradually longer and more complex along both pathways into Siberia.
  • However, songs have also become different in structure, resulting in distinct differences in songs between the Siberian forms.
Song patterns changed as new species emerged.

The birds distinguish between these differences; males respond aggressively to tape recordings of their own songs, thinking that another male has invaded their territory, but they do not respond to songs of the other form. In most species of songbirds, songs play an important role in mate choice; usually, only males sing, and females listen to songs when deciding which male to choose as a mate.12 Speciation is essentially the evolution of reproductive isolation between two populations, and song differences can cause reproductive isolation. Hence, the geographical variation in songs of greenish warblers provides a rare illustration of how gradual change in a trait can cause speciation.

Demonstrations of evolution

Greenish warblers and Ensatina salamanders illustrate three fundamental ways that ring species can teach us about evolution:

Ring species provide strong evidence for evolution.
  • Ring species provide strong evidence for evolution causing the appearance of new species, demonstrating that many small changes can eventually accumulate into large differences between distinct species. Some critics of evolutionary theory think that evolution can only cause limited change within a species and cannot lead to the evolution of new species. Ring species show that they are wrong; variation between species is qualitatively similar, though different in degree, to variation within a species.
Complete geographical isolation is not necessary to produce new species.
  • Ring species allow a reconstruction of the history and causes of divergence during speciation, since spatial variation may illustrate change through time. Without the rings of populations connecting the terminal forms, we would have little understanding of the history of divergence of greenish warbler songs or Ensatina color patterns.

  • Ring species provide evidence that speciation can occur without complete geographic isolation. As discussed at the beginning of this article, the prevailing view of speciation has been that two populations must become geographically isolated, such that they do not exchange genes, before speciation can occur (this process is called ‘allopatric speciation’). Ring species, however, show that the ends of a long chain of interbreeding populations can diverge to the point that they do not directly interbreed, even though genes can travel between them through the intermediate populations (in other words, they are connected by ‘gene flow’). This aspect of ring species has been rather controversial, and critics have argued that some apparent examples of ring species, such as Ensatina, have breaks in gene flow.13

Rarity of ring species

There are few clear examples of ring species.

Since we can learn so much from ring species, it is unfortunate that few examples are known. At least 23 cases have been proposed, but most of them are not such clear examples as the salamanders and warblers.14 Most of the proposed cases have major gaps in distribution in the chain of populations connecting the terminal forms, and some cases appear to have more than one species boundary in the ring of populations. However, most of the cases have one thing in common: in one place, there are clearly two species, while in another area the boundary between species is difficult to determine.

Ring species are rare for several reasons:14,15

One reason ring species are rare is because they require an unbroken ring of suitable habitat.
  • Their formation requires unusual geographic situations, in which a species can expand around a geographic barrier through a continuous ring of suitable habitat. The range expansion must occur slowly enough that the two expanding fronts have time to diverge before they meet on the other side of the barrier, and the size of the barrier must be large compared to the distance that individuals disperse.

  • The taxonomic rules that are used by biologists to classify organisms create a bias against recognizing ring species. Under these rules, a ring species must be classified either as a single species or as two species. Both classification schemes conceal the fact that there is gradual variation between reproductively isolated forms.

  • Ring species might be rare because many of them were destroyed before they could be discovered, as will be described below.

Importance of conservation

Most conservation is aimed at species rather than within-species diversity.

While ring species teach us about evolution, they also provide lessons about the importance of habitat conservation. They demonstrate that species can differ substantially between different parts of their ranges. Ensatina salamanders and greenish warblers are each usually classified as a single species, even though each contains populations that differ at the between-species level. As the commonly-used phrase ‘endangered species’ reveals, most conservation efforts are directed at the species level, and species are often eliminated from much of their range before they receive legal protection. However, within-species variation is an important component of biodiversity. Whenever a species is eliminated from part of its range, unique traits that were only found in that area are lost forever. Not only are the species and its traits lost from that area, but the particular story of evolution that could be learned from it is lost as well.

Conclusion: If we continue to destroy habitat, we may not be able to discover and study new ring species.

It is not a coincidence that the best two examples of ring species are both found in relatively undisturbed mountainous habitat. Ring species might have existed at one time in habitats such as the grasslands of central North America, but those would now be destroyed or severely altered because of the impact of agriculture. Large areas of greenish warbler habitat are being deforested, particularly in China, India, and Nepal, and this process might continue to the point that future biologists would not be able to recognize greenish warblers as a ring species. Undoubtedly, many undiscovered ring species are being destroyed now because of human activity, and many more will be lost in the future if we fail to protect their habitats.

Darren E. Irwin, Ph.D., conducted his doctoral research on the greenish warbler ring species while a graduate student at the University of California, San Diego. He is continuing his research on speciation in warblers at the Department of Animal Ecology of Lund University, Sweden, where he is funded by an International Research Fellowship from the National Science Foundation.
http://ucsdnews.ucsd.edu/newsrel/science/mcwarbler.htm

Ring Species: Unusual Demonstrations of Speciation

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Understanding Evolution

Your one-stop source for information on evolution. Learn the facts in Evolution 101, browse the resource library, read about evolution in the news, or discover a wealth of materials to help educate others about evolution and related concepts—it’s all right here! http://evolution.berkeley.edu

Salamander ring species

More information regarding the Ensatina ring species, along with illustrations of the various forms and a map showing their distributions in California.
http://www.pbs.org/wgbh/evolution/library/05/2/l05205.html

Greenish warbler ring species

An article from the San Francisco Chronicle that discusses greenish warblers, along with a full-color map showing its distribution in Asia.
http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2001/03/26/MN172778.DTL

“Speciation and Biodiversity”

Edward O. Wilson examines species’ needs in order to survive and to form new species.
http://www.actionbioscience.org/biodiversity/wilson.html

“Species, Speciation and the Environment”

Niles Eldredge looks at environment’s role in speciation and extinction patterns.
http://www.actionbioscience.org/evolution/eldredge.html

BioScience Articles

  • » “Darwin’s Finches: Multiply and Subtract.” Irwin mentions Darwin’s Finches as an example of a single species that evolves over time. Read Robert Zink’s review of the book, “How and Why Species Multiply: The Radiation of Darwin’s Finches,” in the January 2009 issue of BioScience, and learn more about speciation, as well as specifics about this important bird species. Free to read.
    http://caliber.ucpress.net/doi/full/10.1525/bio.2009.59.1.13
  • » “Beaks, Adaptation, and Vocal Evolution in Darwin’s Finches.
    Darwin’s finches are well known for their remarkable diversity in beak form and function. Field studies have shown that beaks evolve by natural selection in response to variation in local ecological conditions. Podos and Nowicki posit a new hypothesis: as a consequence of beak evolution, there have been changes in the structure of finch vocal signals. Read the abstract of this June 2004, BioScience article, or log in to purchase the full article.
    http://caliber.ucpress.net/doi/abs/10.1641/0006-3568%282004%29054%5B0501%3ABAAVEI%5D2.0.CO%3B2

Read a book

To learn more about speciation, a fascinating, very understandable, and very enjoyable read is Frogs, Flies, and Dandelions. Speciation — The Evolution of New Species by Menno Schilthuizen. This book gives an excellent introduction to the major theories regarding speciation and summarizes many recent findings.

Wildlands Project

This group works to protect and restore natural habitats in North America. They emphasize the importance of connecting existing nature reserves into networks of wilderness, which is essential for the preservation of geographically variable species.
http://www.twp.org/

The Nature Conservancy

This organization has protected millions of acres of habitat around the world, but much more needs to be protected. Support of this organization will help it protect more natural communities.
http://nature.org/

ActionBioscience.org original lesson

This lesson has been written by a science educator to specifically accompany the above article. It includes article content and extension questions, as well as activity handouts for different grade levels.

Lesson Title: Evolution in a Ring
Levels: high school - undergraduate
Summary: This lesson engages students in critical thinking about ring species and how the process of speciation occurs. Students work on distribution maps, write news stories, consider “What if?” scenarios…and more!

Download/view lesson. (To open the lesson’s PDF file, you need Adobe Acrobat Reader free software.)

Useful links for educators

Useful links for student research

In addition to the links in the “learn more” section above:

  1. Grant, P. R., and B. R. Grant. 2002. “Unpredictable evolution in a 30-year study of Darwin’s finches.” Science 296: 707-711.
  2. Reznick, D. N., F. H. Shaw, F. H. Rodd, and R. G. Shaw. 1997. “Evaluation of the rate of evolution in natural populations of guppies (Poecilia reticulata).” Science 275: 1934-1937.
  3. Jordan, D. S. 1905. “The origin of species through isolation.” Science 22: 545-562.
  4. Mayr, E. 1942. Systematics and the Origin of Species. Dover Publications, New York.
  5. Stebbins, R. C. 1949. “Speciation in salamanders of the plethodontid genus Ensatina.” University of California Publications in Zoology 48: 377-526.
  6. Wake, D. B., and K. P. Yanev. 1986. “Geographic variation in allozymes in a ‘ring species,’ the plethodontid salamander Ensatina eschscholtzii of western North America.” Evolution 40: 702-715.
  7. Moritz, C., C. J. Schneider, and D. B. Wake. 1992. “Evolutionary relationships within the Ensatina eschscholtzii complex confirm the ring species interpretation.” Systematic Biology 41: 273-291.
  8. Wake, D. B., and C. J. Schneider. 1998. “Taxonomy of the plethodontid salamander genus Ensatina.” Herpetologica 54: 279-298.
  9. Ticehurst, C. B. 1938. A Systematic Review of the Genus Phylloscopus. Trustees of the British Museum, London.
  10. Irwin, D. E. 2000. “Song variation in an avian ring species.” Evolution 54: 998-1010.
  11. Irwin, D. E., S. Bensch, and T. D. Price. 2001. “Speciation in a ring.” Nature 409: 333-337.
  12. Catchpole, C. K., and P. J. B. Slater. 1995. Bird Song: Biological Themes and Variations. Cambridge University Press, Cambridge.
  13. Highton, R. 1998. “Is Ensatina eschscholtzii a ring-species?” Herpetologica 54: 254
  14. Irwin, D. E., J. H. Irwin, and T. D. Price. 2001. “Ring species as bridges between microevolution and speciation.” Genetica 112-113:223-243.
  15. Wake, D. B. 2001. “Speciation in the round.” Nature 409: 299-300.

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