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. Ring Species (gene flow around a barrier).png
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. Ring species diagram.svg
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.

Contents

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". [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 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. [5] 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. [6] 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; [7] [8] but again, some authors such as Jerry Coyne consider this classification incorrect. [4] Finally in 2012, the first example of a ring species in plants was found in a 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 Ensatina eschscholtzii ring species.jpg
Ensatina salamanders example of ring species
Speculated evolution and spread of the greenish warbler, Phylloscopus trochiloides. .mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{} 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 Greenish warbler ring.svg
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

Related Research Articles

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species. He also identified sexual selection as a likely mechanism, but found it problematic.

<span class="mw-page-title-main">Subspecies</span> Taxonomic rank subordinate to species

In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species have subspecies, but for those that do there must be at least two. Subspecies is abbreviated as subsp. or ssp. and the singular and plural forms are the same.

<span class="mw-page-title-main">Gull</span> Seabirds of the family Laridae in the suborder Lari

Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and distantly related to auks, and even more distantly to waders. Until the 21st century, most gulls were placed in the genus Larus, but that arrangement is now considered polyphyletic, leading to the resurrection of several genera. An older name for gulls is mews, which is cognate with German Möwe, Danish måge, Swedish mås, Dutch meeuw, Norwegian måke/måse, and French mouette, and can still be found in certain regional dialects.

<i>Larus</i> Genus of birds

Larus is a large genus of gulls with worldwide distribution.

<span class="mw-page-title-main">Greenish warbler</span> Species of bird

The greenish warbler is a widespread leaf warbler with a breeding range in northeastern Europe, and temperate to subtropical continental Asia. This warbler is strongly migratory and winters in India. It is not uncommon as a spring or early autumn vagrant in Western Europe and is annually seen in Great Britain. In Central Europe large numbers of vagrant birds are encountered in some years; some of these may stay to breed, as a handful of pairs does each year in Germany.

<span class="mw-page-title-main">European herring gull</span> Species of bird

The European herring gull is a large gull, up to 66 cm (26 in) long. It breeds throughout the northern and western coasts of Europe. Some European herring gulls, especially those resident in colder areas, migrate further south in winter, but many are permanent residents, such as in Ireland, Britain, Iceland, or on the North Sea shores. They have a varied diet, including fish, crustaceans, as well as some plants, and are also scavengers, consuming carrion and food left by or stolen from humans.

<span class="mw-page-title-main">Lesser black-backed gull</span> Species of bird

The lesser black-backed gull is a large gull that breeds on the Atlantic coasts of Europe. It is migratory, wintering from the British Isles south to West Africa. However, it has increased dramatically in North America, especially along the east coast. Formerly just a winter visitor to North America, it has increased and occurs in large numbers some winters and birds are now recorded year-round. However, there is serious concern about decline in many parts of its range. The species is on the UK Amber List because the UK is home to 40 per cent of the European population and more than half of these are found at fewer than ten breeding sites.

Allopatric speciation – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow.

<span class="mw-page-title-main">Sympatric speciation</span> Evolution of a new species from an ancestor in the same location

In evolutionary biology, sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap so that they occur together at least in some places. If these organisms are closely related, such a distribution may be the result of sympatric speciation. Etymologically, sympatry is derived from Greek συν (sun-) 'together' and πατρίς (patrís) 'fatherland'. The term was coined by Edward Bagnall Poulton in 1904, who explains the derivation.

<span class="mw-page-title-main">Peripatric speciation</span>

Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from exchanging genes, it can often be difficult to distinguish between them., and peripatric speciation may be considered one type or model of allopatric speciation. The primary distinguishing characteristic of peripatric speciation is that one of the populations is much smaller than the other, as opposed to allopatric speciation, in which similarly-sized populations become separated. The terms peripatric and peripatry are often used in biogeography, referring to organisms whose ranges are closely adjacent but do not overlap, being separated where these organisms do not occur—for example on an oceanic island compared to the mainland. Such organisms are usually closely related ; their distribution being the result of peripatric speciation.

<i>Ensatina</i> Species of amphibians

The ensatina is a species complex of plethodontid (lungless) salamanders found in coniferous forests, oak woodland and chaparral from British Columbia, through Washington, Oregon, across California, all the way down to Baja California in Mexico. The genus Ensatina originated approximately 21.5 million years ago. It is usually considered as monospecific, being represented by a single species, Ensatina eschscholtzii, with several subspecies forming a ring species.

<span class="mw-page-title-main">Species complex</span> Group of closely related similar organisms

In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each other, further blurring any distinctions. Terms that are sometimes used synonymously but have more precise meanings are cryptic species for two or more species hidden under one species name, sibling species for two species that are each other's closest relative, and species flock for a group of closely related species that live in the same habitat. As informal taxonomic ranks, species group, species aggregate, macrospecies, and superspecies are also in use.

<span class="mw-page-title-main">Parapatric speciation</span> Speciation within a population where subpopulations are reproductively isolated

In parapatric speciation, two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange genes. This mode of speciation has three distinguishing characteristics: 1) mating occurs non-randomly, 2) gene flow occurs unequally, and 3) populations exist in either continuous or discontinuous geographic ranges. This distribution pattern may be the result of unequal dispersal, incomplete geographical barriers, or divergent expressions of behavior, among other things. Parapatric speciation predicts that hybrid zones will often exist at the junction between the two populations.

In biology, a cline is a measurable gradient in a single characteristic of a species across its geographical range. Clines usually have a genetic, or phenotypic character. They can show either smooth, continuous gradation in a character, or more abrupt changes in the trait from one geographic region to the next.

<span class="mw-page-title-main">Vega gull</span> Species of bird

The Vega gull, East Siberian gull, or East Siberian herring gull is a large gull of the herring gull/lesser black-backed gull complex which breeds in Northeast Asia. Its classification is still controversial and uncertain. It is variously treated as a separate species, as a subspecies of the American herring gull or included with both the American herring gull and European herring gull in L. argentatus. The Mongolian gull, Larus mongolicus, was formerly regarded as a subspecies.

<span class="mw-page-title-main">Hybrid speciation</span> Form of speciation involving hybridization between two different species

Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was thought to be particularly difficult to achieve, and thus hybrid species were thought to be very rare. With DNA analysis becoming more accessible in the 1990s, hybrid speciation has been shown to be a somewhat common phenomenon, particularly in plants. In botanical nomenclature, a hybrid species is also called a nothospecies. Hybrid species are by their nature polyphyletic.

<span class="mw-page-title-main">Species</span> Basic unit of taxonomic classification, below genus

A species is a population of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour, or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. About 14% of these had been described by 2011. All species are given a two-part name, a "binomial". The first part of a binomial is the genus to which the species belongs. The second part is called the specific name or the specific epithet. For example, Boa constrictor is one of the species of the genus Boa, with constrictor being the species' epithet.

Many types of polymorphism can be seen in the insect order Lepidoptera. Polymorphism is the appearance of forms or "morphs" differing in color and number of attributes within a single species. In Lepidoptera, polymorphism can be seen not only between individuals in a population but also between the sexes as sexual dimorphism, between geographically separated populations in geographical polymorphism and also between generations flying at different seasons of the year. It also includes the phenomenon of mimicry when mimetic morphs fly alongside non-mimetic morphs in a population of a particular species. Polymorphism occurs both at a specific level with heritable variation in the overall morphological design of individuals as well as in certain specific morphological or physiological traits within a species.

<span class="mw-page-title-main">Ecological speciation</span>

Ecological speciation is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species. Ecological factors can include changes in the environmental conditions in which a species experiences, such as behavioral changes involving predation, predator avoidance, pollinator attraction, and foraging; as well as changes in mate choice due to sexual selection or communication systems. Ecologically-driven reproductive isolation under divergent natural selection leads to the formation of new species. This has been documented in many cases in nature and has been a major focus of research on speciation for the past few decades.

<span class="mw-page-title-main">Nonecological speciation</span>

When speciation is not driven by divergent natural selection, it can be said to be nonecological, so as to distinguish it from the typical definition of ecological speciation: "It is useful to consider ecological speciation as its own form of species formation because it focuses on an explicit mechanism of speciation: namely divergent natural selection. There are numerous ways other than via divergent natural selection in which populations might become genetically differentiated and reproductively isolated." It is likely that many instances of nonecological speciation are allopatric, especially when the organisms in question are poor dispersers, however sympatric nonecological speciation may also be possible, especially when accompanied by an "instant" loss of reproductive compatibility, as when polyploidization happens. Other potential mechanisms for nonecological speciation include mutation-order speciation and changes in chirality in gastropods.

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Irwin, Darren E.; Irwin, Jessica H.; Price, Trevor D. (2001). "Ring species as bridges between microevolution and speciation". Genetica. 112/113: 223–243. doi:10.1023/A:1013319217703. PMID   11838767. S2CID   7108000.
  2. ↑ Dawkins, Richard (2004). The Ancestor's Tale. Houghton Mifflin. p. 303. ISBN   0-618-00583-8.
  3. ↑ Brown, Rob. "'Same Species' vs. 'Interfertile: concise wording can avoid confusion when discussing evolution".
  4. 1 2 3 4 Coyne, Jerry A.; Orr, H. Allen (2004). Speciation. Sinauer Associates. pp. 102–105. ISBN   0-87893-091-4.
  5. ↑ Liebers, D.; De Knijff, P.; Helbig, A. J. (2004). "The herring gull complex is not a ring species". Proceedings of the Royal Society B: Biological Sciences. 271 (1542): 893–901. doi:10.1098/rspb.2004.2679. PMC   1691675 . PMID   15255043.
  6. ↑ Irwin, Darren. "The greenish warbler ring species".
  7. ↑ "A closer look at a classic ring species: The work of Tom Devitt". Understanding Evolution. 29 April 2021.
  8. ↑ This species ring forms the subject of "The Salamander's tale" in Richard Dawkins' The Ancestor's Tale , 2004.
  9. 1 2 Cacho, N. Ivalú; Baum, David A. (7 September 2012). "The Caribbean slipper spurge Euphorbia tithymaloides: the first example of a ring species in plants". Proceedings of the Royal Society B: Biological Sciences. 279 (1742): 3377–3383. doi:10.1098/rspb.2012.0498. PMC   3396892 . PMID   22696529.
  10. 1 2 3 4 5 6 Pereira, Ricardo J.; Wake, David B. (November 2015). "Ring species as demonstrations of the continuum of species formation". Molecular Ecology. 24 (21): 5312–5314. Bibcode:2015MolEc..24.5312P. doi: 10.1111/mec.13412 . PMID   26509692. S2CID   206182763.
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  22. ↑ Moritz, C.; Schneider, C. J.; Wake, D. B. (1 September 1992). "Evolutionary Relationships Within the Ensatina Eschscholtzii Complex Confirm the Ring Species Interpretation". Systematic Biology. 41 (3): 273–291. doi:10.1093/sysbio/41.3.273. S2CID   49267121.
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  28. ↑ Liebers, Dorit; de Knijff, Peter; Helbig, Andreas J. (7 May 2004). "The herring gull complex is not a ring species". Proceedings of the Royal Society of London. Series B: Biological Sciences. 271 (1542): 893–901. doi:10.1098/rspb.2004.2679. PMC   1691675 . PMID   15255043.
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  37. ↑ Patten, Michael A.; Pruett, Christin L. (March 2009). "The Song Sparrow, Melospiza melodia, as a ring species: Patterns of geographic variation, a revision of subspecies, and implications for speciation". Systematics and Biodiversity. 7 (1): 33–62. Bibcode:2009SyBio...7...33P. doi:10.1017/S1477200008002867. S2CID   154943.
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