In
biologyBiology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, a
ring species is a connected series of neighboring populations, each of which can interbreed 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 flowIn population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
between each "linked" species. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing a "ring".
Ring species provide important evidence of
evolutionEvolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
in that they illustrate what happens over time as populations genetically diverge, and are special 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.
Richard DawkinsClinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
observes that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension."
Ring species also present an interesting case of the
species problemThe species problem or species concept is a mixture of difficult, related questions that often come up when biologists identify species and when they define the word "species"....
, for those who seek to divide the living world into discrete
speciesIn biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. After all, 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, the ring species' distal populations will be recognized as two distinct species.
Formally, the issue is that
interfertile "able to interbreed" is not a
transitive relationIn mathematics, a binary relation R over a set X is transitive if whenever an element a is related to an element b, and b is in turn related to an element c, then a is also related to c....
– if A can breed with B, and B can breed with C, it does not follow that A can breed with C – and thus does not define an
equivalence relationIn mathematics, an equivalence relation is a relation that, loosely speaking, partitions a set so that every element of the set is a member of one and only one cell of the partition. Two elements of the set are considered equivalent if and only if they are elements of the same cell...
. A ring species is a species that exhibits a counterexample to transitivity.
Explanation of the diagram
The coloured bar to the right shows a number of natural populations, each population represented by a different colour, varying along a
clineIn biology, an ecocline or simply cline describes an ecotone in which a series of biocommunities display continuous gradient...
(a gradual change in conditions which gives rise to slightly different characteristics predominating in the organisms that live along it). Such variation may occur in a straight line (for example, up a mountain slope) as is shown in
A, or may bend right around (for example, around the shores of an ocean), as is shown in
B.
In the case 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 is shown in
C, the differences that have accumulated along the cline are great enough to prevent interbreeding (represented by the gap between pink and green on the diagram). The interbreeding populations in this circular breeding group are then collectively referred to as a ring species.
Problem of definition
The problem, then, is whether to quantify the whole ring as a single species (despite the fact that not all individuals can interbreed) or to classify each population as a distinct species (despite the fact that it can interbreed with its near neighbours). Ring species illustrate that the species concept is not as clear-cut as it is often thought to be.
Larus gulls
A classic example of ring species is the
LarusLarus is a large genus of gulls with worldwide distribution . Many of its species are abundant and well-known birds in their ranges...
gulls' circumpolar species "ring". The range of these gulls forms a ring around the
North PoleThe North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
, which is not normally transited by individual gulls.
The Herring Gull
L. argentatus, which lives primarily in
Great BritainGreat Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
and
IrelandIreland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
, can hybridize with the
American Herring GullThe American Herring Gull or Smithsonian Gull is a large gull which breeds in North America. It is often treated as a subspecies of the European Herring Gull but is now regarded as a separate species by some authorities.Adults are white with gray back and wings, black wingtips with white spots,...
L. smithsonianus, (living in
North AmericaNorth America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
), which can also hybridize with the Vega or East Siberian Herring Gull
L. vegae, the western subspecies of which, Birula's Gull
L. vegae birulai, can hybridize with
Heuglin's gullHeuglin's Gull or Siberian Gull, Larus heuglini, is a seabird in the genus Larus. It is closely related to the Lesser Black-backed Gull, Larus fuscus, and is often classified as a subspecies of it...
L. heuglini, which in turn can hybridize with the Siberian
Lesser Black-backed GullThe 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...
L. fuscus. All four of these live across the north of
SiberiaSiberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
. The last is the eastern representative of the Lesser Black-backed Gulls back in north-western
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, including Great Britain.
The Lesser Black-backed Gulls and Herring Gulls are sufficiently different that they do not normally hybridize; thus the group of gulls forms a continuum except where the two lineages meet in Europe.
However, a recent genetic study entitled
The herring gull complex is not a ring species has shown that this example is far more complicated than presented here (Liebers
et al., 2004): 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 which belong in the same
superspeciesA superspecies is a group of at least two more or less distinctive species with approximately parapatric distributions. Not all species complexes, whether cryptices or ring species are superspecies, and vice versa, but many are...
complex, such as
Yellow-legged GullThe Yellow-legged Gull , sometimes referred to as Western Yellow-legged Gull , is a large gull of Europe, the Middle East and North Africa, which has only recently achieved wide recognition as a distinct species...
L. michahellis,
Glaucous GullThe Glaucous Gull is a large gull which breeds in the Arctic regions of the northern hemisphere and the Atlantic coasts of Europe. It is migratory, wintering from in the North Atlantic and North Pacific oceans as far south as the British Isles and northernmost states of the USA, also on the Great...
L. hyperboreus and
Caspian GullCaspian Gull is a name applied to the gull taxon Larus cachinnans, a member of the Herring Gull/Lesser Black-backed Gull complex.- Description :...
Larus cachinnans.
Other examples
There are other examples. The
EnsatinaEnsatina eschscholtzii is a complex of plethodontid 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.-As a ring species:The...
salamanderSalamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...
s form a ring round the Central Valley in
CaliforniaCalifornia is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. The
Greenish WarblerThe Greenish Warbler and Green Warbler are widespread leaf-warblers throughout their 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...
(
Phylloscopus trochiloides) forms a species ring, around the
HimalayasThe Himalaya Range or Himalaya Mountains Sanskrit: Devanagari: हिमालय, literally "abode of snow"), usually called the Himalayas or Himalaya for short, is a mountain range in Asia, separating the Indian subcontinent from the Tibetan Plateau...
(Alström 2006).
See also
- Cline (biology)
- Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
- Parapatric speciation
Parapatry is a term from biogeography, referring to organisms whose ranges do not significantly overlap but are immediately adjacent to each other; they only occur together in the narrow contact zone, if at all. This geographical distribution is opposed to sympatry & allopatry or peripatry...
- Dialect continuum
A dialect continuum, or dialect area, was defined by Leonard Bloomfield as a range of dialects spoken across some geographical area that differ only slightly between neighboring areas, but as one travels in any direction, these differences accumulate such that speakers from opposite ends of the...
- Cryptic species
- Species group
A species group is an informal taxonomic rank into which an assemblage of closely related species within a genus are grouped because of their morphological similarities and their identity as a biological unit with a single monophyletic origin.-Use:...
- Species problem
The species problem or species concept is a mixture of difficult, related questions that often come up when biologists identify species and when they define the word "species"....
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