Everything about Ring Species totally explained
In
biology, a
ring species is a connected series of neighboring populations that can interbreed with relatively closely related populations, but for which there exist at least two "end" populations in the series that are too distantly related to interbreed. Often such non-breeding-though-genetically-connected populations co-exist in the same region thus creating a "ring". Ring species provide important evidence of
evolution 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.
Ring species also present an interesting problem for those who seek to divide the living world into discrete
species, as well as for those who believe that evolution doesn't create new species. 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 becomes two distinct species.
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
cline (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 a lake), 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
genetic 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 isn't as clear-cut as it's often understood to be.
Larus gulls
Larus gulls circumpolar species "ring". The range of these gulls forms a ring around the
North Pole. The
Herring Gull, which lives primarily in
Great Britain and
Ireland, can
hybridize with the
American Herring Gull (living in
North America), which can also interbreed with the Vega or
East Siberian Herring Gull, the western subspecies of which,
Birula's Gull, can hybridize with
Heuglin's gull, which in turn can interbreed with the
Siberian Lesser Black-backed Gull (all four of these live across the north of
Siberia). The last is the eastern representative of the
Lesser Black-backed Gulls back in north-western
Europe, including Great Britain. However, the Lesser Black-backed Gulls and Herring Gull are sufficiently different that they don't normally interbreed; thus the group of gulls forms a continuum except in Europe where the two lineages meet. A recent genetic study has shown that this example is far more complicated than presented here (Liebers
et al, 2004). This example only speaks of classical Herring Gull - Lesser Black-Backed Gull complex and doesn't include several other taxonomically unclear examples which belong in the same
superspecies complex, such as
Yellow-Legged Gull,
Glaucous Gull and
Caspian Gull.
Other examples
Other examples include:
Further Information
Get more info on 'Ring Species'.
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