In 1905, Leonhard
Stejneger suggested that a population might expand
along two pathways around a geographic barrier such as a mountain
range. Individuals might gradually diverge so much along these
pathways that when they re-met on the other side of the barrier
they would act as different species.
In 1954, Arthur
Cain (of Cepaea fame) called such systems 'ring
species'.
Although they are very
rare, we can potentially learn a lot from ring species. The problem
is - as so often happens - in the details. Arguably the best studied
are:
Herring
Gull superspecies around North Pole
Herring
Gulls Larus argentatus and Lesser
Black-backed Gulls L. fuscus in Britain are easy
to tell apart but hybridise occasionally. They form part of
an interbreeding chain around the Arctic. But it appears that
they are not the ends of a ring:
The
surprising conclusion of recent genetic studies is that American
and European Herring Gulls - usually treated merely as geographical
races - are not closely related.
Instead
of being a ring species, 'Herring Gulls' seem to have spread
eastwards from two refugia: the NE Atlantic and the area around
the Aral and Caspian Seas.
These
birds might become a ring species if the Lesser Black-backed
Gull Larus fuscus colonizes N America, where records
have increased dramatically recently.
The
paper explaining the full complexity
Greenish
Warblers around Himalayas
These distant relatives
of Chiffchaffs Phylloscopus collybita form a ring around
the Himalayas.
Where the Greenish
Warbler P. trochiloides and Two-barred Greenish
Warbler P. plumbeitarsus meet they do not interbreed.
As their names suggest, the two forms have one or two wing-bars,
i.e. they are also morphologically distinct.
The main RIM is behavioural:
they do not recognise each other's songs.
More
details
More
from Darren Irwin who studied them
Great
Tits around Himalayas
The Great
Tit Parus major is a familiar garden bird in
Britain. It has long been claimed to form a ring around the
Himalayas with limited interbreeding at opposite ends.
I think that this
hypothesis is now drowning in the face of recent
evidence.
Californian
Salamanders around Central Californian Valley
Ensatina species.
Details
from the Evolution Library.
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to Darwinian Evolution Index
Page last updated by
David on 14 November 2006
