In biology, 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 flow 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 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, in which the intermediates have become extinct. Richard Dawkins observes that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension."
Formally, the issue is that interfertile "able to interbreed" is not a transitive relation – 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 relation. A ring species is a species that exhibits a counterexample to transitivity.
Read more about Ring Species: Problem of Definition
Famous quotes containing the words ring and/or species:
“It is engend’red in the eyes,
With gazing fed; and fancy dies
In the cradle where it lies.
Let us all ring fancy’s knell.
I’ll begin it—Ding, dong, bell.”
—William Shakespeare (1564–1616)
“Genius detects through the fly, through the caterpillar, through the grub, through the egg, the constant individual; through countless individuals the fixed species; through many species the genus; through all genera the steadfast type; through all the kingdoms of organized life the eternal unity. Nature is a mutable cloud which is always and never the same.”
—Ralph Waldo Emerson (1803–1882)