Ring species can be found where a complex of species, sub-species and closely related members of the same genus exist over a large range and where various local varieties or subspecies have evolved to suit local conditions or to adapt to opportunities in local ecological niches. They are a special form of the phenomenon known to biology as a cline, which is where a species gradually changes across a wide geographical range so that an expert can generally recognise roughly where a specimen was from. We see this in many species, especially insects, birds, plants and some mammals. A ring species is a cline where the ends of the range meet to form a ring.
A few examples should illustrate this but a search on Google will yield several more examples.
This group of salamanders (Ensatina escholtzii) is believed to have evolved as the original species from Oregon and Washington extended its range southward to the San Joaquin Valley in California and into southern California. As the species moved down either side of the valley, separated by the valley floor, they occupied and adapted to the various opportunities and threats the new niches presented. By the time they had reached southern California they had formed two quite distinct forms: one (E. e. klauberi ) having dark blotches in a camouflage pattern; the other (E. e. eschscholtzii) less strongly marked and with yellow eyes which may mimic those of the poisonous western newt. These two sub-species coexist in some areas but do not, and apparently cannot, interbreed.
So, to a southern Californian taxonomist unaware of the situation further north, these two salamanders are different species, filling all the requirements of that classification. Yet, following each species north on either side of San Joaquin Valley we find other varieties with which interbreeding occurs naturally until, at the northern end of the valley we find a single species.
What we have is speciation in progress with evidence of the intermediate forms preserved. All we would need to happen is that the subspecies either side of the valley went extinct and left no geological record and we would have speciation without known transitional forms - something which creation 'scientists' would then wave jubilantly as 'proof' of creation.
Larus Gulls. The Larus gulls form a northern circumpolar complex of gulls from the North Atlantic, particularly Britain to Scandinavia, across the northern coast of Siberia and across the Bering Straits to Canada. At the point where the ends of this ring close, in the British Isles, the two ends behave like two different species - the lesser black-backed gull and the herring gull.
But again, as with the Escalina salamanders, we find a series of subspecies which can and do interbreed with their neighbours. Again we see speciation in progress and probably more to the point, we see what happens when complete speciation does not occur because isolation for sufficient time has not occurred and may not be possible. A dynamic equilibrium is achieved partway towards speciation by frequent 'contamination' of the gene-pool by hybridization.
|Yellow: P. t. trochiloides, Orange: P. t. obscuratus, Red: P. t. plumbeitarsus, Green: P. t. ludlowi, Blue: P. t. viridanus|
Once again we see a species diversifying either side of a geographical barrier, forming several sub-species which can interbreed with their neighbours, until they meet on the far side of the barrier, at which point, even though they co-exist, no interbreeding occurs and they behave exactly like different species.
|Western Green Warbler (P.t.viridianus)|
And this of course is how we can tell that creationism is wrong and evolution is right. The test is simple: you just check to see which describes reality and which would require you to ignore reality in order to cling to a belief in it.
Creation pseudo-scientists would prefer you to adopt the latter approach. I'm sure you can work out why.