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A ring species is a species that spit off in to two or more groups and while each group can interbreed with nearby groups, they cannot interbreed with more distant groups. And when two distant lines come together, it forms a ring where the creatures at either end cannot breed.
There are several examples of ring species throughout the world.
A common example of this is called the greenish warbler (found in Asia). In this case they got separated by the Tibetan Plateau and as you go around it, there are different varieties that are found in different areas. And while they all interbreed with neighboring groups, where the two lines meet they are different enough that they can’t interbreed.
Alleged Problem for Definition of Kind
Evolutionists have made a big deal about how can they be the same kind since they can't interbreed. But this comes from a misunderstanding of the definition of the word "Kind."
A kind is a group of animals that share a common ancestry. And while the presence of interbreeding indicates that two or more organisms are of the same kind, the absence of interbreeding does not mean that the organisms are of different kinds.
Evolutionists will often assume that all organisms share a common ancestor, therefore concluding that all organisms are really the same kind by the definition given above. This assumption however, is without evidence. It's also an instance of "your theory doesn't work under my theory, so your theory must be wrong."
Problems for Definition of Species
A species is commonly defined as a group of organisms that can interbreed. Ring species presents a problem for this definition because while neighboring populations can interbreed with each other, the populations at each of the ends of the ring cannot, therefore making it impossible to classify the two populations as different species even though they cannot interbreed.
Flood Model Relevance
Ring Species are exactly what one would expect from the post Flood diversification. And they're an excellent example of how the relatively few annuals on Noah's ark could have produced all of the observed diversity of today's land animals.