What is a key characteristic of the 'ring species' concept?

A key characteristic of the 'ring species' concept is that it illustrates gradual divergence without a clear boundary. In a ring species, populations of a single species are spread out in a geographical ring, where adjacent populations can interbreed, but populations at the ends of the range do not interbreed due to genetic changes that have occurred over time. This gradual change across a geographical continuum makes it difficult to draw distinct lines between species, as there are populations that are genetically distinct yet connected through others that are similar.

The concept emphasizes the continuity of species and variation in traits through geographical distance rather than portraying species as being distinctly separated by clear boundaries. This highlights the complexity of speciation and challenges traditional views on how species are defined, especially in cases where intermediate forms exist.