Cline (biology) - Ring Species

Ring Species

Ring species are a distinct type of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline. Ring species present an interesting problem for those who seek to divide the living world into discrete species, as chain species are closely related to speciation (in this case, parapatric).

In the case of Larus gulls, the habitats of the end populations even overlap, which introduces questions as to what constitutes a species: nowhere along the cline can a line be drawn between the populations, but they are unable to interbreed. However a recent study (Liebers et al., 2004) has provided genetic evidence that this example is far more complicated than presented here, and likely does not constitute a typical ring species.