Abstract A ring species is a monophyletic group whose range has extended around a geographic barrier to produce a ringshaped distribution. The populations that make up the ring should be contiguous and without barriers to gene flow anywhere in the ring except where the terminal populations are sympatric but reproductively isolated from one another. Few, if any of the species that have been described as ring species, actually meet all of these requirements. The best documented example of a species that exhibits all of these traits is the greenish warbler complex ( Phylloscopus trochiloides ). However, even species like the herring gull complex or Ensatina salamanders that fail to exhibit some of the characteristics of a true ring species offer opportunities to study important evolutionary processes. These species are particularly helpful in understanding how microevolutionary changes can create two unique species, how speciation can occur in spite of gene flow, and how geographic speciation with or without adaptive divergence can occur. Key Concepts:Few organisms exhibit all of the requirements to be considered a true ring species.Ring species are helpful tools that can be used to help elucidate evolutionary processes.The preponderance of avian and mammal ring species may be an ascertainment bias.
