Ring Species and Speciation

Abstract

Abstract A ring species is a monophyletic group whose range has expanded around a geographic barrier producing a ring‐shaped distribution. Populations that make up the ring should be contiguous and without barriers to gene flow except at one location where two reproductively isolated populations co‐occur. Ring species that meet this definition provide an opportunity for studying how speciation occurs through the gradual accrual of differences leading to reproductive isolation. However, few if any of the species that have a ring‐shaped distribution meet these requirements. The most studied species, greenish warbler Phylloscopus trochiloides and Ensatina eschscholtzii salamanders, fail to exhibit all of the characteristics of a strict ring species. However, the study of species with ring distributions has provided information about the processes that cause population divergence through time, and the use of new genomics and modelling tools could provide valuable insights into how geographic speciation, with or without adaptive divergence, could 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 terrestrial and temperate ring species may be an ascertainment bias. New methods in genomics and modelling will provide answers to questions about ring species formation and the processes of speciation. Species from a wide range of taxa, other than birds and mammals, are now being examined as possible ring species.