Reexamining Phylloscopus trochiloides complex as a ring species: a refugial counter‐hypothesis

Abstract

The idea of ring species has been viewed as a demonstration of the potential for species‐level differentiation produced by isolation by distance in otherwise continuous populations. Although the concept is attractive, the list of empirical examples has narrowed significantly as detailed studies have been carried out. One of the few examples still cited among birds is the warbler species complex Phylloscopus trochiloides, which comprises P. trochiloides sensu stricto, P. plumbeitarsus, and P. nitidus. The ring comprises a series of populations surrounding the Tibetan Plateau in which a species‐level break is apparent at the north end of the ring. Although recent theoretical treatments have indicated that such differentiation is possible in theory, recent genomic analyses indicated multiple differentiated populations around the ring unlikely to have resulted from isolation by distance. Here, we complement the genomic studies to date with paleodistributional projections of ecological niche models that provide further evidence for multiple, isolated refugia around the ring during and since the Late Pleistocene. As such, this group should not be interpreted as exemplifying an isolation‐by‐distance mechanism of speciation, and should no longer be considered as a ring species: genomic and geographic evidence coincide in indicating that its differentiation took place in allopatry.