RING SPECIES (conclusions)
Ring species may be more common than realized but have been eliminated through taxonomic practice and by application of excessively strict criteria (Monahan et al., 2012). Alternatively, ring species may be rare because they require a specific biogeographic backdrop combined with a well-matched ecology (dispersal relative to geographic range),and because climatic shifts may cause them to exist only for a short period of time. Theoretical models suggest that ring species may be short lived, because even in the absence of shifting ranges and allopatry they are expected to fall apart into a complex of reproductively isolated units (Gavrilets, 2004). Whatever the case, ring species are important, not because they represent a mode of speciation, but because they are a premier illustration of the gradual (and usually messy;Fig.3B) process of species formation. Ensatina salamanders, for example, exhibit geographic variation among populations, isolation by distance, divergent phenotypes, and secondary contacts characterized by admixture, tension zone dynamics, occasional hybridization, and complete reproductive isolation. In Ensatina, the ring species scenario has been upheld in that Ensatina evolved in the north and dispersed southward down two distributional axes; and (ii) at the southern terminus between these distribution arms, reproductive isolation is nearly complete, whereas elsewhere reproductive isolation is less advanced.
The Greenish Warbler fulfills similar criteria in that it is a nearly complete ring characterized by a high level of reproductive isolation at one point in the ring. While Mayr considered ring species the ‘‘perfect demonstration of speciation’’ under the geographic speciation paradigm (Mayr,1942, 1963), we have argued here that ring species are preeminent examples of Darwinian species formation. They exemplify how the same evolutionary processes that create patterns of geographic variation gradually accumulate in evolutionary lineages. By contrast, geographic speciation is focused on the necessity of geographic isolation for lineage differentiation (Mayr, 1942).
The ideal ring species is an unbroken chain of populations that evolved from a single ancestral population. There should be intergradation throughout, save for complete reproductive isolation at the point of terminal overlap, and the components of an ideal ring species should never have experienced even a single period of allopatry. Like all models, the ideal ring species is too simplified to describe any species complex accurately, and no ideal ring species are known. However, we side with Mayr in advocating for a focus on fundamental evolutionary processes—the interaction between biogeographic history and the evolution of reproductive isolating mechanisms—over a focus on strict criteria. Ensatina and the Greenish Warbler have both been characterized by periods of allopatry, have geographic breaks in their rings, and exhibit low levels of hybridization among terminal forms, yet they remain ring complexes. There has been much pressure in recent years to revamp the taxonomy of Ensatina. If this were done, we argue that Ensatina would remain a ring complex, because the biogeographic scenario and the patterns of reproductive isolation would remain intact.
The requirement that ring species are necessarily linked to taxonomy is a throwback to the BSC (re: Biological Species Concept), with reproductive isolation as a necessary property, as opposed to a contingent property, of species.
Complete text of publication
(See the Greenish Warbler)