The formation of ring species might provide an explanation of how speciation can occur despite ongoing gene flow. However, few species fit all of the criteria of a classic ring species that formed via isolation by distance around a barrier. Population genetic analyses and ecological niche models were used to examine a ring of song sparrow (Melospiza melodia) subspecies that surround the Sierra Nevada in North America. Eight models were compared that included both geography-based and ecology-based scenarios of ring formation. Song sparrows do fit some aspects of a classic ring species that formed via expansion around a barrier; however, admixture rather than complete reproductive isolation occurred when populations met at the terminus of the ring in southern California. Nichemodels show that variation among subspecies is likely to reflect adaptation to local conditions coupled with current limitations to gene flow across ecotones and that birds are likely to have expanded from a refugium in the southwestern United States. Given that simple isolation-based models often fail to explain many ring species patterns, alternative models that incorporate ecological factors might provide a better explanation of how most ring species formed. Isolation and subsequent partitioning of populations by ecotones can be important drivers of geographic variability in ring species.
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Vol. 64 • No. 3