Genomic divergence in a ring species complex

Two species may be fully reproductively isolated at the point they meet, but be connected by continuous gene flow by a longer route around the back of a geographical barrier; such a ring species complex exists for the greenish warbler, and genomics shows that there have been several historical breaks in gene flow along the continuum, as well as some introgression between the end forms. Ringing the changes in speciation Ring species are rare occurrences of 'speciation by distance', where a series of neighbouring populations in one locale centred on a geographical barrier is connected through interbreeding, though at the same time the ring includes two or more 'end' populations that cannot interbreed. Here, the authors use genome-wide analysis to investigate the greenish warbler ring species complex, a classic ring including non-interbreeding forms linked by a chain of populations encircling the Tibetan plateau. The data confirm that there is indeed a ring, but some of the details diverge from the idealized case: there have been several historical breaks in gene flow along the continuum, as well as some introgression between the end forms. Ring species provide particularly clear demonstrations of how one species can gradually evolve into two, but are rare in nature 1 , 2 , 3 . In the greenish warbler ( Phylloscopus trochiloides ) species complex, a ring of populations wraps around Tibet. Two reproductively isolated forms co-exist in central Siberia, with a gradient of genetic and phenotypic characteristics through the southern chain of populations connecting them 4 , 5 , 6 . Previous genetic evidence has proven inconclusive, however, regarding whether species divergence took place in the face of continuous gene flow and whether hybridization between the terminal forms of the ring ever occurred 7 , 8 , 9 . Here we use genome-wide analyses to show that, although spatial patterns of genetic variation are currently mostly as expected of a ring species, historical breaks in gene flow have existed at more than one location around the ring, and the two Siberian forms have occasionally interbred. Substantial periods of geographical isolation occurred not only in the north but also in the western Himalayas, where there is now an extensive hybrid zone between genetically divergent forms. Limited asymmetric introgression has occurred directly between the Siberian forms, although it has not caused a blending of those forms, suggesting selection against introgressed gene