Data from: Genomic divergence in a ring species complex

Ring species provide particularly clear demonstrations of how one species
can gradually evolve into two, but are rare in nature. 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. 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. 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 genes in the novel genetic background. Levels of reproductive
isolation and genetic introgression are consistent with levels of
phenotypic divergence around the ring, with phenotypic similarity and
extensive interbreeding across the southwestern contact zone and strong
phenotypic divergence and nearly complete reproductive isolation across
the northern contact zone. These results cast doubt on the hypothesis that
the greenish warbler should be viewed as a rare example of speciation by
distance, but demonstrate that the greenish warbler displays a continuum
from slightly divergent neighbouring populations to almost fully
reproductively isolated species.