Rapid, complete reproductive isolation in two closely-related Zosterops White-eye bird species despite broadly overlapping ranges

Examining what happens when two closely-related species come into secondary contact provides insight into the later stages of the speciation process. The Zosteropidae family of birds is one of the most rapidly speciating vertebrate lineages. Members of this family are highly vagile and geographically widespread, raising the question of how divergence can occur if populations can easily come into secondary contact. On the small island of Kolombangara, two closely-related non-sister species of White-eyes, Zosterops kulambangrae and Z. murphyi, are distributed along an elevational gradient and come into secondary contact at mid-elevations. We captured 134 individuals of both species along two elevational transects. Using genotyping-by-sequencing data and a mitochondrial marker, we found no evidence of past hybridization events and strong persistence of species boundaries, even though the species have only been diverging for approximately two million years. We explore potential reproductive...