Narrow hybrid zones in spite of very low population differentiation in neutral markers in an island bird species complex.

Patterns of phenotypic and genic frequencies across hybrid zones provide insight into the origin and evolution of reproductive isolation. The Reunion grey white-eye, Zosterops borbonicus, exhibits parapatrically distributed plumage colour forms across the lowlands of the small volcanic island of Reunion (Mascarene archipelago). These forms meet and hybridize in regions that are natural barriers to dispersal (rivers, lava fields). Here, we investigated the relationship among patterns of differentiation at neutral genetic (microsatellite) markers, phenotypic traits (morphology and plumage colour) and niche characteristics across three independent hybrid zones. Patterns of phenotypic divergence revealed that these hybrid zones are among the narrowest ever documented in birds. However, the levels of phenotypic divergence stand in stark contrast to the lack of clear population neutral genetic structure between forms. The position of the hybrid zones coincides with different natural physical barriers, yet is not associated with steep changes in vegetation and related climatic variables, and major habitat transitions are shifted from these locations by at least 18 km. This suggests that the hybrid zones are stabilized over natural dispersal barriers, independently of environmental boundaries, and are not associated with niche divergence. A striking feature of these hybrid zones is the very low levels of genetic differentiation in neutral markers between forms, suggesting that phenotypic divergence has a narrow genetic basis and may reflect recent divergence at a few linked genes under strong selection, with a possible role for assortative mating in keeping these forms apart.