From Pleistocene islands to the Mid-Sierra Filter Zone: Phylogeography and hybridization in a Philippine-endemic avian genus ( Sterrhoptilus )

The complex geological and geographic history of the Philippine Archipelago has shaped its terrestrial biodiversity and patterns of endemism. Some explanations for the structuring of Philippine diversity, such as the oceanic origin of the islands or Pleistocene Aggregate Island Complexes (PAICs), are well-supported and accepted predictors for patterns of biodiversity. Other factors, such as the recently documented Mid-Sierra Filter Zone, remain underexplored and in need of additional research. In the present study, we employ restriction site-associated DNA sequencing (RADseq) to reconstruct the evolutionary history of the Philippine-endemic bird genus Sterrhoptilus. We corroborate the paraphyletic relationship of the two black-crowned species (Sterrhoptilus nigrocapitatus and S. affinis) recently recovered by Hosner et al. (2018) and find that Sterrhoptilus broadly follows phylogenetic patterns predicted by PAICs. We also learn more about within-PAIC diversity and uncover numerous hybrid individuals in central Luzon, a well-known population turnover boundary for many taxa. The pattern of backcrossed individuals and unsampled geographic areas give an incomplete picture of the hybrid zone but support the possibility of a broad region of overlap and hybridization between two species in the Mid-Sierra Madre.