Hybridization in headwater regions, and the role of rivers as drivers of speciation in Amazonian birds

This is the peer reviewed version of the following article: Weir, J. T., Faccio, M. S., Pulido‐Santacruz, P., Barrera‐Guzmán, A. O., & Aleixo, A. (2015). Hybridization in headwater regions, and the role of rivers as drivers of speciation in Amazonian birds. Evolution, 69(7), 1823-1834.which has been published in final form at https://doi.org/10.1111/evo.12696. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Many understory birds and other groups form genetically differentiated subspecies or closely related species on opposite sides of major rivers of Amazonia, but are proposed to come into geographic contact in headwater regions where narrower river widths may present less of a dispersal barrier. Whether such forms hybridize in headwater regions is generally unknown, but has important implications to our understanding of the role of rivers as drivers of speciation. We used a dataset of several thousand single nucleotide polymorphisms to show that seven taxon pairs that differentiate across a major Amazonian river come into geographic contact and hybridize in headwater regions. All taxon pairs possessed hybrids with low numbers of loci in which alleles were inherited from both parental species, suggesting they are backcrossed with parentals, and indicating gene flow between parental populations. Ongoing gene flow challenges rivers as the sole cause of in situ speciation, but is compatible with the view that the wide river courses in the heart of Amazonia may have driven interfluvial divergence during episodes of wet forest retraction away from headwater regions. Taxa as old as 4 Ma in our Amazonian dataset continue to hybridize at contact zones, suggesting reproductive isolation evolves at a slow pace.The following funding sources are acknowledged: a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (402013‐2011) and a UTSC VPR‐Research Competitiveness Fund (JTW), a Mitacs Student Fellowship (MF), a FAPESPA (ICAAF 23/2011), and several CNPq grants to AA (‘‘INCT em Biodiversidade e Uso da Terra da Amazônia’’ # 574008/2008‐0; # 471342/ 2011–4; and # 310880/2012‐2)