Environmental variability and hybridization in Red-breasted and Red-naped Sapsuckers

Background/Question/Methods

Biologists study natural hybrid zones because they allow us to understand the degree of reproductive isolation between species and to study the different barriers to reproduction between two forms, particularly in non-laboratory settings. However, empirical hybrid zones are difficult to replicate or study under varying environmental conditions because they are often geographically limited and singular in occurrence. Replicate transect analysis, wherein multiple hybrid zones among two species are compared across independent transects, is seldom possible but can be a powerful tool in decoupling intrinsic (e.g. genetic incompatibilities) from extrinsic barriers (e.g. environmental factors) and identifying environmental features associated with reproductive barriers. Here, we have compared genomic and climate data from three replicate hybrid zones among Red-breasted and Red-naped Sapsuckers (Sphyrapicus ruber and S. nuchalis).

Results/Conclusions

We provide evidence that Red-breasted and Red-naped Sapsuckers exhibit variable proportions of admixed birds, inconsistent directionality of back-crossing, and varying cline widths among replicate hybrid zones, despite consistent genomic differentiation in allopatry. These data suggest a strong role of the environment in shaping hybrid zone dynamics. Life on Earth currently faces an onslaught of environmental changes and understanding the role the environment plays in hybridization will be key to forming predictions about species evolution and formation into the future.