Figure 1. Sample distribution for the Northern (yellow), Central (purple) and Southern (teal) transects. Dots indicate sampling localities for birds phenotypically identified as red-breasted (red), red-naped (blue) and hybrid (violet) sapsuckers.
Hybrid zones reveal the strength of reproductive isolation between populations undergoing speciation and are thus a key tool used in evolutionary biology research. Multiple replicate transects across the same hybrid zone offer further insight into the dynamics of hybridization in different environments, clarifying the role of extrinsic forces on the speciation process. Red-breasted and Red-naped Sapsuckers (Sphyrapicus ruber and S. nuchalis) have a long zone of contact over approximately 1,600 km from central British Columbia, Canada to central California, USA. We compared Genotyping-by-Sequencing data from three independent sapsucker hybrid zone transects to compare hybridization dynamics between the same species under variable geoclimatic conditions. We then generated geographic clines of the genomic data to compare hybrid zone widths and used Random Forest models and linear regression to assess the relationship between climate and sapsucker ancestry along each transect. Our results show variation in symmetry and directionality of back crossing, patterns often indicative of moving hybrid zones. We note variable cline widths among transects, indicating differences in the selection maintaining hybrid zone dynamics. Furthermore, Random Forest models identified different variables in close association with sapsucker ancestry across each transect. These results indicate a lack of repeatability across replicate transects and a strong influence of the local environment on hybrid zone dynamics.