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UBC Theses and Dissertations

Reproductive isolation among Sphyrapicus sapsuckers Natola, Elizabeth


The question “How and why do species split?” is the central motivation for this dissertation. Evolutionary biologists have long attributed this speciation process to barriers that interrupt gene flow, such as biological differences between populations that constrain gene exchange and cause isolation among formerly reproducing groups. However, more work is needed to answer what barriers are of key importance. Red-breasted, red-naped, and yellow-bellied sapsuckers are an excellent study system to understand the barriers that cause or maintain reduced gene flow because they are in the early stages of speciation. By studying both population dynamics in hybrid zones where different sapsuckers interbreed and genomic differentiation between species, I describe some of the barriers contributing to sapsucker isolation. In Chapter 2, I ask how the environment affects hybridization rates among sapsuckers. I use genomic data to demonstrate that red-breasted and red-naped sapsuckers hybridize to different extents in different environments, suggesting a strong effect of the environment on hybrid zone dynamics. Chapter 3 describes hybrid zone dynamics in an emergent tri-species hybrid zone. I show that all three species hybridize in one region in central British Columbia. However, they do not collapse into a hybrid swarm, so we propose this isolation is likely due to reduced fitness of hybrids. For Chapter 4 I examine how the level of differentiation between species varies across the genome and find that the Z chromosome shows much higher relative differentiation between species than the rest of the genome and harbors signatures of recurrent selection, high linkage disequilibrium, and likely results in epistasis among Z haplotypes. In Chapter 5, I ask what the genomic basis for sapsucker plumage is, as there are major changes in phenotype between sapsuckers, including a shift from sexually monochromatic to dichromatic forms, which likely affect sapsucker mate choice and mating success. Genome-wide association tests indicate plumage is highly associated with the different Z haplotypes identified in Chapter 4. In summary, within this dissertation I present evidence that post-zygotic isolation likely reduces hybrid fitness via both extrinsic (environmental) and intrinsic (genetic incompatibility) means.

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