Weak Relationship Between Divergence and Selection in a North American Toad Hybrid Zone

Hybrid zones present an excellent opportunity to study the evolution of reproductive isolation between divergent evolutionary lineages as generations of interbreeding and backcrossing produce many different recombinant genotypes. We can then observe patterns of genetic variation within a hybrid zone to understand how selection is acting on these recombinant genotypes. Using genome-wide sequence data, we characterized patterns of introgression within a putative hybrid zone between two species of North American toads (Anaxyrus americanus and Anaxyrus terrestris) to better understand reproductive isolation between these species. Both model based and non-parametric approaches to population structure inference showed that there is likely a substantial level of interbreeding and successful backcrossing at this hybrid zone with admixed individuals located quite far from the center of the hybrid zone. Bayesian genomic cline analysis revealed loci with extreme patterns of introgression relative to other loci which would be expected if these sites were linked to sites under negative selection in a hybrid genomic background. A site-based measure of genetic divergence was found to be weakly correlated with cline parameter estimates. We argue that this weak correlation is consistent with a history of secondary contact following a period of geographic isolation as many highly divergent sites do not have correspondingly high cline parameter estimates consistent with strong selection against introgression. Our findings substantiate previous claims of the existence of a hybrid zone between A. americanus and A. terrestris and highlight the potential for this hybrid zone to further our understanding of the evolution of reproductive incompatibly.