Signatures of hybridization and speciation in genomic patterns of ancestry.

Genomes sampled from hybrid zones between nascent species provide important clues into the speciation process. With advances in genome sequencing and single nucleotide polymorphism (SNP) genotyping, it is now feasible to measure variation in gene flow with high genomic resolution. This progress motivates the development of conceptual and analytical frameworks for hybrid zones that complement well-established cline approaches. We extend the perspective that genomic distributions of ancestry are sensitive indicators of hybridization history. We use simulations to examine the behavior of the number of ancestry junctions-a simple summary of genomic patterns-in hybrid zones under increasingly realistic scenarios. Neutral simulations revealed that ancestry junction number is shaped by population structure, migration rate, and population size. Modeling multiple genetic architectures of hybrid dysfunction, with an emphasis on epistatic hybrid incompatibilities, showed that selection reduces junction number near loci that confer reproductive barriers. The magnitude of this signature was affected by the form of selection, dominance, and genomic location (autosome vs. sex chromosome) of incompatible loci. Our results suggest that researchers can identify loci involved in reproductive isolation by scanning hybrid genomes for local reductions in junction number. We outline necessary directions for future theory and method development to realize this goal.