Fig 1. Schematic of the simplest “hybrid speciation by genetic incompatibility” scenario.

The simplest model hybrid reproductive isolation evolves in a hybrid swarm (S3 Fig.) via fixation of parental genetic incompatibility pairs in opposite directions. Circles indicate the location of incompatibility pairs on chromosomes; yellow shading indicates regions derived from species 1 while blue shading indicates regions derived from species 2. In the first generation, assuming random mating, 50% of individuals will be F₁ hybrids if both species contribute equally to the hybrid population. In subsequent generations, recombination will break up ancestry blocks and selection will drive the fixation of parental genotypes at incompatibility loci. In some proportion of cases, incompatibility pairs will fix for opposite parental species genotypes.