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.
Fig 2.
Hybrid populations rapidly fix for hybrid incompatibility locus pairs.
Selection drives hybrid incompatibility loci to fixation, even when a hybrid population forms at equal admixture proportions (f = 0.5). Black lines show average parent 1 ancestry at a hybrid incompatibility pair (h = 0.5, s = 0.1) in 50 replicate populations of (A) N = 1,000 or (B) N = 10,000 diploid individuals. Gray lines show results for this same population size with no selection. Because of this behavior, two incompatibility pairs may fix for opposite parents, resulting in reproductive isolation of hybrids from both parents (see Fig. 1).
Fig 3.
Relationship between the number of hybrid incompatibility pairs and probability of evolving isolation from both parents.
With an increasing number of hybrid incompatibility pairs, reproductive isolation from both parents increases in likelihood (A) but populations require longer periods of time to reach parental fitness levels (B). In these simulations two to six hybrid incompatibility pairs distinguish the hybridizing species and hybrid populations formed at equal admixture proportions (f = 0.5, 1,000 diploid individuals). Simulations labeled F1 indicate that the selection coefficients were set such that the fitness of F1 hybrids between the two parental species equaled 0.8 regardless of the number of incompatibilities. Results are based on 500 replicate simulations. In (A) whiskers represent two standard errors; in (B) smears represent the means of 1,000 bootstrap samples.
Fig 4.
Hybrid populations rapidly develop reproductive isolation from both parental species, even in the presence of migration.
Once hybrid populations diverge in ancestry at hybrid incompatibility loci from parental populations, individual hybrids have higher fitness on average when they mate with other hybrids in their population compared to either parent. (A) No migration and (B) ongoing migration (4Nm1 = 4Nm2 = 8) from parental populations. Dark points represent the mean fitness, and smears represent the means of 1,000 bootstrap samples. In B, fitness is normalized to the mean fitness of individuals in the parental populations. Simulation parameters: 100 replicates per time point, N = 1000, 20 hybrid incompatibility pairs, s1, s2 and h drawn from distributions (exponential, exponential and uniform, respectively, see details S5 Text).