Hybrid populations rapidly develop reproductive isolation from both parental species, even in the presence of migration.

<p>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 (4<i>Nm</i>₁ = 4<i>Nm</i>₂ = 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, <i>N</i> = 1000, 20 hybrid incompatibility pairs, <i>s</i>_<i>1</i>, <i>s</i>_<i>2</i> and <i>h</i> drawn from distributions (exponential, exponential and uniform, respectively, see details <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005041#pgen.1005041.s005" target="_blank">S5 Text</a>).