This page contains additional material that may be of interest to readers of this manuscript:

Irwin, D.E. 2019. Assortative mating in hybrid zones is remarkably ineffective in promoting speciation. bioRxiv https://doi.org/10.1101/637678

Here is a simulation showing what happens when two populations come into contact, with strong assortative mating but no problems with hybrids (hybrids have high survival just like the parental forms): a complete continuum between the species develops:

In contrast, here is a a simulation in which there is zero assortative mating, but hybrids have 10% lower fitness than “pure” individuals from the two populations. There is lots of hybridization at the start, but the width of the zone stays narrow:

We can fit a cline (the blue line) to how the hybrid index (HI) depends on location, and define the width of the zone as being between HI=0.1 and HI=0.9. We can run the simulation for 1000 generations (with a frame every 5 generations), and see this line fluctuate but the width stays narrow: