DOI: 10.5061/dryad.k98sf7m30
title: Assortative mating in hybrid zones is remarkably ineffective in promoting speciation

description: Partial prezygotic isolation is often viewed as more important than
 partial postzygotic isolation (low fitness of hybrids) early in the
 process of speciation. I simulate secondary contact between two
 populations (‘species’) to examine effects of assortative mating and low
 hybrid fitness in preventing blending. A small reduction in hybrid fitness
 (e.g., by 10%) produces a narrower hybrid zone than a strong but imperfect
 mating preference (e.g., 10x stronger preference for conspecific over
 heterospecific mates). In the latter case, rare F1 hybrids find each other
 attractive (due to assortative mating), leading to the buildup of a
 continuum of intermediates. The weakness of assortative mating compared to
 reduced fitness of hybrids in preventing blending is robust to varying
 genetic bases of these traits. Assortative mating is most powerful in
 limiting blending when it is encoded by a single locus, is essentially
 complete, or when there is a large mate search cost. In these cases
 assortative mating is likely to cause hybrids to have low fitness, due to
 frequency-dependent mating disadvantage of individuals of rare mating
 types. These results prompt a questioning of the concept of partial
 prezygotic isolation, since it is not very isolating unless there is also
 postzygotic isolation.
description: This Dryad package contains data files, R code to run the HZAM
 (Hybrid Zone with Assortative Mating) simulation, R code to generate data
 figures, and video animations related to this paper:
 Irwin, D.E. Assortative mating in hybrid zones is remarkably
 ineffective in promoting speciation. American Naturalist, accepted pending
 minor revision. Please see that paper for a full
 description of the model that produced the data. The
 same data, based on the same R code, were presented in this earlier
 bioRxiv preprint: Irwin, D.E. 2019. Assortative mating
 in hybrid zones is remarkably ineffective in promoting speciation. bioRxiv
 doi: https://doi.org/10.1101/637678
description: The files in this package are each described below:
 <strong>HZAM_Irwin_AmNat_paper_figures_script.R</strong><br> This is the R code that can be used to generate the data figures in the paper. It requires the data folder described below. <strong>HZAM_Irwin_AmNat_paper_simulation_data.zip</strong><br> This compressed folder (which should be unzipped before use) contains 2,720 data files containing output of 1,370 HZAM simulations. The code in the "HZAM_Irwin_AmNat_paper_figures_script.R" file uses these data files. <strong>HZAM_release_2.0.R</strong><br> This is the R code that runs HZAM (Hybrid Zone with Assortative Mating) simulations. <strong>HZAM_animation_prefratio0.1_hybridfitness1_K1000_UDLMTL3_gen50.mp4</strong><br> This movie file shows 50 generations of a hybrid zone evolving under conditions of 10x assortative mating, no reduction in hybrid fitness, carrying capacity of 2000*, and 3 loci encoding the mating trait. The left side of Figure 2 in the paper shows three timepoints from this same simulation.  (*note the "K1000" in the file names refer to half of the total carrying capacity) <strong>HZAM_animation_prefratio1_hybridfitness0.9_K1000_UDLMTL3_gen50.mp4</strong><br> This movie file shows 50 generations of a hybrid zone evolving under conditions of no assortative mating, 10% reduction in hybrid fitness, carrying capacity of 2000, and 3 loci encoding the functional trait. The right side of Figure 2 in the paper shows three timepoints from this same simulation. <strong>HZAM_animation_prefratio1_hybridfitness0.9_K1000_UDLMTL3_gen1000.mp4</strong><br> This movie file shows 1000 generations (with an image shown every 5 generations) of a hybrid zone evolving under conditions of no assortative mating, 10% reduction in hybrid fitness, carrying capacity of 2000, and 3 loci encoding the functional trait. The blue line shows the fitted cline, and the gray shading indicates the cline width (see the paper for detailed methodology).