Closely related species of butterfly sampled from southern suture zones in North America exhibit a continuous pattern of gene flow and population difference measures (index values) for autosomes, but not for the Z chromosome; When populations are compared through their Z chromosomes, index values obtained from samples of the same species are separated from those of closely related species by a gap of "missing" values, suggesting a discrete "on--off" criterion for species delimitation. Here, we explore the possibility that some, or all of the index data for suture zones reflects secondary contact between species formed in glacial refugia. We simulate fusion of butterfly populations limited by negative fitness interactions between genes in hybrids, assuming that interactions between autosomes and the Z chromosome are stronger than those among autosomes, and that hybrid fitness effects conform to Haldane's rule. We find that weakly interbreeding populations trace out a path toward equilibrium consistent with the data for butterfly suture zones, in which index values for the Z chromosome lag behind those for autosomes, leading to a similar gap of missing values when species become indistinguishable through their autosomes, but no evidence of a sudden change in index values for the Z chromosome on longer timescales. As a result, we find that the gap can be explained by a process in which the pattern of index data for the Z chromosome is, ultimately, continuous. ### Competing Interest Statement The authors have declared no competing interest.
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