We have added to these data samples from a fifth hybrid zone between H. himera and H. erato cyrbia, two putative incipient species with pre-mating isolation, but no intrinsic post-mating isolation.
Similarly, selection against the introgression of the postman haplotype at the red locus from the postman race into the rayed race in a different hybrid zone could result in genetic hitchhiking of neutral alleles at several sites near the red locus that could be different from the sites that experienced genetic hitchhiking in the first hybrid zone.
We identified environment-dependent and environment-independent combinations and describe here the first vertebrate hybrid zone following the "evolutionary novelty" model defined by Arnold [15], with two types of genotype variation encountered within the same species complex and in the same river.
The first four hybrid zones are all between hybridizing races of H. erato that show no evidence of pre-mating or post-mating isolation [ 8, 10, 33].
The second is from a hybrid zone between Ensatina salamanders in southern California, where Devitt et al. [ 52] inferred that a large proportion of individuals in the hybrid zone were in fact F1 hybrids, based on analysis with NewHybrids.
Because this is the first description of a hybrid zone between polyandrous bird species, it provides a useful counterpoint to studies of hybrid zones among avian species characterized by monogamous or polygynous mating systems, and we believe that intensive studies of this system should provide insight into the interaction of genes, environment, and behavior in the avian speciation process.
In a third case, the moving hybrid zone between Hippolais warblers in western Europe appears completely caused by interspecific interactions [ 78], and is unrelated to the location of preferred habitats based on species distribution models similar to those presented here for jacanas.
Our analyses of phenotypic and genetic markers of jacanas across western Panama and beyond firmly establish hybridization between the two species, and provide the first documentation of a persistent hybrid zone in a simultaneously-polyandrous bird species [ 4].
A mid-ocean hybrid zone was first discovered in deep-sea hydrothermal vent mussels (Mollusca: Bivalvia: Mytilidae) living along the Mid-Atlantic Ridge [ 16].
If this is the case, the Y chromosome introgression is not the first breakdown of the consensus hybrid zone associated with perturbation of the sex ratio, but rather the current incursion of an ongoing arms race between elements distorting the sex ratio.
